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Decadal Survey of Civil Aeronautics: Foundation for the Future (2006)

Chapter: 3 Research and Technology Challenges

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Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
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3
Research and Technology Challenges

The highest priority R&T Challenges for each R&T Area are listed and discussed below. The section for each Area includes a table showing the results of the quality function deployment (QFD) evaluation of R&T Challenges for that area. Each section also discusses general characteristics of high- and low-priority challenges in the relevant Area, R&T Thrusts that encompass multiple Challenges from a given area, and specific Challenges that rank high in national priority, but low in NASA priority. More detailed information for each Challenge appears in Appendixes A to E.

AERODYNAMICS AND AEROACOUSTICS

Introduction

Aerodynamics and aeroacoustics research is required to support development of advanced aeronautical systems. The scope of this R&T Area includes a wide range of fundamental fluid dynamic research ranging from low-speed, low-Reynolds-number flows to hypersonic, chemically reacting flows to aerodynamic issues associated with flight in alternative atmospheres. It does not include aerodynamic issues associated with ground transportation systems or fluid dynamic issues associated with hydrodynamic flows or the space environment.

The QFD process described in Chapter 2 was used to prioritize 19 R&T Challenges related to aerodynamics and aeroacoustics. Table 3-1 and Figure 3-1 show the results. The text that follows describes the 11 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high- and low-priority Challenges, and the R&T Thrusts in this Area.1 Further details on all the challenges, including the rationale for scoring, are found in Appendix A.

In terms of national priority, challenges A1, A2, A3, A6, and A7b all fall within a narrow range. Taking account of the weighting factors and scores, and noting that small changes in many of those elements can produce important changes in the final order, it should be concluded that these challenges are of roughly equal importance.

Top 11 R&T Challenges

A1 Integrated system performance through novel propulsion–airframe integration

Research into improved techniques for propulsion– airframe integration is required to enable greater aircraft flexibility and improve performance, especially as aircraft speeds increase. Improvements in the accuracy of predictions for three-dimensional (3-D) steady and unsteady interactions between external and internal aerodynamics and aeroacoustics would enable the design of advanced aeronautical systems, especially with systems of unconventional design. These interactions include the effects of steady and dynamic distortion on engine operations and the effects of hot, reacting exhaust flows on vehicle aerodynamics. These interactions are particularly important in the design of vertical and short takeoff and landing (V/STOL), extremely short takeoff and landing (ESTOL), supersonic, and hypersonic airplanes.2

1

This chapter describes the top 10 R&T Challenges in each Area, except for the aeronautics and aeroacoustics Area. As shown in Figure 3-1, the NASA priority scores for Challenges A4a through A11 were relatively close, and there was a large difference in the scores for A11 and A12, so in this Area, unlike the remaining four, the top 11 R&T Challenges are described in the aeronautics and aeroacoustics Area.

2

VTOL airplanes can take off and land vertically. This includes tilt-rotors, the AV-8 Harrier, and the Joint Strike Fighter (JSF), for example. VTOL airplanes do not routinely take off or land vertically because of the range-payload penalty associated with the weight limitations of purely vertical operations. Rather, they use any available field length to develop some forward motion and wing lift during takeoff to increase the useful load (fuel plus payload). They tend to land vertically only at the end of the mission, when they are lighter, after burning fuel and/or dropping weapons.STOL airplanes use high-lift systems to take off in less distance than

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

TABLE 3-1 Prioritization of R&T Challenges for Area A: Aerodynamics and Aerocoustics

 

 

 

Strategic Objective

National Priority

Why NASA?

Why NASA Composite Score

NASA Priority Score

 

 

 

Capacity

Safety and Reliability

Efficiency and Performance

Energy and the Environment

Synergies with Security

Support to Space

Supporting Infrastructure

Mission Alignment

Lack of Alternative Sponsors

Appropriate Level of Risk

R&T Challenge

Weight

5

3

1

¼ each

A1

Integrated system performance through novel propulsion–airframe integration

9

3

9

9

9

9

132

3

9

3

9

6.0

792

A2

Aerodynamic performance improvement through transition, boundary layer, and separation control

9

3

9

9

3

3

120

3

9

3

9

6.0

720

A3

Novel aerodynamic configurations that enable high performance and/or flexible multimission aircraft

9

3

9

9

3

1

118

3

9

3

9

6.0

708

A4a

Aerodynamic designs and flow control schemes to reduce aircraft and rotor noise

9

1

3

9

3

1

90

3

9

3

9

6.0

540

A4b

Accuracy of prediction of aerodynamic performance of complex 3-D configurations, including improved boundary layer transition and turbulence models and associated design tools

3

3

9

3

3

3

72

9

9

3

9

7.5

540

A6

Aerodynamics robust to atmospheric disturbances and adverse weather conditions, including icing

9

9

3

1

9

1

112

3

9

3

3

4.5

504

A7a

Aerodynamic configurations to leverage advantages of formation flying

3

1

9

9

3

1

78

3

9

9

3

6.0

468

A7b

Accuracy of wake vortex prediction, and vortex detection and mitigation techniques

9

9

3

1

1

1

104

3

9

3

3

4.5

468

A9

Aerodynamic performance for V/STOL and ESTOL, including adequate control power

9

3

3

1

3

1

76

3

9

3

9

6.0

456

A10

Techniques for reducing/mitigating sonic boom through novel aircraft shaping

3

1

3

9

3

1

60

9

9

3

9

7.5

450

A11

Robust and efficient multidisciplinary design tools

3

3

9

9

3

3

90

3

9

3

3

4.5

405

A12

Accurate predictions of thermal balance and techniques for the reduction of heat transfer to hypersonic vehicles

1

1

3

1

9

9

40

9

9

3

9

7.5

300

A13

Low-speed takeoff and landing flight characteristics for access-to-space vehicles

1

3

1

1

3

9

38

3

9

9

9

7.5

285

A14

Efficient control authority of advanced configurations to permit robust operations at hypersonic speeds and for access-to-space vehicles

1

1

3

1

9

9

40

3

9

3

9

6.0

240

A15

Decelerator technology for planetary entry

1

1

1

1

3

9

28

3

9

9

9

7.5

210

A16

Low-Reynolds-number and unsteady aerodynamics for small UAVs

1

1

3

1

9

3

34

3

9

3

9

6.0

204

A17

Low-drag airship designs to enable long-duration stratospheric flight

1

3

1

3

9

1

42

3

3

3

9

4.5

189

A18

Prediction of communication capability through reentry trajectory and techniques to mitigate impact of communication blackouts

1

1

1

1

9

9

34

3

9

3

3

4.5

153

A19

Aircraft protective countermeasures based on a range of small deployed air vehicles

1

3

1

1

9

1

36

3

3

3

3

3.0

108

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

FIGURE 3-1 NASA and national priorities for Area A: aerodynamics and aeroacoustics.

Minimization of drag by propulsion–airframe integration will reduce fuel burn and CO2 emissions.

A2 Aerodynamic performance improvement through transition, boundary layer, and separation control

Viscous drag at subsonic, supersonic, or hypersonic speeds may be reduced by controlling the onset of boundary layer transition using active control or passive 3-D design concepts. Direct reduction of skin friction drag is possible with extensive laminar flow, which can be achieved with a combination of vehicle shaping and flow control concepts. One example is natural laminar flow using reduced sweep and control of crossflow pressure gradients through shape optimization. A second example is boundary layer manipulation through suction, blowing, or distributed effectors. Related concepts may also be used to reduce separation at high lift and other conditions (e.g., buffet), which improves performance at high-lift conditions. In some conditions of flight, particularly at high lift, a turbulent boundary layer is needed. Active flow control techniques are emerging, including piezoelectric, voice-coil, dielectric barrier discharges, and surface electrical discharges. The potential advantages are clear, but implementation has been hampered by the lack of accurate and efficient methods for prediction (see Challenge A4b) and design and by the difficulty of conducting experiments that require high Reynolds numbers and are sensitive to disturbances such as free-stream turbulence and noise. Work on this Challenge should identify the most promising application domains, control approaches, and actuator concepts and develop efficient methods for design and experimental validation.

A3 Novel aerodynamic configurations that enable high performance and/or flexible multimission aircraft

Most classes of aircraft configuration have remained constant for many years (e.g., the tube and wing of a subsonic transport and the main rotor plus tail rotor of a helicopter). Novel aerodynamic configurations provide substantial opportunities for long-term breakthroughs in aircraft capabili-

conventional aircraft (typically a few thousand feet). Very few STOL aircraft can safely take off on runways shorter than 3,000 ft and none on runways less than 2,000 feet. (This class does not include ultralight aircraft, kit planes, etc. that can operate out of short fields due to their small size but do not have high-lift systems.)ESTOL airplanes would be able to safely take off on runways of 2,000 ft. They would have high-lift systems and thrust-to-weight ratios that are higher than conventional aircraft but not as high as VTOL aircraft. ESTOL aircraft have not yet been developed for commercial or military operations.V/STOL refers to both VTOL and STOL airplanes that convert to fixed-wing flight after takeoff; it does not include helicopters.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

ties. A number of innovative concepts have been proposed in the past and pursued to differing levels. Examples include the blended wing body, canard rotor wing, oblique flying wing, and strut-braced wing. A sustained research program should be promoted to develop novel aircraft configurations, including further development of existing concepts where appropriate, with emphasis on achieving breakthroughs related to the high-priority Strategic Objectives.

Other R&T Challenges would also contribute to enabling novel aerodynamic configurations. Advances in flight mechanics and propulsion–airframe integration (R&T Challenge A1) are required to make advanced concept airplanes viable and robust. Flow control (R&T Challenge A2) could significantly enhance the capability of novel configurations, since it could be assumed a priori in the design process rather than added as an improvement to an existing airplane. Research related to the Common Theme of physics-based analysis tools is needed to move beyond empirical design tools.3 In addition, flight testing is a critical element of a successful research program in novel configurations.

A4a Aerodynamic designs and flow control schemes to reduce aircraft and rotor noise

Many of today’s airports now limit operations because of the noise emitted to the surrounding community. Future passenger growth at many airports will be limited if the noise levels emitted by the newer aircraft are not reduced further, thus adversely affecting capacity. Off-loading the main runway of regional jets by using ESTOL aircraft and rotorcraft, thus reducing congestion for larger passenger aircraft on the main runway, will dramatically increase capacity by allowing more takeoffs and landings at existing airports without increasing demand for runway usage (NRC, 2003; FAA, 2000). However, it will only be possible if these ESTOL aircraft and rotorcraft are quiet. Aerodynamic noise research should be pursued to (1) improve understanding of the underlying flow physics, (2) develop novel technologies, and (3) create improved and validated acoustic prediction and design tools. This research should include a balance of physics modeling, tool development, and experiments. Important physical phenomena that require research include cavity flows, unsteady flow–solid surface interactions, flow separation, rotor dynamic stall, and wake vortex dynamics. Novel needs include quiet, high-lift devices; technologies to enable steep, quiet, slow-approach trajectories; technologies to reduce the strength of vortices shed from the rotor blades and/or vortex/blade position control; integrated advanced control schemes for active rotorcraft noise reduction; and technologies to reduce rotor response to vortex-induced disturbances. Physics-based source noise prediction methods and improved computational aeroacoustic tools are key requirements. Design tools are needed both at the technology level and at the aircraft system level, with particular attention to integrated solutions for aerodynamic and operational issues.

A4b Accuracy of prediction of aerodynamic performance of complex 3-D configurations, including improved boundary layer transition and turbulence models and associated design tools

The aerospace industry lacks computational analysis and design tools that can rapidly and accurately predict complex flow behavior driven by boundary layer transition, flow separation, novel configurations, off-design operations, and multidisciplinary interactions. To meet this need, physics-based design tools must be developed and systematically validated in representative environments. Ideally, these tools should have the following attributes:

  • Adaptive and intelligent self-generating grids that are easily implemented using simple computer-aided design surface instructions, minimal boundary condition definition, and desktop operation.

  • Seamless applicability over the continuum of fluid flows (speed regimes, phase, periodicity) and reference frames.

  • Ability to accurately predict transitional and separated flows, validated through experimentation.

  • Ability to fully describe the state of the fluid at any point in the solution domain, with useful information on the surfaces.

  • Inverse design capability.

The benefit of technologies developed by this Challenge would be enhanced by parallel development of multidisciplinary design tools to address complex nonlinear interactions, and parameter uncertainties and models, while still being computationally efficient (see Challenge A11).

A6 Aerodynamics robust to atmospheric disturbances and adverse weather conditions, including icing

Adverse weather conditions, including storms and icing conditions, significantly reduce the capacity and reliability of the air transportation system. Adverse weather also degrades system safety. This issue is of importance to both civil and military aviation. Research is needed to improve the ability to predict and monitor environmental conditions and develop aerodynamic designs and techniques that are robust to adverse conditions.

At present, wind-shear warning systems are built into commercial aircraft, icing hazards are handled by regulatory constraints on flight operations, and prediction techniques are largely empirical. Low-cost techniques to mea-

3

“Physics-based” refers to the general use of scientific principles in the place of empirical data. It includes the use of principles from chemistry, biology, material science, etc.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

sure upstream environmental conditions should be developed. Examples of promising techniques include microwave, lidar, and laser-acoustic measurement techniques. Efforts to miniaturize and reduce the cost of the measurement equipment should be supported. Techniques to predict and mitigate the impact of adverse environmental conditions on the aircraft operation should be improved. Required improvements include the development of models to predict the impact of multiphase, nonequilibrium situations encountered under icing conditions; validation of icing prediction capabilities to enable a reduction in the high cost of aircraft and helicopter icing certification; and models for the complex-flow, time-dependent, 3-D interactions encountered during wind shear or ambient turbulence on the aircraft flowfield.

A7a Aerodynamic configurations to leverage advantages of formation flying

Formation flight is currently used by military airplanes for a variety of operational reasons, although rarely for drag reduction. Recent breakthroughs in accurate navigation and control make possible extended precision formation flight at cruise and permit exploitation of favorable interference for vortex drag reduction. Although this phenomenon is well known, the magnitude of the potential savings is not widely appreciated. Three airplanes flying in formation and designed to best exploit these effects could reduce vortex drag by more than 50 percent in cruise, a greater reduction than that obtainable by extensive laminar flow control on the wing. This would mean roughly a 20 percent reduction in total drag under identical operating conditions. However, with less induced drag the optimum altitude increases, reducing viscous drag as well. The net result is almost a 30 percent reduction in total drag. Unlike the tight formations required for military applications, drag savings are possible even with longitudinal separations of several miles (Spalart, 1998), reducing safety concerns associated with formation flight.

Initial NASA work on autonomous formation flight has identified some of the technology requirements for achieving these savings, but considerable research remains in both control methodology and aerodynamic design to take most advantage of the concept. Applications to cargo airplanes, rotorcraft, and even supersonic flight are possible but have not been studied extensively. Aerodynamic challenges include vortex location prediction, sensing and control, and wing design for efficient high-lift cruise. Suggested work in this area would result in improved methods for predicting wake vortex evolution; design tools for evaluation and optimization of multiple interacting airplanes; and experimental validation, including flight testing (which is especially important for evaluating real atmospheric effects). The aerodynamic aspects of formation flying are related to R&T Challenges D1 and E2.

A7b Accuracy of wake vortex prediction, and vortex detection and mitigation techniques

Wingtip vortices produced by airplanes present a danger to following aircraft, so airplane designs and techniques that mitigate the strength of these vortices, techniques to locate and determine their strength, and techniques to predict their propagation and decay are important factors in minimizing aircraft separation and enhancing safety.4 (Since aircraft lift is intimately tied to the production of circulation, these vortices cannot be completely eliminated.) Currently, aircraft separation standards are set by conservative estimates of the wake vortex trajectory (generally a sinking trajectory, but also affected by local weather conditions) and decay rate. Techniques to measure the characteristics of upstream wake vortices include lidar and laser-acoustic techniques, but these technologies are currently expensive (limiting their use to larger aircraft) and are less reliable than desired.

Research into techniques to predict the formation, trajectory, and decay of vortices needs to be performed. This includes development and validation of numerical methods to accurately predict the trajectory and dissipation of vortices, integration of local weather prediction techniques into existing larger-scale weather models, demonstration of low-cost techniques for locating and measuring the strength of wake vortices for both ground-based and aircraft-based applications, and investigation of airplane designs that mitigate the strength of wake vortices.

A9 Aerodynamic performance for V/STOL and ESTOL, including adequate control power

The development of ESTOL regional jets able to operate from 2,000 ft runways and taxiways and to cruise in existing air traffic corridors will significantly reduce congestion problems on the main runways of hub airports. V/STOL aircraft will be able to operate from taxiways and other paved areas at major airports, further relieving congestion. In responding to natural disasters and carrying out military operations, low-cost VTOL tactical transports would be able to operate from short, austere landing fields near the focus of attention (e.g., the location of injured civilians or troops, battle areas, and landslides).

Development of an efficient high-lift system is not the most important enabling technology for ESTOL airplanes. Conventional aerodynamic control surfaces become ineffective at the low landing and takeoff speeds of ESTOL airplanes (on the order of 65 knots). The challenge is to generate the forces needed for pitch trim and to control the aircraft at these slow speeds. It is also important to develop a thrust vectoring and reversing nozzle technology that not only provides the required lift but can also be integrated into a low-

4

The scope of this Challenge does not include and would not directly apply to helicopter blade wakes.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

drag configuration. (ESTOL airplanes require much more thrust than conventional or STOL airplanes, but not as much as VTOL airplanes.) In addition, wing design and fuselage shaping are needed to reduce cruise drag in the transonic regime for ESTOL regional jets.

An important task for research related to rotorcraft and VTOL airplanes is to improve hovering and cruise efficiency. Reductions in downward forces in near-hovering flight dramatically improve the payload capability of tilt-rotor and powered-lift aircraft. Active control of large separation regions on these aircraft through blowing, zero-mass effectors, and integrated mechanical devices are promising methods for reducing download. Active twist control of the rotor also allows the rotorcraft to be designed to better match the hover and cruise design conditions, thereby improving efficiency. Active control of separation regions and smart design guided by high-fidelity codes will decrease cruise drag and improve the performance of VTOL airplanes.

A10 Techniques for reducing/mitigating sonic boom through novel aircraft shaping

Safe, efficient, cost-effective, environmentally acceptable supersonic flight over land remains elusive nearly 60 years after airplanes broke the sound barrier. The principal remaining problems are sonic boom mitigation, public acceptance, and sustained supersonic flight performance. Today, federal regulations prohibit civil supersonic flight over land. If this regulatory barrier can be overcome, it will probably stimulate investment that would overcome the other barriers and help usher in a new era of time-critical air travel. Building on the recent in-flight validation of NASA’s theory of shaped sonic boom persistence, a robust and comprehensive plan of research for technology maturation and tool development should be pursued to determine if practical supersonic airplanes can be developed whose sonic boom is acceptable to the public (Pawlowski et al., 2005). Such a plan should comprise public sonic boom acceptability determination; community exposure testing; aircraft shaping techniques that result in a low-amplitude, acceptable acoustic signature with minimal performance impact; critical propulsion-airframe integration technologies commensurate with low-boom design; aircraft and acoustic scaling methodologies; sensitivities to off-design conditions under a variety of atmospheric conditions; rapid and inverse computational design tools that address multiple design constraints; systematic validation through ground and flight tests; and metrics to assess progress and guide continuation according to plan. This Challenge is closely tied to Challenge B8.

A11 Robust and efficient multidisciplinary design tools

Multidisciplinary design tools are pervasive in aeronautics. More effective multidisciplinary tools would likely shorten the design cycle time for conventional aircraft and facilitate the discovery of new highly integrated aircraft designs with better performance than conventional designs. The development of physics-based models for this design environment is addressed in R&T Challenge A4b. This Challenge is associated with the research required to efficiently and effectively integrate multidisciplinary design tools of varying fidelity and numerical complexity into a seamless design environment. Research is also needed on automated techniques for handling and propagating parameter uncertainties throughout the design to allow development of robust aircraft designs.

High-Priority R&T Challenges and Their Associated Thrusts

Some of the high-priority R&T Challenges significantly impact multiple Strategic Objectives; others are high priority because NASA possesses unique capabilities to address them. In particular, R&T Challenges that significantly improve capacity or safety and reliability scored high due to the relevant weightings. The principal factors affecting an increase in capacity relate to expanding the operational capabilities near airports, expanding flight capabilities under adverse weather conditions, and enabling an expansion of operation from smaller airports. The expansion of operations near airports will require research into noise reduction and aircraft wake physics. Expansion of operations under adverse weather conditions will require research associated with techniques to monitor and then mitigate adverse environmental conditions, including icing, wind shear, and free-stream turbulence. Expansion of operations from smaller airports involves research on shortened takeoffs and landings and the associated noise reduction.

The development of improved physical models and design tools for aerodynamic and aeroacoustic phenomena and techniques aimed at understanding and providing the option of controlling these phenomena rank high in the R&T Challenge prioritization. Mastery of these Challenges will enable significant advances in the performance and operability of aircraft through development of improved and possibly revolutionary designs and reduction of design margins associated with uncertainties.

The following four R&T Thrusts describe threads of commonality among the R&T Challenges within the aerodynamics and aeroacoustics Area.

Improved understanding and control of the fundamental physics of aerodynamic and aeroacoustic phenomena

Complex fluid dynamic processes often present barriers to improved aircraft performance, so a better understanding of these phenomena is required. These processes can occur across significant spatial and temporal scales and involve interactions with processes that come under the purview of other disciplines. With a deeper knowledge of the funda-

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

mental physical phenomena, effective techniques will likely evolve to control these processes, enabling improved aircraft performance.

Accurate and robust multidisciplinary design tools

Aeronautics is fundamentally multidisciplinary, so many aspects of aerodynamics and aeroacoustics are impacted by cross-discipline factors. Multidisciplinary aerodynamic and aeroacoustic design tools are needed that are accurate and robust yet cost-effective in terms of computing time and computational resources.

Sensing and responding to the external environment

Development of aircraft systems that respond dynamically to the local environment could significantly improve capacity and safety. With measurement techniques to sense the local environment ahead of an aircraft and allow it to respond accordingly, aircraft spacing can be reduced and operations in adverse weather can be expanded, with no degradation of safety.

Revolutionary aerodynamic configurations

Even though the basic design of civil aircraft has remained remarkably stable for many decades, it is not clear that the configuration has already been optimized. The steering committee believes that improved understanding and control of fluid dynamic phenomena will result in novel aircraft designs offering revolutionary advances in performance and operability in all mission areas.

Low-Priority R&T Challenges

No attempt was made to compile and assess all possible aerodynamic and aeroacoustic issues. All of the Challenges described above are relevant to fundamental aeronautics of civil aircraft. The aerodynamic Challenges that ranked low in the prioritization were largely research areas that support national or homeland security or the NASA space mission but minimally impact Strategic Objectives directly related to the performance of the air transportation system. Examples of these Challenges include hypersonic vehicle technologies, small UAVs, and stratospheric airships. These Challenges could play a vital role in NASA’s space exploration mission and in matters of national and homeland security; however, they ranked low in terms of both national and NASA priority for this report, where the focus is on civil aeronautics.

Hypersonic technologies appear in Challenges throughout the prioritization list. Challenges associated with the development of a more complete understanding of hypersonic issues, such as transition, turbulence, and separation phenomena and the development of techniques to control these phenomena, are included in high-priority R&T Challenges that encompass multiple speed regimes. Challenges specific to hypersonic vehicles, such as low-speed handling characteristics, are rated much lower.

PROPULSION AND POWER

Introduction

This section describes key R&T Challenges and Thrusts associated with aircraft propulsion and electrical power generation that should be addressed via basic and applied research to advance national civil aeronautics capabilities. These advances will permit the U.S. aeronautics enterprise to bring highly competitive products to market and improve the national capacity to move people and goods quickly and affordably with minimal energy usage and environmental impact.

Historically, paradigm shifts in propulsion capability have enabled significant advances in aircraft performance. The replacement of water-cooled piston engines with radial, air-cooled engines enabled the great airframe advances of the first half of the 20th century, while those in the second half were greatly expedited by the gas turbine engine. The gas turbine will very likely continue to be the dominant means of propulsion for both civilian and military aircraft for the next half century. With oil prices at historic highs and increasingly stringent noise and emissions regulations, gas turbine designers face formidable obstacles to create more fuel efficient, cleaner, and quieter engines. Opportunities abound for significant advances, with current gas turbine performance still well below theoretical limits. For example, improvements in overall efficiency and, concomitantly, fuel economy, of more than 30 percent appear attainable (Koff, 2004) but will only occur with significant advances in high-temperature materials and rotating machinery aerodynamics. With advances in information technology, sensor miniaturization, and modeling, intelligent engines capable of self-diagnosis and adaptation, similar to those in the automotive realm, are in the offing. Advances in information technology are also driving electrical power demands for both flight systems and passenger needs—that is, entertainment and productivity. The desire for rapid yet affordable transcontinental and intercontinental travel will continue to motivate research into supersonic flight engines; it is difficult to imagine commercial aviation being restricted to subsonic flight regimes 50 years from now. Airbreathing engine technology also has the potential to contribute significantly to the development of reusable higher payload fraction, access-to-space vehicles. Technical progress will be greatly expedited by the use of validated, physics-based computational simulation tools, which will permit designers to optimize designs and greatly minimize the number of design cycles typical of empirical design-build-test-redesign approaches.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

The QFD process described in Chapter 2 was used to prioritize 16 R&T Challenges related to the Area of propulsion and power. Table 3-2 and Figure 3-2 show the results. The text that follows describes the 10 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high- and low-priority Challenges, and the R&T Thrusts in this Area. Further details on all Challenges, including the rationale for scoring, are found in Appendix B.

Top 10 R&T Challenges

B1a Quiet propulsion systems

Public concerns over the environmental impact of aircraft and airport operations—primarily noise and emissions— have prompted increasingly strict legal and regulatory requirements, which can severely constrain the ability of civil aviation to meet national and global needs for mobility, increased market access, and sustained economic growth. Aircraft noise concerns include takeoff and landing noise; taxi and engine run-up noise; flyovers at cruise altitude over very quiet areas; and sonic booms associated with supersonic flight.

Figure 3-3 shows how the impact of aviation noise on people living around airports has declined in the United States. It contrasts the growth of air travel with the reduction in the number of people exposed to 65-decibel (dB) day-night average sound level (DNL), which is what the federal government has defined as the “significant noise level.” Since 1975, the number of persons exposed to significant noise levels has greatly declined, with the transition of commercial aircraft to quieter models even as air travel has grown dramatically. The availability of low-noise technologies, such as high-bypass-ratio engines, contributed significantly to this transition. Assuming the industry’s continued recovery, and given the goal of doubling capacity over the next 10 to 35 years, the dramatic improvements in noise exposure in the last two decades are unlikely to persist. The environmental impact of aircraft noise is projected to remain roughly constant in the United States for the next several years and then increase as air travel growth outpaces expected technological and operational advancements (Waitz et al., 2004). Furthermore, the public currently reports considerable annoyance even when DNLs are below 65 dB. Regulatory actions to limit or reduce noise exposure will likely lead to even more stringent limits.

Future abatement efforts may need to reduce allowable noise levels to as low as 55 dB DNL in both the United States (NASA, 2003) and Europe (ACARE, 2001). Meeting future noise targets will be extremely challenging and will require continued fundamental research in noise phenomena and advanced propulsion technologies. The development of validated, physics-based noise prediction tools by NASA will greatly aid the development of quieter engines. Research is needed to reduce the noise of engine systems, including fan noise, jet noise, and core noise. Research should also encompass systems analysis; advanced concepts, such as adaptable chevrons; the community impact of aircraft noise; and improved metrics to quantify and mitigate these impacts.

B1b Ultraclean gas turbine combustors to reduce gaseous and particulate emissions in all flight segments

Emissions from aircraft constrain the growth of aviation due to their environmental impacts and potential human health consequences. For example, airports located in air quality nonattainment or maintenance areas increasingly find that air emissions add to the complexity, length, and uncertainty of the environmental review and approval of expansion projects (Akin et al., 2003).

Key pollutants of concern include oxides of nitrogen and sulfur (NOx and SOx), carbon monoxide (CO), unburned hydrocarbons (UHCs), hazardous air pollutants, and particulate matter (PM). In addition, emissions of CO2 and water vapor (H2O) in the upper troposphere and stratosphere are of concern because of their potential impact on Earth’s climate (IPCC, 1999). Both CO2 and H2O are inherent combustion products of hydrocarbon fuels, and their emissions can only be reduced through improvements in overall cycle efficiency (see R&T Challenge B4)—or a change in fuels. Emissions of NOx, CO, UHC, and PM from the combustor can be reduced through the development of ultraclean combustion approaches, a critical step to mitigate the environmental impacts of aviation.

Low NOx emissions can be achieved with both rich- and lean-burning combustor designs. Lean combustion concepts have received substantial market penetration through their widespread implementation in land-based gas turbine applications over the last two decades. The key technical issues associated with these combustors concern unsteady combustion phenomena, including combustion instability, flame blow-off, flashback, and autoignition. Although combustors run lean overall, the majority of commercial aircraft engines run rich in the front end. The key issues associated with them are PM emissions and quench zone mixing (Lefebvre, 1999).

B3 Intelligent engines and mechanical power systems capable of self-diagnosis and reconfiguration between shop visits

In the future, advances in sensing, control, and information technology will lead to engines that are more sophisticated and more intelligent. Research thrusts should investigate how more intelligent systems can (1) improve engine health diagnostics and remedial actions in flight, (2) optimize the mission, and (3) use flight data to improve maintenance on the ground. For current engines, the focus will be very much on diagnostics. Better physics-based modeling will be essential. Development of better computational fluid dynamics (CFD) tools, life-prediction tools, and steady-state

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

and dynamic performance checks will be keys to success. Reducing in-flight shutdowns by a factor of 3 and unscheduled engines removals and delays and cancellations by a factor of 5 should be achievable and would reduce maintenance costs by 50 percent. Requirements include (1) smaller sensors with better response and higher operating temperatures and (2) better materials with narrower property tolerances. This should increase disk and airfoil life by 50 percent.

Intelligent engine development will include active control of many engine components: combustor control to permit operation with leaner burners, leading to lower NOx emissions; compressor active stall control to allow operation at higher pressure ratios, leading to higher fuel efficiency; and closed-loop clearance control to increase turbine efficiencies and extend on-wing life by 3 years.

B4 Improved propulsion system fuel economy

The fuel economy of gas turbine propulsion systems is a function of engine efficiency, propulsion-induced drag, and propulsion weight. Overall engine efficiency is the product of the efficiency of creating hot, high-pressure gases (thermal or cycle efficiency), the efficiency of transferring energy from the hot high-pressure gases to a more desirable form (transfer efficiency), and the efficiency of creating thrust from the engine fan and core flows (propulsion efficiency). The thermal efficiency for a gas turbine (Brayton cycle) is primarily a function of overall engine pressure ratio. That is, as long as the turbine can tolerate the inlet temperature corresponding to a given pressure ratio, the overall pressure ratio sets the efficiency of the cycle. Figure 3-4 illustrates very clearly that state-of-the-art gas turbines have not reached the theoretical limits of thermal efficiency. The technologies identified in the figure have the potential to improve the thermal efficiency of gas turbines, to significantly increase fuel economy, and to decrease the environmental impact of the air transportation system.

Transfer efficiency is determined by the component efficiencies of the fan and low-pressure turbine and the losses of the shaft bearings. High-efficiency, low-pressure turbines need high rotor speeds, but highly efficient fans require low rotor speeds. Therefore, engines with high transfer efficiency must have reduction gearboxes or other technologies that permit different rotor speeds for the fan and low-pressure turbine.

Propulsion efficiency is a function of the difference between the velocity of engine exhaust and the forward velocity of the aircraft. Increasing the mass flow of air through the system at slower speed improves propulsion efficiency and decreases noise. However, this increases the diameter of the engine, which increases friction and flow blockage. Since larger engines will also be heavier, the use of composites or other lightweight materials for construction of the large structural pieces of the turbofan will also be necessary.

As shown in Figure 3-4, improving thermal efficiency by 15 percent requires advances in several technologies: 3D aerodynamics, active flow control, cooled cooling air and a thermal management system, multiwalled cooling, and ceramic matrix composites (CMCs) and intermetallics. Over the long term, advances in all three efficiencies (thermal, transfer, and propulsion) should be able to improve fuel economy by 30 percent relative to the GE-90 for large commercial engines and 30 percent relative to T700/CT7 for small engines.

B5 Propulsion systems for short takeoff and vertical lift

The utilization of V/STOL airplanes and increased use of helicopters could greatly increase the capacity of civil aviation by allowing more takeoffs and landings at existing airports without increasing demand for runway usage (NRC, 2003). V/STOL airplanes include tilt-wing aircraft, tilt-rotor aircraft, vertical-lift fan aircraft, and blown-wing aircraft. Currently, the fuel economy of V/STOL propulsion systems is not on par with that of fixed-wing commercial airplanes. Propulsion systems for all new aircraft must also demonstrate extremely high levels of reliability. Propulsion systems for V/STOL aircraft are in an early state of development or do not exist for civil aircraft. In addition, engine-out strategies need to be developed and verified for certification.

This Challenge should support development of V/STOL and helicopter propulsion systems with fuel economy comparable to future small commercial aircraft—namely, 20 percent better than the CT7 family of engines that are currently in production for small conventional aircraft. Many of the same technologies that apply to large and small engines for conventional aircraft also apply to V/STOL propulsions systems. However, additional technologies such as high-efficiency, angled gearboxes; high-efficiency reduction gear-boxes; large-bleed systems; thrust vectoring systems; noise reduction both inside and outside the aircraft; fan-tip-driven turbines; and high-power clutch systems will be required to put V/STOL airplanes into affordable, large-scale commercial service with minimal environmental impact.

There are three major technology efforts to be undertaken in support of V/STOL airplanes for civil aviation. The first and most important is to demonstrate an engine sized for most helicopters or for UAVs (roughly 3,000-shaft horsepower) that meets the fuel economy goals. The important characteristics of this demonstration engine are to achieve overall pressure ratios of 25:1 or 30:1 and turbine inlet temperatures of 2800°F. This will require some combination of the following technologies: (1) new compressor disk materials, (2) greatly improved turbine cooling configurations, (3) new turbine blade alloys and coatings, (4) component aerodynamics designed with the latest computational models, and (5) highly effective, low-pressure-drop dirt separation. Such an engine would benefit helicopters as well.

Second, the powertrain system of most V/STOL airplanes (as well as helicopters) will consist of shafting with speed reduction gearboxes, angled gearboxes, and perhaps

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

TABLE 3-2 Prioritization of R&T Challenges for Area B: Propulsion and Power

 

 

 

Strategic Objective

National Priority

Why NASA?

NASA Priority Score

 

 

Capacity

Safety and Reliability

Efficiency and Performance

Energy and the Environment

Synergies with Security

Support to Space

Supporting Infrastructure

Mission Alignment

Lack of Alternative Sponsors

Appropriate Level of Risk

Why NASA Composite Score

R&T Challenge

Weight

5

3

1

¼ each

B1a

Quiet propulsion systems

9

1

3

9

3

1

90

3

9

3

9

6.0

540

B1b

Ultraclean gas turbine combustors to reduce gaseous and particulate emissions in all flight segments

9

1

3

9

3

1

90

3

9

3

9

6.0

540

B3

Intelligent engines and mechanical power systems capable of self-diagnosis and reconfiguration between shop visits

3

9

3

3

3

1

82

3

9

3

9

6.0

492

B4

Improved propulsion system fuel economy

3

1

9

9

3

1

78

3

9

3

9

6.0

468

B5

Propulsion systems for short takeoff and vertical lift

9

1

3

3

3

1

72

3

9

3

9

6.0

432

B6a

Variable-cycle engines to expand the operating envelope

3

1

9

3

3

9

68

3

9

3

9

6.0

408

B6b

Integrated power and thermal management systems

3

1

9

3

3

9

68

3

9

3

9

6.0

408

B8

Propulsion systems for supersonic flight

3

1

3

1

9

9

50

9

9

3

9

7.5

375

B9

High-reliability, high-performance, and high-power-density aircraft electric power systems

1

3

9

3

3

3

62

1

9

3

9

5.5

341

B10

Combined-cycle hypersonic propulsion systems with mode transition

1

1

3

1

9

9

40

9

9

3

9

7.5

300

B11

Alternative fuels and additives for propulsion that could broaden fuel sources and/or lessen environmental impact

3

1

3

9

3

1

60

3

3

3

9

4.5

270

B12

Hypersonic hydrocarbon-fueled scramjet

1

1

3

1

9

9

40

9

3

3

9

6.0

240

B13

Improved propulsion system tolerance to weather, inlet distortion, wake ingestion, bird strike, and foreign object damage

3

9

3

1

3

1

76

3

3

3

3

3.0

228

B14

Propulsion approaches employing specific planetary atmospheres in thrust-producing chemical reactions

1

1

1

1

1

9

26

3

9

9

9

7.5

195

B15

Environmentally benign propulsion systems, structural components, and chemicals

1

1

1

9

3

1

44

3

3

3

3

3.0

132

B16

Reduced engine manufacturing and maintenance costs

3

3

3

3

3

1

52

3

1

1

3

2.0

104

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

FIGURE 3-2 NASA and national priorities for Area B: propulsion and power.

FIGURE 3-3 Actual and predicted exposure to significant noise (65-dB day-night average sound level) and enplanement trends for the United States, 1975-2005. SOURCE: C. Burleson, FAA, “Aviation environmental challenges,” Presentation to Panel B, December 13, 2005.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

FIGURE 3-4 Considerable gas turbine fuel efficiency improvements are still possible. SOURCE: J. Stricker, Air Force Research Laboratory, Private communication to panel member D. Crow, February 2006.

clutch systems. The technology goal is to demonstrate highly reliable gearboxes with transfer efficiencies of about 99.8 percent and a power-to-weight ratio of 50 hp per pound. Reliable clutch operation would enable many new types of V/STOL aircraft.

Thirdly, engine-assisted wing lift, such as the blown wing, offers the simplest, most energy efficient short takeoff. Wing aerodynamics need to be developed, and the bleed or suction locations and quantities required need to be demonstrated for blown-wing V/STOL airplanes.

B6a Variable-cycle engines to expand the operating envelope

Variable-cycle engines have two or three flow paths through the engine, variable vanes, and variable exhaust nozzles, all of which allow them to vary engine bypass ratios and pressure ratios. They can improve the performance of both military and civil aircraft in many flight regimes by changing the bypass ratio and pressure ratio as a function of speed, altitude, and mission requirements. For the long-range Joint Strike Fighter (JSF), this should permit a twofold increase in rapid response radius, an eightfold increase in loiter capability, and a 30 percent reduction in gross weight. For a JSF follow-on aircraft, a 25 percent increase in lift and a 10-25 percent increase in range, depending on the mission, appear possible.

Variable-cycle engines have the potential to increase subsonic engine fuel economy. They also appear attractive for a supersonic commercial aircraft that has to accommodate stringent takeoff noise requirements and still achieve reasonable performance at supersonic speeds. For access to space, variable-cycle engines could provide a large reduction in payload costs as well as marked safety improvements.

This Challenge requires the development of numerous technologies: integrated thermal management approaches; reliable air-to-fuel heat exchangers; low-pressure-drop air-to-air heat exchangers; improved JP-8 heat sink capability; CMC technologies and associated life-prediction tools for operation above 2400°F; complex shape fabrication; high-speed bearings; improved turbine cooling; better engine health predictions; probabilistic life analysis; in-flight data analysis; low-emission, high-temperature combustors; variable-geometry fan systems; and improved airframe-engine integration. This Challenge would benefit from the development of intelligent engines (Challenge B3).

B6b Integrated power and thermal management systems

Efficiency can be enhanced by integrating and optimizing, at the vehicle level, the traditionally severable airframe power and thermal management systems. “Integration” refers to physical, functional, and requirements integration of

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

key propulsion and power system components, by combining them into fewer, multifunctional units all tied together in a more-electric architecture (see Challenge B9). Key components and functions include engine starting; electrical power generation, power conditioning, and routing; air cycle environmental control; avionics, fuel, and oil cooling; ventilation; flight control actuation; and overall vehicle and propulsion system thermal management, especially waste heat recovery and/or rejection. For example, engine start, auxiliary power, and environmental control systems may be combined into an airframe-mounted integrated power package that is physically coupled to the engine through power extraction and waste heat recovery. In this integrated approach, flight control systems are likely to be driven by electric or electrohydrostatic actuators, and thermal management is addressed in a seamless, system-level fashion. At the propulsion system level, electric power must be generated and integrated with airframe needs in the most efficient manner. This may be by a generator mounted on the shaft of the low-pressure turbine or, eventually, by fuel-cell-driven generators distributed within the airframe.

Today’s modeling tools are derived from legacy approaches in which numerous component suppliers individually design, develop, and validate their product based on component-level requirements and specifications. New modeling and simulation infrastructures are necessary to use modeling tools in a system-level design framework, accommodating multiple platforms across multiple sites. A robust modeling framework is necessary to justify the system-level benefit of a given integrated component that may weigh or cost more than a traditional component or have different or enhanced functionality.

B8 Propulsion systems for supersonic flight

Commercially viable supersonic propulsion remains an elusive goal. Key issues include system performance and efficiency, the current ban on civil supersonic flight over the continental United States (14 CFR 91 ¶817), and Stage 4 noise standards.

Particularly for supersonic flight, propulsion systems development needs to be integrated with the design of the rest of the aircraft in a multidisciplinary effort to find an optimal trade-off between performance, efficiency, noise, emissions, and thermal management. Engine–airframe integration becomes more critical as the flight speed increases. This Challenge requires validated physics-based numerical simulation codes for component-level analysis and the improvement of multidisciplinary, system-level design tools for vehicle analysis.

Gas turbine research topics of interest include

  • Variable-cycle engines optimized for both subsonic and supersonic flight with low specific fuel consumption, high thrust-to-weight ratios (T/Ws), and low noise.

  • Lightweight, low-noise, efficient inlets and nozzles that also reduce wave drag and help to shape the sonic boom efficiently.

  • Integrated airframe and propulsion controls to actively reduce vibration mode interactions between the engine and the plane (NIA, 2005).

  • Noise and emissions data to validate models for sonic boom signature and determine its effect on humans (psychoacoustics), to assess the interaction of combustion products with ozone, and to help establish or confirm noise and emissions regulations.

  • Electric actuation systems to eliminate the need for high-temperature hydraulic actuation systems.

  • Active flow control to improve engine efficiency, reduce noise, and enable different airframe–propulsion integration concepts.

  • Combustion process physics: modeling and experimental validation of injection, mixing, ignition, finite-rate kinetics, turbulence–chemistry interactions, and combustion instability to improve efficiency and life.

  • Advanced materials and coatings (including high-temperature alloys for compressor and turbine disks) that meet requirements for operating temperature, service life, strength, and propulsion system noise.

  • Alternative engine cycles for supersonic flight that might replace or enhance traditional gas turbines.

Many of these technologies are discussed in other R&T Challenges; much of the research proposed for subsonic engines will build a foundation for supersonic flight.

B9 High-reliability, high-performance, and high-power-density aircraft electric power systems

Future aircraft power systems must be able to meet the demands of more-electric aircraft (MEA). Future aircraft will progressively replace more and more mechanical and hydraulic systems with electrical systems, and electrical loads imposed by conventional systems will also continue to grow, to improve performance, convenience, and reliability. The higher power requirements of conventional loads are being driven by advances in avionics as well as by passenger entertainment and productivity needs. For example, the electric power demand on Boeing’s 787 is nearly 1 MW, which is double that of the Boeing 777 and many times that of the first U.S.-built commercial jet, the Boeing 707 (Ames, 2005). The growth of new MEA loads is being driven by advances in the capabilities of electric actuators and controls, and it is being enabled by the development of more flexible and reliable aircraft generators. This Challenge can be met by improving key components and system-level technologies:

  • Tenfold increase in power density for electric generators and motors suitable for aircraft use.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×
  • Fivefold increase in energy and power density of suitable batteries and hybrid storage systems (e.g., the battery–ultracapacitor).

  • An order of magnitude lighter optimized power system architectures (including, for example, a DC power bus, remotely controlled loads, and a wireless system control).

  • Intelligent power management and distribution (PMAD) using advanced system models and wireless sensors or sensorless control technologies for graceful degradation and failsafe operation.

  • Advanced analysis and simulation tools for multi-converter power systems that can predict new modes of system dynamics and instability.

B10 Combined-cycle hypersonic propulsion systems with mode transition

The primary NASA hypersonics mission is for access to space in support of the Space Exploration Initiative and in placing and maintaining scientific payloads in low Earth orbit. A two-stage-to-orbit (TSTO) vehicle using a hydrogen-fueled, airbreathing first stage and a hydrogen-fueled rocket second stage could double the payload fraction to low Earth orbit relative to a two-stage, hydrogen-fueled rocket.5 This would greatly reduce the cost of putting a payload into orbit. In addition, airbreathing hypersonic vehicles offer airplanelike operations, with increased safety and efficiency, more robust operation, and greater mission flexibility than rockets. A secondary mission for NASA hypersonics is to provide synergy with DoD programs in the development of missiles for time-critical targets; global strike and rapid resupply aircraft; and routine, on-demand access to space.

One combined-cycle hypersonic propulsion system under study for access to space is a turbine-based combined-cycle (TBCC) system. In order to design complex, combined-cycle hypersonic propulsion systems, experimentally validated, physics-based tools must be developed and refined because steady, full-enthalpy, clean air conditions cannot be reproduced in hypersonic ground test facilities. Experiments must be conducted on unit problems (e.g., jet injection into a supersonic stream) that contain the relevant flow physics but are amenable to simulation. Facility upgrades, such as for long-duration, high-temperature testing of engine materials and structures, should be completed in order to conduct the unit experiments under near-realistic flight conditions. Advanced diagnostics must be developed and used to obtain detailed databases in unit-problem experiments for complete validation of the computational tools, which can then be used for the vehicle design. Multiple-point validations are needed to verify that the tools produce results that can be extrapolated to conditions not available on the ground. Ultimately, flight testing must be conducted in order to obtain results under realistic operating conditions. Experiments should be flown on low-cost, suborbital rockets instead of expensive flight vehicles.

High-Priority R&T Challenges and Their Associated Thrusts

The rationale for the assignment of scores for each R&T Challenge is provided in Appendix B. In this section, the rationale for scoring will be discussed more generally. Table 3-2 shows that the top 10 R&T Challenges were all very relevant to NASA’s mission, while those below the top 10 were less well aligned (with the exception of extraterrestrial planetary propulsion, which is clearly a NASA mission). In general, NASA has considerable infrastructure to support all the Challenges, with the exception of electric power systems, and NASA is particularly well equipped to conduct supersonic and hypersonic R&T. Other than propulsion in the atmospheres of extraterrestrial planets, industry, DoD, or, in a few cases, some other government agency will support R&T relevant to the high-priority Challenges. DoD, for example, has historically been a very strong supporter of V/STOL research. However, in the procurement-driven environment in which industry and the DoD live, time pressures often preclude achieving fundamental understanding, and empiricism must be resorted to when problems arise. Even though NASA may not be the only sponsor for some R&T, it can distinguish its research support by developing a fundamental understanding of phenomena, a strong commitment to physics-based modeling, and extensive validation of those models. All 10 high-priority Challenges entail moderate to high risk, which is the appropriate level for NASA R&T.

Not surprisingly, all of the top 10 Challenges involve gas turbine engines, with a strong focus on subsonic operations, the only flight regime currently supporting commercial capacity. V/STOL propulsion systems rank high for their potential to improve capacity, but this will not happen unless significant improvements are made in noise, fuel economy, and reliability. The top 10 Challenges will increase the efficiency of future aircraft, with greater levels of systems integration and optimization offering benefits not possible on aircraft designed component by component. Advances in information technology will lead to intelligent propulsion systems that invoke variability to optimize mission performance. These advances will increase demand for onboard electrical power, which will require electric power generation and distribution systems with more power and higher efficiency. Global air transportation is unlikely to be permanently confined to subsonic flight. Supersonic propulsion technologies will have strong synergies with DoD supersonic aircraft and space launch missions. In addition, many supersonic technologies will also be used to improve the performance of subsonic aircraft components and systems.

5

P. Buckley, AFRL, “Payload mass fraction vs. staging velocity for TSTO vehicles to 51.7° orbit,” Presentation to the DoD Technology Area Review and Assessment on March 29, 2004.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

Combined-cycle hypersonic propulsion systems are expected to enable reusable launch vehicles with higher pay-load fractions and to benefit DoD as well.

The following four R&T thrusts describe threads of commonality among the R&T Challenges in the propulsion and power Area:

  • High-temperature materials and structures.

  • Validated physics-based modeling and simulation.

  • Systems integration.

  • Intelligent, adaptive systems.

Most of the Challenges in this area, regardless of rank, fall into one of these Thrusts, which are very important and will require significant investment of resources.

High-temperature materials and structures

Advanced materials are a key enabling technology for aeronautical and space vehicles and play a particularly critical role in propulsion systems. New developments in materials and processes for the production of these materials can deliver important improvements in performance, efficiency, safety, and reliability and can enable major advances in engine cycle design. In addition to developing materials with higher use temperatures, there is very significant payoff for high-temperature materials with (1) lower density or higher specific strength, (2) greater resistance to the combustion environment, (3) higher damage tolerance and predictable modes of degradation and failure, and (4) multifunctionality.

Significant NASA investment in materials is absolutely crucial for continued advances in subsonic, supersonic, and hypersonic propulsion and for continued U.S. leadership in advanced propulsion systems.

Gas turbines will continue to dominate civil aviation in the next few decades. Fuel costs, safety, and noise will drive major improvements in efficiency and reliability. Overall efficiency improvements will require higher pressure ratios for the overall cycle, higher turbine inlet temperatures, improvements in fan efficiency, and weight reduction in the large structural engine components. To achieve this, a number of materials developments must occur, including stronger compressor disk materials, higher temperature turbine disk and airfoil materials, and thermal barrier coating systems with higher temperature capability and increased reliability. For larger fan and structural components, low-density intermetallics and improved polymeric composites are needed. Over the past decade NASA has provided leadership and worked cooperatively with engine manufacturers in the development of advanced superalloy turbine disks and single-crystal airfoil alloys that will significantly improve the performance of the next generation of commercial engines. Continued support for research on airfoil and disk materials (including new processing approaches) with temperature capabilities 100°F to 200°F greater than current alloys is a high priority, since a broad exploration of new superalloys, refractory alloys, and intermetallics is beyond the scope and resources of any single engine manufacturer.

NASA has also contributed substantially to the fundamental knowledge base on oxidation of superalloys and coatings and the performance of bond coat/yttria-stabilized zirconia thermal barrier coating systems. Breakthroughs are needed in new ceramics and intermetallic bond coats for thermal barrier coating systems. New testing methodologies should be developed for these coatings to simulate engine environments, including the high thermal gradients that are characteristic of the turbine airfoil.

The development of intelligent engines will also require progress in life prediction, materials diagnostics, and multifunctional materials to enable computation-based life-prediction tools and complementary new approaches to in situ materials diagnostics.

Advances in supersonic and hypersonic propulsion will permit more efficient cross- and intercontinental travel and access to space, respectively. As Mach number increases, propulsion system temperatures escalate rapidly and oxidation becomes a major difficulty, particularly for air-breathing engines. The ceramics, CMCs, and high-temperature metallics (with active cooling) needed for these propulsion systems remain at low technology readiness levels. Materials systems in need of further development include carbon– carbon and carbon–silicon carbide composites, refractory alloys (rhenium-, niobium-, or molybdenum-based), and nickel alloys. Innovation in processing, joining, and close integration of materials with propulsion system design is essential. Significant progress in supersonic or hypersonic flight will require substantial investment in ultrahigh-temperature ceramics, CMCs, and high-temperature metallics. No single U.S. industrial organization has the expertise to make the major breakthroughs in materials that are required.

Validated physics-based modeling and simulation

With the advances in computational speed, power, and affordability of the last two decades, aeronautics researchers have turned increasingly to computational simulation codes to model the complex physical and chemical conditions inherent in aircraft propulsion and power systems. Industry is appropriately enamored of the possibility of using computational simulation to reduce significantly both the cost and time of product development, to optimize system designs, and to increase reliability. Academic and government researchers also value the potential to attack more complex problems. Computational simulations generally employ a number of physics-based models within the governing conservation and state equations. Examples of models already in use include combustion–turbulence interactions, subgrid turbulence models in large eddy simulation (LES) codes, effects of unsteadiness in steady-state compressor codes, reduced-order chemical kinetic mechanisms, and droplet–

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

flow interactions. These physics-based models often contain adjustable parameters that are grossly calibrated to empirical data sets; the data sets themselves are often incomplete, particularly with regard to boundary conditions, prompting further untested assumptions to be incorporated. The entire codes themselves are often not validated in detail except for comparing their code predictions to input and output measurements. The codes often do not work well when the design space changes considerably, prompting more tweaking of the adjustable parameters. Nevertheless, within their applicable ranges, the computational simulation codes have enabled technical progress, as witnessed by the state of aircraft propulsion today. Unfortunately, the applicable range limits themselves are often not well understood. NASA and its partners can greatly advance aircraft propulsion and power by developing and validating the constitutive physics-based models.

Physics-based models are readily assimilated by industry into their proprietary product system design codes. Research into physics-based models can be conducted jointly by NASA, industry, and academia since it is fundamental in nature, publishable, and shareable. It is work that takes time to mature, yet advances can readily be translated into practice as they occur. Validation involves the design of experimental facilities of appropriate scale and the use of advanced, nonperturbing diagnostics to measure parameters accurately in space and time to rigorously ascertain model fidelity. It is an iterative process culminating in submodels whose accuracy and range of applicability are well established.

Systems integration

This R&T Thrust is intended to support a clear trend in aeronautics design—namely, the movement toward aircraft system-level integration and optimization of traditionally separate airframe and engine subsystems. Improved systems integration will increase capacity by increasing operating flexibility, enabling the use of shorter runways (by improving the performance of powered lift or thrust vectoring systems), reducing end-user costs, and facilitating the design of commercial supersonic aircraft. Efficiency and safety will also be improved by more functional designs that are robust against adverse operational conditions (icing, wake ingestion, foreign object damage, and temperature extremes). “Integration” in this context refers to the physical, functional, and requirements integration of key propulsion and power components with each other, and with other systems, such as the airframe, the avionics, and the overall air transportation system. Optimization of key metrics (cost, weight, thrust, and fuel consumption) at the system rather than the component level is also included in this Thrust.

Integrated power and thermal subsystems were discussed under R&T Challenge B6b. A second example of the systems integration Thrust can be seen with the inlet and exhaust systems that will be required for innovative air platforms. Blended wing-body concepts have been proposed that use boundary-layer-ingesting engine inlets. This approach reduces the performance of the propulsion system but more than compensates for that loss with vehicle improvements in lift and drag. Similarly, although a variable-cycle engine for supersonic cruise might be heavier than a fixed-cycle engine of comparable thrust, it could also eliminate the need for heavy airframe-mounted, inlet-variable geometry, thereby increasing overall vehicle T/W. A higher degree of systems integration should be evident from the earliest design phases and may necessitate entirely new aircraft or engine architectures. New process modeling and simulation tools, along with business models, must also be developed to enable design and validation of integrated systems in a seamless, multiple-organization environment.

Intelligent, adaptive systems

The development of intelligent, adaptive systems technologies will be a key enabler for civil and military aeronautics and space. These technologies will permit (1) real-time, low-latency health monitoring systems; (2) optimization of the performance of current propulsion systems according to mission requirements and environmental conditions, including active control to enhance performance and avoid anomalous behavior; (3) new sets of tools for extended life and improved maintenance of commercial and military fleets; and (4) totally innovative systems for the future. These technologies involve engine and propulsion systems modeling; improved sensor capabilities; and innovative software for control logic that adjusts engine performance to enhance stability, improve distortion tolerance, minimize noise and emissions, and address deterioration issues in service.

Intelligent, adaptive systems are coming online, principally for the health monitoring of both commercial and military engine systems. This capability will anticipate and prevent failures, using control logic to reconfigure engine operation. This capability will improve time on wing, improve readiness, and reduce operating costs. Intelligent engine technologies are essential to the creation of variable-cycle engines. These engines use variable geometry to optimize performance for the mission takeoff, climb, cruise, descent, and landing.

Intelligent, adaptive technologies are needed to reduce fuel consumption and environmental impact by morphing the aircraft or engine to suit the needs of the moment—for example, a takeoff configuration to address noise requirements and a cruise configuration optimized for fuel burn and low NOx emissions at altitude. Similar technologies will also be used to optimize supersonic and hypersonic engine configurations. For example, intelligent, adaptive engine technologies can be used to optimize a low-noise configuration for takeoff with high bypass ratios and then transition into supersonic or hypersonic configurations. These technologies also have direct application to space vehicles.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

Intelligent, adaptive technologies need to be developed for current and future propulsion systems. With current systems, knowledge management should be developed in areas of software control to provide real-time assessment of the remaining life of critical engine components. In new systems, active control and variable geometry should be used to tailor propulsion flows to reduce sensitivity to inflow distortion, to enhance compressor stability, to control exhaust jet area and vector angle to reduce noise and emissions, and to enhance vehicle performance through powered lift.

Low-Priority R&T Challenges

R&T Challenges B11 and B13 ranked in the top 10 in terms of national priority but not by NASA priority. Challenge B11 (alternative fuels and additives for propulsion that could broaden fuel sources and/or lessen environmental impact) is clearly an important national priority. It was ranked lower as a NASA priority because DOE will need to take the lead in establishing the national infrastructure for an alternative fuel and because the combustion research needed to develop such a fuel will take much less time putting an alternative fuel infrastructure in place. Furthermore, aviation fuels are likely to have a first call on petroleum supplies should they become scarce, so that the use of alternative fuels for aviation is likely to follow their widespread use for ground-based applications, which would place less stringent demands on weight, volume, reliability, safety, and certification of new systems and technologies.

Challenge B13 (improved propulsion system tolerance to weather, inlet distortion, wake ingestion, bird strike, and foreign object damage) is ranked low in terms of NASA priority because the relevant technologies are more mature (and the attendant risk lower) and it is not as relevant to NASA’s mission as the Challenges that scored in the top 10 by NASA priority.

MATERIALS AND STRUCTURES

Introduction

Advances in civil aeronautics materials and structures technologies are often the key enablers for new modalities of operation or regimes of flight. For example, improving jet engine efficiency requires continual introduction of new materials to allow the implementation of advanced aerodynamic concepts and higher operating temperatures to increase propulsion efficiency. A comprehensive multiphysics understanding of materials and structures enables innovative designs. New analysis techniques produce the next generation of design tools, which will allow revolutionary structural concepts to be accelerated into applications.

The assessment of R&T Challenges related to materials and structures was influenced by the globally competitive nature of the aerospace industry, particularly in the civilian aircraft market. New material and structural technologies that would help U.S. industry establish a clear advantage over its global competitors received high marks. R&T Challenges were also ranked bearing in mind global needs in aeronautics. Growth in demand for the movement of passengers and goods, especially against a backdrop of rapid economic development in Asia, calls for significantly increased capacity in the air transportation system. Similarly, environmental concerns related to fuel efficiency led to a focus on materials and structures Challenges for engine development and harvesting of energy from structural components and systems. Improvement in structural performance and efficiency was another key driver, and the assessment focused on design methods and tools required to facilitate such improvement. The changed climate for national and international security was another important factor.

The QFD process described in Chapter 2 was used to prioritize 20 R&T Challenges related to materials and structures. Table 3-3 and Figure 3-5 show the results. The text that follows describes the 10 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high- and low-priority Challenges, and the R&T Thrusts in this Area. Further details on all Challenges, including the rationale for scoring, are found in Appendix C.

Top 10 R&T Challenges

C1 Integrated vehicle health management

Integrated vehicle health management (IVHM) refers to monitoring, assessing, and predicting the health6 of aircraft materials and structures using networks of sophisticated onboard sensors. A fully integrated approach to IVHM relies on a multidisciplinary set of analysis, testing, and inspection tools, including miniaturized sensors and distributed electronics; sophisticated signal processing; data acquisition, integration, and database maintenance; artificial intelligence; damage science; and the mechanics of structures and their failure.

IVHM benefits all classes of aircraft, in all speed regimes and phases of flight. With a national fleet of aging aircraft and infrastructure in an industry with low profit margins, IVHM is increasingly important due to its ability to increase safety and reliability. It can also have a number of benefits for capacity. Decreasing the possibility of unexpected failure could speed the introduction of innovative material systems and structural concepts and enable the use of traditionally high-maintenance (and high-cost) systems, such as rotorcraft. More data would allow better understanding of the stresses experienced by a system, reducing the amount of overdesign motivated by uncertainty. In addition, aircraft could report the predicted lifetimes of their own parts and

6

“Health” in this context implies either an absence of measurable material flaws or an ability to coordinate the growth rate of flaws with the safe life remaining for the element in question.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

TABLE 3-3 Prioritization of R&T Challenges for Area C: Materials and Structures

 

 

 

Strategic Objective

National Priority

Why NASA?

NASA Priority Score

 

 

 

Capacity

Safety and Reliability

Efficiency and Performance

Energy and the Environment

Synergies with Security

Support to Space

Supporting Infrastructure

Mission Alignment

Lack of Alternative Sponsors

Appropriate Level of Risk

Why NASA Composite Score

R&T Challenge

Weight

5

3

1

1/4 each

C1

Integrated vehicle health management

9

9

3

1

9

3

114

9

9

1

9

7.07

98

C2

Adaptive materials and morphing structures

9

3

9

3

9

3

108

9

9

1

9

7.0

756

C3

Multidisciplinary analysis, design, and optimization

9

3

9

1

3

3

96

9

9

3

9

7.5

720

C4

Next-generation polymers and composites

9

3

9

1

9

3

102

9

9

1

9

7.0

714

C5

Noise prediction and suppression

9

1

3

9

3

1

90

9

9

3

9

7.5

677

C6a

Innovative high-temperature metals and environmental coatings

3

9

3

1

9

3

84

9

9

3

9

7.5

630

C6b

Innovative load suppression, and vibration and aeromechanical stability control

3

9

3

1

9

3

84

9

9

3

9

7.5

630

C8

Structural innovations for high-speed rotorcraft

9

1

3

1

9

1

72

9

9

3

9

7.5

540

C9

High-temperature ceramics and coatings

3

1

9

3

3

9

68

9

9

3

9

7.5

510

C10

Multifunctional materials

3

3

9

3

9

9

84

3

9

3

9

6.0

504

C11

Novel coatings

3

9

3

3

1

1

80

3

9

3

9

6.0

480

C12

Innovations in structural joining

3

3

9

1

3

3

66

3

9

3

9

6.0

396

C13

Advanced airframe alloys

9

1

9

1

3

1

84

1

3

1

9

3.5

294

C14

Next-generation nondestructive evaluation

3

9

1

1

3

1

70

3

9

1

3

4.0

280

C15

Aircraft hardening

1

9

1

1

9

1

66

3

3

1

9

4.0

264

C16

Multiphysics and multiscale modeling and simulation

3

3

3

3

3

1

52

3

3

3

3

3.0

156

C17

Ultralight structures

3

1

3

1

3

3

38

3

9

1

3

4.0

152

C18

Advanced functional polymers

1

3

1

1

3

1

30

9

3

3

3

4.5

135

C19

Advanced engine nacelle structures

3

1

3

1

1

1

34

1

9

1

3

3.5

119

C20

Repairability of structures

3

3

3

1

1

1

44

3

3

1

3

2.5

110

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

FIGURE 3-5 NASA and national priorities for Area C: materials and structures.

report the need for replacement parts, reducing operating costs and maintenance downtime. IVHM could quickly diagnose root problems, minimizing flight delays and increasing capacity (Powrie and Fisher, 1999; Simon, 2000). IVHM may also reduce vehicle operating cost and maintenance downtime and can speed the introduction of innovative material systems and structural concepts. Real-time onboard sensor systems that monitor the actual state of materials and structural components enable more efficient use of materials, including novel concepts.

There are two main features of the next generation of IVHM: (1) Sensor packages will be very small and exceedingly lightweight and (2) the reliance on humans to interpret the sensor output and assess the impact on structural integrity will be reduced or eliminated.

Three classes of IVHM systems warrant attention over the next decade, culminating in flight testing of full-scale IVHM systems that detect multisite damage. The first class includes fiber-optic sensor systems that use multiplexed fibers attached to or embedded within the structure, each with numerous sensing sites interrogated in turn by a single electro-optic module. The second class includes locally self-powered, wireless microelectomechanical sensors tiny enough that very large numbers of sensors become practical. Each sensor mote performs a point measurement, so many are used to effectively cover large areas. The third class includes discrete active and passive remotely powered sensor modules (e.g., by means of guided-wave ultrasonic or acoustic emission) that may be large compared to sensor motes but can interpret multimode vibrations or multiphysics parameters (temperature, stress, humidity, etc.) that propagate over relatively long distances within the key structural elements.

Successful application of IVHM also relies on continued research and refinement in fundamental structural mechanics and the mechanics of damage and failure for accurate interpretation of IVHM sensor data and to support autonomous decision making for damage recovery and mitigation.7

C2 Adaptive materials and morphing structures

Use of adaptive materials and morphing structures to change the aircraft shape (outer mold lines) and functions on demand represents a revolutionary approach for enabling optimal performance over a range of flight missions. Morphing wings that change their planform area by up to 50 percent and alter their sweep angle up to 50 degrees are emerging as a viable technology, and the benefits would be

7

See R&T Challenges D4 and D5.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

far more than the simple variable-sweep configurations of the past. However, the costs of incorporating such technology have not yet been evaluated.

During the past 2 years, two prototypes from the Defense Advanced Research Projects Agency (DARPA) Morphing Aircraft Structures program were successfully tested at transonic speeds in NASA Langley’s Transonic Dynamics Tunnel at a scale sufficient to validate the concept. This design included concepts such as stretching skins, sliding skins, and seamless camber change. DARPA has also sponsored flight research of morphing technologies applied at the system level. These tests identified critical long-pole, component technologies that now limit the use of morphing technologies, particularly in heated supersonic flow. Adaptive materials have emerged as the number one component technology need for morphing aircraft. These materials have the ability to radically change the properties of component materials, to facilitate both effective load-carrying abilities and ease of actuation from one shape to another, as well as to change the structural shape, from large variations in wing area to seamless camber-changing.

Adaptive materials may be self-actuated by energy inputs such as light, heat, and electric or magnetic fields. They include heat-activated shape memory alloys like NiTiNOL; ceramics (e.g., lead zirconate titanate); photonically activated, lightweight, flexible shape memory polymers; electrically activated piezoelectrics; and magnetorheological fluids.

This Challenge requires development of commercial, high-speed, morphing airframe concepts, development of structural components such as stretching skins, and accelerated development of a special class of actuatable adaptive materials with lifetimes comparable to those of currently used materials. A fundamental task is to characterize the mechanical response of these inherently nonlinear materials, including hysteresis, fatigue, long-term behavior, and damage behaviors. Analysis and design tools that accurately predict these responses will open the door to even more applications of these revolutionary adaptive structural concepts, which could optimize performance and expand the flight envelope.

C3 Multidisciplinary analysis, design, and optimization

Methods for simulation-based, multidisciplinary design and optimization (MDO) are at the very core of a philosophy that moves away from the build-test-build approach, which has proven to be expensive and ineffective in exploring the aeronautical design space. MDO processes develop synergistic benefits by integrating people, analytical tools, experimentation, and information to design complex structural components and systems (Sobieszczanksi-Sobieski and Haftka, 1997). These approaches allow for development of optimal configurations, topologies, and dimensions for structural members and components to achieve design objectives, and they permit designers to examine the myriad what-if’s that characterize sophisticated designs with interdisciplinary trade-offs.

After almost two decades of R&D, MDO processes for conventional designs have reached a high level of sophistication. In structural designs where the topology or outer mold lines are defined, analytical methods such as the structural finite-element technique, coupled with similar analytical tools for load assessment, provide a high level of success. However, for designs with a multiplicity of topologies, some of which are not well-defined, and for problems where a large number of design parameters and constraints must be considered in the early stages of the design process, MDO methodologies are still underdeveloped (Giesing and Barthelemy, 1998). Major effort must also be directed at including the effects of uncertainty in the design process, as well as increasing the level of detail in representing the structure. New ways of formulating problems that incorporate quantitative reliability measures to facilitate effective design decisions have been considered in this context. The extension of these approaches to large-scale structural and material design problems represents an entirely different level of problem complexity.

Significant new developments are required in both the platforms and the embedded tools that constitute the MDO process. Efficiency and effectiveness of the search process continue to be a problem, particularly in large-dimensionality problems and multimodal or disjointed search spaces. Current platforms are ill equipped to efficiently parse the vast amounts of data associated with the design process. There is a marked need for developing analysis modules for the search process to query in the design process. Such analysis modules must be based on the physics of the problem or on inferences derived from experimental data. While digital designs have enabled tight manufacturing tolerances and manufacturers can incorporate cost models, explicit mathematical modeling of manufacturing processes, repair, and environmental impact must be better integrated into the MDO process. These analysis tools must be developed at multiple levels of granularity and precision, to coincide with the appropriate stage of the design process. The numerical efficiency of these tools is paramount, and alternative paradigms that take advantage of a new generation of parallel computational hardware must be sought (Giesing and Barthelemy, 1998; Sobieszczanksi-Sobieski and Haftka, 1997). Uncertainty modeling in a data-lean environment, specifically for new concepts, continues to be an issue in this regard. There is a similar dearth of computationally efficient methods for reliability assessment, particularly in situations where uncertainty distributions do not conform to standard forms or where components or elements exhibit discrete behavior. The propagation of uncertainty in complex and highly coupled multidisciplinary systems needs to be modeled, and tools for design and optimization in a nondeterministic environment continue to be computationally intractable, especially when applied to design problems involving a large number of nondeterministic variables, parameters, and design constraints. Furthermore, the inclusion of risk and reli-

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

ability analysis in the design process would yield a time-dependent description of risk associated with structural and material systems in service, facilitating decisions that enhance vehicle availability and reliability.

Use of commercial tools in optimization is not enough to advance the state of the art in MDO. Optimization is only one piece of the analysis, design, and optimization triad. It is the tightly integrated development of analysis and optimization tools that furthers the potential of MDO methods. In the aerospace arena, such expertise is unique to NASA. Additional gains can be realized with NASA working in close collaboration with researchers from academia and industry. A number of synergistic benefits could also be achieved by developing this aspect in concert with health-monitoring technologies (see R&T Challenge C1).

C4 Next-generation polymers and composites

Over the past 50 years, polymeric composites have revolutionized and improved the performance of aircraft structures. Future needs for enhanced structural performance, high-temperature capability, and durability can only be met by the next generation of polymer-based composites. Next-generation composites will take advantage of improved high-temperature polymeric matrices, new reinforcement materials, hybrid reinforcement approaches, improved joining technology, and science-based manufacturing with controlled 3-D placement of reinforcements. This Challenge includes development of tougher, higher-temperature adhesives for joining, innovative fillers to enhance performance, and new core materials for ultralightweight sandwich construction. It also includes development of repair techniques to restore structural integrity to damaged composite structures. The development of next-generation composites is dependent on three capabilities: multiscale modeling that links nano- and microstructure to structural composite response; science-based processing techniques that account for resin chemistry, cure kinetics, and flow physics to guide placement and distribution of the different reinforcement phases; and structural and mechanical testing to evaluate both the design and processing parameters. This next generation of composites will significantly improve structural efficiency, safety, and high-temperature performance; reduce data scatter; increase damage tolerance (e.g., delamination); and improve manufacturability (e.g., by eliminating hand lay-up). These composites will likely incorporate adaptive materials and multifunctional concepts, thus providing the enabling materials needed for visionary concepts in nacelle components, wing structures, and fuselage materials.

C5 Noise prediction and suppression

Local communities in this country and abroad are becoming extremely aggressive in passing stringent noise regulations, in order to substantially reduce the impact of aircraft noise. Takeoffs and landings at many airports have been restricted. The ability to reduce aircraft noise thus becomes an environmental as well as an operational constraint. Regulations passed recently in the European Union regarding noise inside commercial aircraft point to the need for cabin noise control as well as external noise control. There are a variety of promising materials and structures approaches that could be developed and validated in the next decade to substantially reduce both exterior and interior noise.

Noise is a multidisciplinary phenomenon. Effective noise control techniques must take into account multiple types of aerodynamic and acoustic excitations. Therefore, structural prediction tools must be integrated with computational aeroacoustic and fluid dynamic prediction tools for a fully coupled solution to the problem of structural noise. To validate these predictions, systematic tests should be carried out to measure noise signatures for a range of flight conditions in controlled environments such as anechoic wind tunnels. This should be followed by selective flight test of full-scale systems to measure noise signatures from the ground as well as inside the airframe.

Advanced materials for larger, stronger fan blades and higher-temperature turbine blades, together with the development of very-high-bypass-ratio engines, will be the biggest single factor in reducing external noise produced by jet aircraft. Advances in strong, lightweight composite nacelle structures, smart materials, and active structures would also reduce engine noise. Variable-geometry-chevron nozzles, which could be driven by the shape memory alloy NiTiNOL, have been demonstrated to reduce noise during takeoff and then reconfigure themselves to a more efficient shape for cruise (Calkins and Butler, 2004).

Major strides in noise suppression can also be achieved using advanced materials and active and passive structural techniques. Promising approaches include nanotechnology to enhance structural damping (noise absorption); morphing or tailored structures for laminar flow and noise source control; and multifunctional active composite structures with improved noise signature control, structural strength, health monitoring, and thermal insulation. The structural weight of additional materials or devices used for noise suppression is a key factor; with expanding advancements in smart structures technology and rapid miniaturization in data processing techniques, active noise control within the aircraft cabin appears more promising. Development of both sorts of noise suppression devices will be a key step to quieting current aircraft (interior and exterior) and could provide an impetus for an explosion of civil applications of rotorcraft and fuel-efficient prop-rotor aircraft.

C6a Innovative high-temperature metals and environmental coatings

Advanced high-temperature metallic turbine material systems (i.e., alloy substrates for the turbine blade, disk, and

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

shroud, plus necessary environmental coatings) are critical to advancing the next generation of jet engines. These engines will power future subsonic and supersonic fixed-wing airplanes and rotorcraft, while enabling reduced operating costs and improved engine safety and reliability. Metallic material systems with higher operating temperatures will improve engine cycle efficiencies. Dramatic improvements in these materials are possible, but development has been retarded by the high cost of R&D given the current highly iterative nature of alloy design. For instance, intermetallic silicides may enable considerably higher operating temperatures than nickel superalloys; advanced disk alloys may greatly reduce creep and fatigue; and protective coatings with superior resistance to environmental degradation could significantly extend the service life of hot section components. But the length of time to develop these materials, often a decade or two, and the risk that success will not be achieved have been a huge disincentive to aggressive development.

The most difficult technical issue is the need to develop material systems that possess improved performance at higher temperatures while maintaining stability for tens of thousands of operating hours in an environment that is highly oxidative, corrosive, and erosive. However, strides are being made in materials modeling capability, driven by the success of new computational tools and ever-increasing desktop computer processing capability. The application of models to guide the advancement of these materials is just beginning, but it is becoming apparent that these tools can cut development time by half and focus alloy development on the most promising approaches, reducing development risk and cost (NRC, 2004). The drawbacks to new material development would be obviated by the ability to replace experiments with computer simulations, as is done with computational fluid dynamics. The goal of this Challenge is to provide the underlying technologies for material modeling tools that can predict properties of new high-temperature metallic materials and associated protective coatings. The effort would include generation of the necessary fundamental data, complemented by testing that simulates realistic jet engine operating conditions to validate the models. In concert with industry, these tools would then be applied to the development of innovative propulsion materials.

C6b Innovative load suppression, and vibration and aeromechanical stability control

This Challenge will minimize the impact of vibratory loads in aircraft using innovative passive and active techniques. It will also examine innovative techniques to increase aeromechanical stability margins in all flight modes.

Current aircraft use numerous passive devices to increase passenger comfort and to safeguard the functioning of key structural components and instruments. Some modern rotorcraft, such as the Sikorsky S-92 and Bell-Boeing V-22, have made successful use of active vibration control, as well. Additionally, the flight envelope is sometimes restricted due to low stability margins for some aircraft. The objective of this research is to couple advanced CFD methodology with comprehensive structural analysis, including multibody formulation, nonlinear structural and inertial couplings, and interactions between the flow and the structure to predict aeromechanical stability, vibratory loads, and vibration signatures at different stations in the airframe. To validate predictions, systematic tests in wind tunnels should be carried out using dynamically scaled and full-scale models to measure vibration loads and damping of different modes for a range of flight conditions. Selective full-scale flight tests should be carried out to measure vibratory loads and stability at level and maneuvering flight conditions. Innovative active and passive techniques should be developed to minimize vibration and increase stability margin. Finally, multidisciplinary optimization should be exploited to develop efficient, low-vibration, aeromechanically stable aircraft.

C8 Structural innovations for high-speed rotorcraft

One revolutionary vision is a next-generation, high-speed, high-lift rotorcraft that can cruise at over 250 knots, and that is “neighborly” quiet, runway independent, and economically competitive with a Boeing 737 aircraft (Johnson et al., 2006). Advances required to achieve such a vehicle include innovative rotor designs, active vibration and load control, variable-speed rotor technologies, active noise control, rotor morphing, lightweight and crash-absorbing airframe technologies, advanced composites with high damage tolerance, advanced transmission systems, diagnostics and prognostics of drive trains and rotor head systems, increased autonomy and maneuverability, and enhanced handling qualities. Reliable, comprehensive aeromechanics and technology tools must be developed and validated systematically, through dynamically scaled and full-scale tests in wind tunnels. This vision provides opportunities to incorporate many disruptive and nondisruptive technologies in rotorcraft design, with an enormous payoff in performance and life-cycle cost compared with existing helicopters.

C9 High-temperature ceramics and coatings

Advanced structural ceramics, including oxide-, carbide-, nitride- and boride-based systems, are characterized by high strength, stiffness, hardness, corrosion resistance, and durability. Such ceramics retain these properties at high temperatures, making them ideal for a wide range of demanding applications, including engine components for subsonic aircraft (combustor liners, exhaust-washed structures, high-temperature ducts, heat exchangers, and nacelle insulation) and airframe and propulsion systems for high-speed vehicles. The primary benefit of structural ceramic materials is the ability to withstand higher temperatures, which improves propulsion system efficiency, increases lifetime, enables

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

higher operating speeds, and expands the margin of safety in airframe applications (NRC, 1998).

Oxide composites with operating temperatures as high as 1250°C and lifetimes of thousands of hours in highly oxidizing combustion or reentry environments are very suitable for some engine components, warm structures, and thermal management components.

Nonoxide composites made of silicon carbide reinforced either with carbon fibers or a combination of carbon fibers and silicon carbide fibers are capable of operating temperatures of 1300°C-2000°C for short times in highly oxidizing environments or for much longer times near the lower end of the thermal range when protected with environmental barrier coatings. Furthermore, because nonoxide fibers exhibit higher strength and better strength retention than oxide fibers, they are being widely researched for application in combustion environments as well as for hot structures of hypersonic and reentry vehicles.

Refractory metal (e.g., hafnium or zirconium) carbides and borides are capable of surviving thermal excursions up to 2000°C-2500°C for short times with little material recession, making them a strong candidate (in either monolithic form or as a composite matrix) for the sharp leading edges of hypersonic vehicles.

During the last 10 years, significant progress has been made in the processing, development, and demonstration of many ceramic systems for specific applications. Oxide composites deriving damage tolerance from highly porous matrices have been commercialized, and other systems with novel fiber coatings have been demonstrated in subscale testing for reentry vehicle thermal protection systems. Silicon carbide matrix processing approaches have advanced significantly, with systems produced by chemical vapor infiltration, melt infiltration, and preceramic polymer infiltration all having been demonstrated in subscale testing for jet or rocket engine components. NASA Glenn has led efforts to fabricate and test jet engine components such as exhaust nozzle liners, combustor liners, and turbine airfoils with silicon carbide matrix composites. Rocket nozzles fabricated from silicon carbide materials have been rig tested, and NASA Ames has demonstrated the ability to reproducibly fabricate refractory metal carbide and boride systems. Despite the above successes, component fabrication is not often taken much beyond the prototyping stage. Advancing the state of the art for high-temperature ceramics suitable for aeronautical applications requires research in several key areas: fabrication and testing; modeling; and attachment methods.

Insufficient fabrication and testing experience deprives designers of confidence in the long-term behavior of these materials and in the design rules for translating material characteristics into component designs. Modeling tools to predict component life for these materials are inadequate. This causes inaccurate performance and cost assessments and further limits the use of ceramic materials. Since these materials are only considered for niche applications, no economy-of-scale cost savings can be anticipated. This could be alleviated through the development of better design tools, a more thorough understanding of the effects of process variations, and more efficient approaches to commercial fabrication. Work is also needed to develop robust methods of attaching hot components to warm and cool structures as well as to develop textile approaches that can integrate complex component architectures with key features such as stiffeners, sensors, and cooling features.

C10 Multifunctional materials

Materials that possess multifunctional behavior combine electronic, magnetic, chemical, thermal, and mechanical properties at the macro, micro, or atomic level. These materials present unique opportunities for integrating communication, actuation, sensing, self-healing, and energy-harvesting functionalities into lightweight, load-bearing structures. Multifunctional materials enable a wide range of benefits, including improved aircraft telecommunications (wired, wireless, and optical); enhanced potential capabilities and flexibility for electronic and optoelectronic platforms, such as agile phased array and multifunctional radar systems; structural prognosis and nondestructive evaluation; self-sensing and self-repair; and local power generation through energy harvesting. The use of structural elements to provide new functions to aircraft platforms increases structural efficiency and enables new aircraft capabilities.

While the most research to date has been on materials with coupled electromechanical domains, a much broader vision is possible. Recent discoveries of electrochromic, magnetoelectric, and thermomechanical materials show substantial promise for future multifunctional materials.

High-Priority R&T Challenges and Their Associated Thrusts

High-priority R&T Challenges in the materials and structures Area had major relevance to at least one of three high-priority Strategic Objectives: capacity, safety and reliability, and efficiency and performance. Most of the Challenges in this Area were judged to have little or no relevance to energy and the environment, which is the fourth highest-priority Strategic Objective, although one was judged to have major relevance.

The most highly ranked R&T Challenges are those that could radically change the way new aircraft are designed, manufactured, and maintained. The key to success for all of these Challenges is interdisciplinary collaboration. Such a strategy will derive full benefit from NASA’s extensive infrastructure in (1) materials development and characterization and (2) structural analysis, optimization, and testing. For example, the highest priority materials and structures Challenge is integrated vehicle health management (IVHM). Success will require a multidisciplinary set of analysis, testing, and inspec-

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

tion tools, including miniaturized sensors and distributed electronics; sophisticated signal processing; data acquisition, integration, and database maintenance; artificial intelligence; damage science; and the mechanics of structures and their failure. Multiple aspects of materials science, structural design, and aeronautics are brought to bear on the problem of assuring vehicle health. IVHM holds the promise of reducing vehicle cost, weight, and maintenance downtime as well as speeding the introduction of new material systems and structural concepts. In addition, IVHM has the strongest influence on both of the most highly weighted Strategic Objectives: improved capacity and enhanced safety and reliability.

Four R&T Thrusts describe threads of commonality among the R&T Challenges within the materials and structures Area. Most of Challenges in this area, regardless of rank, fall into one of these Thrusts, described below.

Visionary materials and structures concepts

Visionary aeronautical concepts often depend on new materials that possess unprecedented properties or behavior and allow aircraft designers to consider new flight regimes, aircraft configurations, and operational paradigms, such as high-speed rotorcraft and morphing aircraft with the ability to change their outer mold lines. Visionary concepts also hinge on innovative structural components, often made possible with newly developed materials, alloys, and coatings. This R&T Thrust is fundamental to NASA’s aeronautics mission of enabling revolutionary concepts and innovative designs. New structural concepts take advantage of emerging analytical design tools and advanced structural materials, and they promise to significantly reduce the weight of structures while maintaining structural integrity and improving efficiency. New material concepts include next-generation polymers, metals, and composites, as well as advanced functional polymers and adaptive and multifunctional materials and coatings.

Comprehensive multilevel predictive methodologies for design and analysis

The second R&T Thrust for materials and structures technology is developing and understanding the multiscale and multiphysics behavior of aircraft materials and structures in a comprehensive manner and then bringing together previously separate design and analysis methodologies and tools, starting from the initial design concept all the way through to operation. This thrust moves beyond the realm of existing MDO techniques to incorporate key aspects of risk-based design and reliability. With new tools that allow systematic inclusion of the effects of uncertainty, whether in loading, material behavior, or mission requirements, this process would yield a rational approach for quantifying the risk associated with a certain design and allow for meaningful trade-off studies to be performed among competing design concepts. These tool sets would revolutionize aircraft design by reducing weight, noise, and vibration and by increasing stability control. NASA could have a unique role in developing and benchmarking such tools at the precompetitive stage, prior to their adoption by industry.

Novel technologies for improved structural efficiency and safety

Many of the R&T Challenges for materials and structures relate to improving structural efficiency and safety. Some, like ultralight structures and advanced joining, reduce weight directly. Alternatively, functionality can be added to a structural component or to the materials in the component to increase overall design efficiency. This Thrust includes adaptive structures that change shape and functions on demand, allowing efficient, multipoint adaptability for optimal performance. Also included are materials that perform dual roles in systems by virtue of their ability to serve as structural elements and to generate power, manage thermal loads, or impart some other additional functionality. IVHM, nondestructive evaluation (NDE), and vibration control use advances in sensing and actuation technology to reduce requirements on the structures themselves. This Thrust also includes aircraft hardening (increasing survivability of an aircraft in the event of an explosion or biological or chemical threat), which should enable new levels of safety.

Materials and structures for extreme environments

Materials and structures suitable for use in extreme environments are relevant to many R&T Challenges. Extreme environments are characterized by very high temperatures, chemical reactions, and erosive and/or corrosive conditions. They can be found in engine interiors and in the supersonic and hypersonic speed regimes. Expanding the operational envelope of each class of structural materials (polymers, metals, ceramics, and composite systems) would increase efficiency and safety margins for airframe and engine materials and structures.

Low-Priority R&T Challenges

The QFD process identifies areas of high national and NASA priority. In general, these two scores were highly correlated. However, as seen in Figure 3-5, two R&T Challenges in the top 10 by national priority did not make the top 10 by NASA priorities: novel coatings and advanced airframe alloys.

C11 Novel coatings

Novel coatings had only middling scores when it came to supporting infrastructure available at NASA and lack of alternative sponsors, primarily because these coatings were, for the most part, either underdeveloped or well developed.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

Underdeveloped coatings (including self-sensing, acoustically active, and functionally graded coatings) are still in the very fundamental research stage. Rather than focus on them directly, NASA should establish partnerships with universities to develop these coatings. The well-developed coatings (including superhydrophobic and ice shredding coatings) already have significant commercial potential and should be handled by industry.

C13 Advanced airframe alloys

Airframe alloys had low scores in supporting infrastructure and lack of alternative sponsors. Industry has significant interest, resources, facilities, and expertise to address this Challenge, whereas NASA’s own capability has eroded owing to the retirement of expert personnel. Several new and promising metallic materials can be used for critical structural applications. NASA can make a significant contribution to developing these alloys by collaborating with universities doing fundamental multiscale physics research necessary to gain a fundamental understanding of their material behavior. This knowledge could then be leveraged with industry’s efforts to enable the design of materials for specific properties. This will dramatically shorten the development cycle for new alloys by focusing limited resources on the most promising candidates.

Other low-priority challenges

Some materials and structures R&T Challenges scored low because they did not fit within the decadal time frame that is the scope of this survey, they were already being pursued by industry or other government agencies, or they were not viewed as major contributors to civil aviation. Many of these Challenges address emerging technologies, and relevant research will yield useful products, but their utility to and impact on commercial aviation is either limited or unknown. Most intriguing is advanced functional polymers (Challenge C18), which includes self-healing polymers for passive repair of damage, reversible liquid crystal adhesives, light-harvesting polymers for collecting solar energy, superabsorbent polymers for flame retardation, and mechanochromic polymers that can change color in response to damage. A logical role for NASA in this dynamic and diverse area would be to vet new materials and devices and develop criteria for long-term materials investment.

R&T Challenges such as innovations in structural joining (Challenge C12), advanced airframe alloys (Challenge C13), aircraft hardening (Challenge C15), ultralight structures (Challenge C17), advanced engine nacelle structures (Challenge C19), and repairability of structures (Challenge C20) are also worthy of note since they could improve current civil aircraft design and also apply to military aircraft. In some cases, these Challenges are regarded as natural candidates for company investment, not cutting-edge NASA efforts. Joining, repair, and lightweight design aspects of air-frames are also addressed in the highly ranked multidisciplinary analysis and the new composites tasks. Ultralightweight structural concepts benefit from the integration of advanced composites, adaptive materials, multifunctional materials, and multidisciplinary structural optimization. Thus, even though these topics received low rankings on their own, they will be addressed in a broader, integrated context in the more highly ranked tasks.

Similarly, advances in NDE are synergistic and closely allied with IVHM efforts. NDE provides input to prognosis and life-prediction systems, and its development fits well with NASA’s mission in terms of aviation safety. NDE facilitates the development and insertion of new materials and structures and processes by ensuring that they are manufactured according to specifications and behave as designed when they are put into service. However, NDE is also an active area of research in DoD and in nonaerospace industries. For the next generation of NDE, the data-acquisition hardware is of less interest than the new tools for interpretating the multiphysics NDE measurement data. NASA should work in collaboration with academia to develop analysis tools for automating the interpretation of the multiphysics NDE measurement data. This role would provide NASA a unique focus, different from the work currently sponsored by others.

Multiphysics and multiscale modeling and simulation (Challenge C16), while important to the efficient design of future materials and structures, has many contributors from several federal agencies as well as academia. It lacks a clear, focused set of objectives and a clear tie to NASA’s mission. The long time frame associated with R&D in multiphysics, multiscale modeling makes this Challenge more suited to academia.

DYNAMICS, NAVIGATION, AND CONTROL, AND AVIONICS

Introduction

In this report, dynamics is defined as the motion characteristics of the aircraft due to the forces that act on it. Navigation generally refers to determination of the aircraft’s state (i.e., its position, velocity, and attitude rates) in three dimensions at a particular time. Control is closely linked to guidance; together they refer to determination of the aircraft’s future state and the processes for reaching and staying on a specified trajectory. Finally, avionics consists of aviation electronics, both onboard and off-, which implements navigation, guidance, control, surveillance, communications, and other functions. Avionics includes the development, production, and use of aviation electronics, including both hardware and software.

The QFD process described in Chapter 2 was used to prioritize 14 R&T Challenges related to dynamics, navigation, and control, and avionics. Table 3-4 and Figure 3-6 show the re-

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

TABLE 3-4 Prioritization of R&T Challenges for Area D: Dynamics, Navigation, and Control, and Avionics

 

 

 

Strategic Objective

National Priority

Why NASA?

NASA Priority Score

 

 

 

Capacity

Safety and Reliability

Efficiency and Performance

Energy and the Environment

Synergies with Security

Support to Space

Supporting Infrastructure

Mission Alignment

Lack of Alternative Sponsors

Appropriate Level of Risk

Why NASA Composite Score

R&T Challenge

Weight

5

3

1

1/4 each

D1

Advanced guidance systems

9

9

9

3

3

3

132

9

9

3

9

7.5

990

D2

Distributed decision making, decision making under uncertainty, and flight-path planning and prediction

9

9

9

3

3

3

132

3

9

3

9

6.0

792

D3

Aerodynamics and vehicle dynamics via closed-loop flow control

1

9

9

3

3

3

92

9

9

3

9

7.5

690

D4

Intelligent and adaptive flight control techniques

3

9

9

3

3

9

108

3

9

3

9

6.0

648

D5

Fault-tolerant and integrated vehicle health management systems

3

9

3

1

3

9

84

9

9

3

9

7.5

630

D6

Improved onboard weather systems and tools

9

9

3

1

1

1

104

9

9

3

3

6.0

624

D7

Advanced communication, navigation, and surveillance technology

9

9

9

3

3

3

132

3

9

3

3

4.5

594

D8

Human-machine integration

3

9

9

1

3

3

96

3

9

3

9

6.0

576

D9

Synthetic and enhanced vision systems

3

9

3

1

1

3

76

9

9

3

3

6.0

456

D10

Safe operation of unmanned air vehicles in the national airspace

3

9

3

1

9

1

82

3

9

3

3

4.5

369

D11

Secure network-centric avionics architectures and systems to provide low- cost, efficient, fault-tolerant, onboard communications systems for data link and data transfer

9

9

9

1

9

3

132

3

3

1

3

2.5

330

D12

Smaller, lighter, and less expensive avionics

1

3

9

3

3

9

68

3

3

3

3

3.0

204

D13

More efficient certification processes for complex systems

3

9

9

1

1

3

94

3

1

1

3

2.0

188

D14

Design, development, and upgrade processes for complex, software-intensive systems, including tools for design, development, and validation and verification

3

9

3

1

1

3

76

1

3

1

1

1.5

114

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

FIGURE 3-6 NASA and national priorities for Area D: dynamics, navigation, and control, and avionics.

sults. This section describes the 10 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high- and low-priority Challenges, and the R&T Thrusts in this Area. Further details on all the Challenges, including the rationale for the scoring, are found in Appendix D.

Top 10 R&T Challenges

D1 Advanced guidance systems

Advanced guidance systems consist of subsystems and processes (hardware and software) assembled for the purpose of providing an aircraft, spacecraft, or other dynamic system with desired state trajectories. These trajectories can be defined using either discrete or continuous data and can include information such as current velocity, acceleration, time of arrival, and desired position. The determination of the desired trajectory usually takes into account mission-dependent constraints, which can include obstacles (such as terrain, wake vortices, or other aircraft), hazards (such as weather), coordination with other aircraft (such as cooperative and multiaircraft guidance, formation flight, or swarming), and regulatory constraints (such as airspace class restrictions) (Doebbler et al., 2005).

State-of-the-art guidance systems enable aircraft to follow waypoints, perform automatic obstacle avoidance, and fly in formation with other aircraft (Schierman et al., 2004). Additional research is needed to develop guidance algorithms and mature them into flight-ready systems,8 to develop improved reconfigurable and adaptive guidance systems, and to develop advanced guidance systems for UAVs. One concern, for example, is the need to develop improved technologies to avoid controlled flight into terrain, particularly in the case of all-weather operation of advanced rotorcraft. Some important research is inhibited by the limited number of programs and facilities capable of implementing and flying these systems on real aircraft. Also, certification and regulatory issues must be resolved so that the air trans-

8

R. Duren, associate professor, Baylor University, “Avionics research challenges,” Presentation to Panel D on November 15, 2005.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

portation system can take advantage of the full capabilities of current and future guidance systems for piloted aircraft and UAVs.

Advanced guidance systems have the potential to greatly improve the capacity, safety, and efficiency of the air transportation system. In addition, they can enhance the performance of many existing and future military systems.

D2 Distributed decision making, decision making under uncertainty, and flight path planning and prediction

Improving the decision-making process used by pilots and aircraft systems, when coupled with improvements in flight-path planning and prediction, has been theorized as an effective approach to improving air transportation system capacity and safety. This Challenge has the potential to significantly improve the timeliness of real-time decisions to alter flight paths in the dynamic environment of congested airspace (Ding et al., 2004; Helbing et al., forthcoming; Rong et al., 2002). Coordinated decision making, which includes the direct exchange of data among different aircraft and the deconfliction of flight paths without the need to rely on ground-based controllers, addresses the inherent limitations of centralized air traffic control systems in terms of uncertainty and fault tolerance. A coordinated, distributed approach to decision making increases air transportation system reliability and safety by distributing control and mission management capabilities among multiple agents. It also allows for rapid response to changing dynamics and minimizes vulnerability to system failures.

Automated systems can help improve decision making and flight path planning. Levels of automation ranging from “pilot aid” (that is, systems that advise pilots to take specific action) to “fully autonomous” are achievable but have not yet been developed to the point where they can support high levels of automation for civil aircraft. Until now, coordinated distributed algorithms for constraint reasoning (for example, to optimize flight paths) have not been applied to the air transportation system because implementation with such a complex system would require aircraft to exchange a large number of messages, which raises substantive communications, bandwidth, and man–machine interface issues.

This Challenge should address the needs of a wide variety of conventional and unconventional aircraft types, including those with no distributed decision-making capability. Aircraft types of interest include commercial airliners, general aviation aircraft, civil helicopters, military aircraft, and UAVs.

This Challenge also has the potential to be of great benefit when applied to complex, nonaviation systems that operate in dynamically changing environments and require high-quality, real-time decision making.

D3 Aerodynamics and vehicle dynamics via closed-loop flow control

Closed-loop flow control appears to offer tremendous promise in improving aerodynamic performance. For example, active flow control approaches should allow the airfoil lift:drag (L/D) to remain high over large changes in angle of attack.9 Flow control R&T could also be used to develop a spoiler-aileron to replace complex and heavy control surfaces and to reduce or eliminate turbulent flow over aircraft surfaces to reduce skin-friction drag. These applications could lead to new aircraft configurations (Chavez and Schmidt, 1994).

The mechanization of flow control systems may require a large number of distributed sensors measuring pressure or shear stress over the wing and changes in the boundary layer. Actuation might be accomplished by morphing the wing or introducing devices that induce sucking or blowing along the wing. These distributed sensors and actuators are coordinated so that control is obtained over large flight regimes, angles of attack, and attitudes.

Distributed sensing and actuation would also permit structures to be self-aware for health monitoring, thereby increasing system reliability. Airframe and engine structures could be monitored for changes in behavior.

Some of the techniques developed by this Challenge may also advance modeling and design capabilities applicable to morphing aircraft (Tandale et al., 2005; Valasek et al., 2005). Heretofore, aircraft have generally been fixed-frame structures. Morphing aircraft would be designed with distributed actuation and controls and with mechanization as an inherent property. They would lead to new capabilities and concepts in aircraft design. Examples include (1) biomorphic aircraft, such as ornithopters, that could maneuver robustly in complex environments and (2) hunter-killer aircraft that change shape to optimize performance for different tasks (e.g., surveillance, reconnaissance, and ground attack). Morphing technology might also enable aircraft capable of perching.

D4 Intelligent and adaptive flight control techniques

The missions and capabilities of future aircraft, both manned and unmanned, will be more multifunctional than those of the current generation of specialized aircraft. Achieving aggressive performance targets in range, payload, reliability, safety, noise, and emissions will require a total system that is integrated to a far higher level than existing aircraft. R&D for military aircraft has been able to push the

9

The flow over the specially shaped GLAS II airfoil remains naturally separated at the rear of its upper surface over a wide range of angles of incidence; in the absence of active control, its L/D does not exceed 25. At an incidence angle of 10 degrees, its L/D is nearly 500 (Glauert, 1945; Glauert et al., 1948).

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

technological envelope associated with intelligent and adaptive flight control techniques farther than R&D for civil aircraft because of different safety limits. In the far term, as it advances, application of military technology to civil aircraft may be possible.

The vehicle management systems (VMS) paradigm offers the most promising path to realizing goals related to this Challenge. VMS takes a top-down systems approach to specifying, designing, and validating the aircraft as a single system with highly integrated inner and outer loops. It thereby unifies the traditionally separate fields of propulsion control, flight control, structural control, noise control, emissions control, and health monitoring. The current state of the art in VMS uses traditional feedback control, consisting of measurements of vehicle states such as airspeed, altitude, angle of attack, and linear and angular acceleration (Jaw and Garg, 2005). By incorporating an online learning capability to cope with new and unforeseen events and situations and nonlinear adaptive control, in which the controller self-tunes to maintain stability and tracking in the presence of disturbances and changing vehicle parameters, an intelligent and adaptive VMS can be developed with the promise of significant advances in capability, safety, and supportability (Tandale and Valasek, 2003).

Significant advances in the state of the art are required to develop an intelligent and adaptive VMS. Current nonlinear adaptive control approaches assume that (1) sensor information is reliable and (2) known nonlinearities can be modeled as slowly varying parameters that affect the system linearly. However, advanced actuators for flow control and structural control will have characteristics that are much more nonlinear than those of conventional control actuators. Control laws and control actuator allocation are currently treated as separate problems, such that optimization of the integrated control law is difficult or impossible. Finally, the problem of multiple correlated, simultaneous failures remains unsolved. Approaches that use analytic redundancy to finding failed sensors generally assume that aircraft dynamics have not changed, while adaptive or reconfigurable control approaches assume that sensor information is reliable. On an affordable aircraft with limited or no sensor redundancy, it is difficult or impossible to tell the difference between a degraded sensor and damage to the aircraft that changes the way it flies.

D5 Fault-tolerant and integrated vehicle health management systems

Development of IVHM system technologies is key to the acceptance of the automation needed in the transformation of the air transportation system. The technology provides an increased capability to accurately discover and assess system faults and reconfigure or recover from them. Although highly integrated, health management aspects consist of related components: fault detection and isolation, recovery and reconfiguration, and condition-based maintenance (CBM). In addition, modeling plays an important role in the development of these functions (Garg, 2005; Litt et al., 2005; Tandale and Valasek, 2006).

Fault detection, isolation, recovery, and reconfiguration

Fault detection, isolation, recovery, and reconfiguration involve processes and approaches that enable robust detection of faults from measured or estimated error residuals and isolation of faults with minimal latency in the presence of noise and environmental effects during aircraft operation. Fault detection, isolation, recovery, and reconfiguration are platform specific and should cover all flight regimes and mission types. Recovery and reconfiguration systems are developed with regard to the possibilities of faults, the nature of the latency of the fault detection and isolation system, and the controls available for recovery and reconfiguration. Redundancy management strategies for avionics and the airframe directly influence options for recovery and reconfiguration.

Condition-based maintenance

CBM involves maintenance processes and capabilities derived from real-time assessment of aircraft system conditions obtained by software from embedded and redundant sensors. The combination of software and sensors can create important communications and bandwidth problems. More robust diagnostics and prognostics are needed to achieve the goal of CBM, which is to perform maintenance only on evidence of need to prevent a failure from reducing aircraft availability. In addition, CBM includes processes that couple real-time assessment of system and component performance with ground- and air-based logistics to improve aircraft system readiness and maintenance practices. CBM is a form of proactive equipment maintenance that forecasts incipient failures. CBM also aims to ensure safety, equipment reliability, and reduction of total ownership cost. Fault tolerance is achieved when CBM is married to decision strategies for safe and reliable operation of manned and unmanned aircraft.

Modeling

Physics-based models of sensors, actuators, avionics, components, and vehicle flight dynamics contribute to the development of methods for forecasting aircraft system performance and, thereby, help uncover faults. In addition, these models can be used for examining architectures and control strategies to reconfigure systems and ensure safety and reliability.

An aircraft is a very complex system. While individual fault-tolerant functions can be set up for each subsystem, the value of fault-tolerant designs is maximized when the sys-

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

tem is modeled as a whole, since the behavior of each subsystem can influence that of other subsystems. The advantage of working with a total system model lies in the ability to discover a fault through its effects on other parts of the system before the fault is discovered in the individual subsystem itself. One primary thrust of fault-tolerant technology development is to identify system models that characterize the behavior of systems properly without developing an overly detailed and unnecessary representation. In other words, an optimum system is not a collection of optimized subsystems.

To advance the state of the art in fault-tolerant aircraft systems, fundamental R&T is required in the three topics above to develop a more robust image of the state, or health, of an aircraft in the presence of uncertainty. With a better model of itself the aircraft can trace back system anomalies through the multitude of discrete state and mode changes to isolate aberrant behavior. Fault-tolerant systems combine simple rule-based reasoning, state charts, model-free monitoring of cross-correlations among state variables, and model-based representations of aircraft subsystems. Together, these models form a hybrid system model. Advances in computing resource technology have allowed hybrid system models to run in real time.

Fault-tolerant aircraft systems, coupled with CBM, may improve aircraft safety and reduce aircraft life-cycle maintenance and ownership costs. Critical research tasks include developing (1) robust and reliable hardware and software tools for monitoring components, detecting faults, and identifying anomalies; (2) prognosis analysis tools for predicting the remaining life of key components; (3) approaches for recovering from detected faults, including reconfiguration of the flight control system for in-flight failures of manned and unmanned aircraft; and (4) low-cost, lightweight, wireless, self-powered sensors with greater memory and processing capability.

D6 Improved onboard weather systems and tools

Pilots—and the avionics software that provides in-flight four-dimensional trajectory replanning and commands to the pilot or autopilot—require additional weather information to minimize the impact of weather on the control of flight in high-density traffic. Basic research is needed to determine the most cost-effective way of integrating real-time weather information into four-dimensional integrated control of flight. This information might include information from data links with ground sites and other aircraft and weather video from ground stations and satellites (Bokadia and Valasek, 2001; Lampton and Valasek, 2005, 2006).

Other aircraft could provide information about geospatial position, wind, icing conditions, turbulence, lightning, and precipitation, as well as imagery from radars and other sensors. Data links with the ground could provide actual and forecast information on winds at different flight levels, pressure, icing potential, precipitation, ground-level temperatures, weather fronts, severe weather, airport surface conditions, and other information from significant meteorological information reports (SIGMETs); pilot reports (PIREPs); meteorological aviation reports (METARs), terminal area forecasts (TAFs), imagery from satellites and radars, and so on.

D7 Advanced communication, navigation, and surveillance (CNS) technology

The capacity of the air transportation system is dependent on minimum spacing requirements for safe operation. Minimum spacing depends on many factors, including the capability of each aircraft to precisely fly a predetermined, geospatially time-referenced flight path.

Advanced, integrated, accurate, secure, and reliable CNS capabilities are required for network-centric operations, which can increase capacity in very high density airspace. Each aircraft may be considered a node in a network-centric, distributed, fault-tolerant ATM system. Communications between nodes (aircraft to aircraft, aircraft to ground, and aircraft to satellite to ground) must be highly reliable. (For example, the probability of a missed or incorrect message should be less than 10−7 per flight hour, depending on the consequence of the fault.) Safe, secure, accurate, and certifiable CNS technologies that provide required capabilities are needed.

More precision aircraft navigation, coupled with the precise six-dimensional10 guidance algorithms used in advanced flight management systems, will enable reduced spacing between aircraft operating en route and in the terminal airspace. CNS system functions must be tightly coupled in terms of information integrity, and they should allow pilots to operate cooperatively with ground systems without controllers continuously in the control loop. The CNS should transmit navigation, guidance, and other sensor data to other aircraft and ground operation centers via multichannel data links while, at essentially the same time, they receive similar information about other aircraft, the weather, airport conditions, etc. This information can prevent accidents by revealing the current and future status of other aircraft, weather phenomena, terrain, buildings, and vehicles on the ground at airports. This Challenge should also increase the affordability of onboard avionics to encourage aircraft owners and operators to procure more capable avionics. This Challenge encompasses the following CNS issues:

  • Communications issues.

  • Fault-tolerant network connectivity and security.

  • Dynamic network control and reconfiguration.

  • Quality of service.

10

The six dimensions refer to three position coordinates and three velocity vectors to define aircraft location, speed, and direction of motion.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×
  • Spectrum allocation and usage.

  • Adequate communication bandwidth.

  • Required communications capability as a function of geospatial location and phase of flight.

  • Navigation issues.

    • High-precision, six-dimensional estimate of aircraft state as a function of time.

    • Integration of satellite navigation with other navigation modes.

    • Navigation system capability, including reliability and quality of input signals.

    • Functional integration of navigation system with guidance and flight control systems to ensure high-integrity, integrated control of flight during automatic and manual modes.

  • Surveillance issues.

    • Capability of data links to provide accurate time-referenced data from navigation systems, guidance systems, and other sensors when interrogated by external systems or periodic broadcast.

    • Handling of multiple, simultaneous interrogations using multiple channels to provide high-integrity, secure data.

    • Processing and reacting to incoming data about other aircraft, hazardous weather, etc.

    • Continuous improvement in situational awareness through advanced sensors, communication links, and human–system interfaces.

D8 Human–machine integration

The ever-increasing demand for air transportation, combined with the rapid pace of technological change, poses significant challenges for effective integration of humans and automation. For the foreseeable future, humans will continue to play a central role in key decision-making tasks that directly influence the efficiency and safety of civil aviation. As technology evolves, it may be anticipated that the role of humans and the nature of their task will change accordingly. In order to maintain or improve on existing standards of performance and safety, it is critical that the allocation of functions between humans and automation and the design of the human–machine interface be optimized based on a solid foundation of scientific principles that reflect our best understanding of human sensory, perceptual, and cognitive processes. Human–machine integration should remain an important element of NASA research directed toward civil aeronautics applications.11 However, the emphasis should be shifted from development and testing of specific input and output devices toward more fundamental research involving modern instruments that measure brain physiology. Research should also include voice command and recognition technology, coupled with increased machine contextual understanding, to reduce workload. This will help define the future role of humans in complex, highly automated systems. Key research topics include human–machine integration methods, tools, and integration technologies for vehicle applications.

D9 Synthetic and enhanced vision systems

Synthetic and enhanced vision systems provide an out-the-window view of terrain, obstacles, and traffic. These systems can also be used as flight crew interfaces for flight trajectory and planning operations (Kelly et al., 2005). The synthetic vision systems that use databases to generate terrain and obstacles require high-fidelity, high-integrity information and a self-healing capability. Enhanced vision systems use forward-looking sensors such as infrared, radar, and laser ranging to allow the flight crew to visualize the real world when visibility is hindered. Currently, vision systems are limited by weather, human factors issues, and other issues. New sensors and improved sensor fusion are needed.

A combined synthetic and enhanced vision system has future potential as a navigation, approach, and landing sensor. The ability to “see” the airport in poor weather has the potential to reduce the likelihood of a go-around. Information fusion that exploits the capabilities of sensors and compensates for their deficiencies is needed, and the immature state of this art represents the most difficult obstacle to achieving these benefits.

Synthetic and enhanced vision systems are also intended to aid airport surface operations in poor weather, reducing runway occupancy and taxiing errors and reducing gate-to-gate travel time. Research topics of interest are as follows:

  • Database integrity and quality.

  • Information fusion.

  • Object detection and avoidance.

  • Human–machine interface issues.

  • Verification of accuracy, fault tolerance, and reliability.

D10 Safe operation of unmanned air vehicles in the national airspace

The use of UAVs for a variety of civil applications (e.g., farming, communications relays, border monitoring, power line and pipeline monitoring, and firefighting) will continue to increase. Flight operations of military UAVs in civil airspace are also expected to increase. To facilitate these operations, UAVs should be integrated into the air transportation system. This requires them to be at least as safe as manned aircraft.

Most UAV technologies, capabilities, and processes are shared with manned aircraft and require research in several key topics, including the following four:

11

J. Vagners, professor emeritus, aeronautics and astronautics, University of Washington, Presentation to Panel D on November 15, 2005.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×
  • Aircraft. Automation, system upgrade issues, and communications systems, all of which are distinct from those for manned aircraft.

  • Human–machine interaction. Function allocation, human interface design, situational awareness, training, and required level of proficiency in the remote operation of the aircraft.

  • Maintenance and support. In matters where UAVs differ distinctly from traditional aircraft.

  • Flight operations. Sense- or see-and-avoid issues, person-to-person interfaces between operators and controllers, assurance of positive control of the aircraft (especially with highly automated UAVs that are not directly controlled by ground-based operators in real time), and automated contingency management.

High-Priority R&T Challenges and Their Associated Thrusts

R&T Challenges that significantly impact multiple strategic objectives or for which NASA possesses unique capabilities ranked high on the technology prioritization list. All of the top 10 Challenges received the maximum score for relevance to safety and reliability, and seven of the top 10 also received the maximum score for relevance to capacity and/or efficiency and performance. None of the top 10 received the maximum score for energy and the environment, one received the maximum score for relevance to national and homeland security, and two received maximum scores for support to space. Most of the Challenges, regardless of rank, fall into one of five R&T Thrusts that describe threads of commonality among the R&T Challenges within the dynamics, navigation, and control, and avionics Area. These thrusts are discussed next.

Increased integration

Avionics systems are becoming more integrated within individual aircraft, and the control of aircraft flights is more tightly integrated in the air transportation system as a whole. The future air transportation system will see increased integration and information sharing among components of the air transportation system—including individual commercial, business, and general aviation aircraft; ATM facilities; and operation centers for passenger airlines, air cargo operators, and the military. Capacity increases can be achieved, for example, by reducing separation between aircraft, but this could threaten safety. An individual aircraft requires information on the relative position of other aircraft and ground hazards, which may be fixed (terrain and buildings) or moving (aircraft on taxiways and runways). Functional as well as information integration will be needed. All of this will require fault-tolerant, integrated, secure, reliable, flight-critical communications.

Multifunctional, highly integrated guidance and control

The missions and capabilities of future aircraft, both manned and unmanned, will be more multifunctional than the current generation of specialized aircraft. Achieving aggressive performance targets in range, payload, reliability, safety, and emissions will require aircraft to be much more integrated than existing aircraft systems.

Distributed decision making and control

Most scenarios for the future air transportation system envision increased distribution of decision making. Currently, most decision making is centralized and ground-based, with air traffic controllers responsible for coordinating the movement of aircraft in the air and the FAA center at Herndon, Virginia, responsible for national flow control. While centralization has advantages in terms of ensuring safety, it is inflexible and it limits decision making by the pilots and airlines. It also has inherent limits—for example, the mental limits of individual human controllers—that contribute to capacity problems in the system.

Technological advances such as advanced communication systems, satellite navigation, and sophisticated decision-making technologies could further distribute decision making and control among various airspace systems and move these tasks from the ground to the air. These changes, however, will require much greater complexity and functionality in the airborne systems, and important questions must be answered before these changes can become a reality. Relevant questions include how to ensure safety in such a distributed environment, how to provide reliable and efficient communications, how to develop and implement sophisticated decision-making algorithms, and how to define and implement appropriate human–machine interactions.

Intelligent use of automation

Used intelligently, automation has the potential to greatly enhance the safety and efficiency of civil aviation. The rapid evolution of technologies for sensing, processing, and communicating information enables designers to consider new systems with unprecedented levels of automation. The current trend toward increased automation is introducing fundamental, qualitative changes in human roles and tasks. In some cases, these changes have assigned humans tasks for which they are ill-equipped, such as monitoring highly automated processes.

NASA has substantial facilities and expertise that could be applied to this Thrust. Historically, these resources have primarily been used to develop and demonstrate specific system- and subsystem-level solutions to particular operational or safety problems. Transitioning these point designs to practical applications has been problematic. A more productive use of NASA’s considerable capabilities would be

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

for NASA to become a provider of basic research products, enabling technologies, and system engineering tools to support system development by industry and certification by the FAA. There is a compelling need for a focused program of research that will yield practical, validated technologies, processes, and tools to support effective human–machine integration in civil aviation.

Revolutionary vs. evolutionary approaches

Aeronautics research can use revolutionary and/or evolutionary approaches. A revolutionary approach allows the researcher to look for the best solution to a problem (assuming “best” can be properly defined) using the latest technology available without concern for the technology that is currently fielded. The evolutionary approach looks for a solution that is a derivative of or an incremental improvement to a current system.

NASA plays an important role in aeronautics with its capability to do revolutionary research. By following a revolutionary approach to ATM and avionics, for example, NASA can set a goal for the end state—a picture of what the system might look like in 10, 15, or 25 years. NASA can use its modeling and simulation expertise and its proof-of-concept flight demonstration capabilities to predict the system efficiencies for the end state. It can set the long-term vision for aeronautics and the air transportation system. This role requires communication and interaction with the FAA, DHS, DoD, and other members of the Joint Planning and Development Office that is defining the nature of the Next Generation Air Transportation System and the research program necessary to make it a reality. The FAA, which must implement modifications to the system in an evolutionary manner, can develop its system roadmap in part by using the NASA end state.

Low-Priority R&T Challenges

Four R&T Challenges were not in the top 10. Three of the four (D12, smaller, lighter, and less expensive avionics; D13, more efficient certification processes; and D14, design, development, and upgrade processes for complex, software-intensive systems) had a significant impact on only one or two Strategic Objectives. For example, Challenge D14 is very relevant to the safety and reliability Strategic Objective but had only a minimal or modest impact on the other five Objectives.

The fourth R&T Challenge that did not make the top 10 was D11, secure, network-centric avionics architecture and systems. This Challenge has a significant effect on four of the Objectives, and it tied for the highest score in terms of national priority. However, it ranked low in terms of NASA priority because it scored worse than all of the top 10 Challenges in terms of alignment with the NASA mission and the availability of alternative sponsors.

Challenges D13 and D14 also scored in the top 10 in terms of national priority but not in terms of NASA priority. Challenge D13 is important because all new technologies must be certified before they can be put in service. D14 is important because many new systems will be software intensive. However, these Challenges scored low in terms of “Why NASA?” because (1) other organizations in government, industry, and academia are already working on relevant technologies and (2) NASA has relatively little infrastructure or expertise to contribute. However, because Challenges D11, D13, and D14 scored high as a national priority, some part of the national civil aeronautics effort should support relevant R&T.

INTELLIGENT AND AUTONOMOUS SYSTEMS, OPERATIONS AND DECISION MAKING, HUMAN INTEGRATED SYSTEMS, AND NETWORKING AND COMMUNICATIONS

Introduction

Aeronautics research encompasses much more than airframes and engines. For many years NASA has been in the forefront of discovering how human beings interface with aviation hardware. NASA has also been a leader in development of autonomous systems and communications interfaces. Accordingly, R&T Challenges in the Area of intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications focus on issues associated with the air transportation system of today and tomorrow as a complex interactive system; issues associated with the performance of systems in individual aircraft are addressed in the preceding sections.

The Next Generation Air Transportation System (NGATS) Joint Planning and Development Office (JPDO) is striving to achieve eight key capabilities (NGATS JPDO, 2005, pp. 7-9):

  • Network-enabled information access, which will give decision makers throughout the air transportation system quick access to the critical information they need in normal and emergency conditions.

  • Performance-based services, which will maximize the performance of all categories of aircraft.

  • Weather assimilated into decision making, which will take advantage of improved probabilistic weather information.

  • Layered, adaptive security, which will be more efficient, more effective, and less intrusive.

  • Broad-area precision navigation, which will allow pilots to make precision landings at airports that do not have control towers, radar, or an instrument landing system (ILS).

  • Aircraft trajectory-based operations, which will include automatic, continuous analysis of trajectories to increase capacity and assure safe separation of aircraft.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×
  • Equivalent visual operations, which will allow pilots and controllers to see the same picture, enabling controllers to delegate some tasks to pilots.

  • Super-density operations, which will use advanced capabilities, including detection and avoidance of hazardous wake vortices, to enable closely spaced and converging approaches in the air as well as more efficient airport ground operations.

This study did not assess and does not necessarily endorse the above set of capabilities. However, many of the capabilities would be supported by R&T Challenges in this Area. These Challenges also encompass the basic and applied research necessary to establish a proper balance between automated and human-centric system configurations and operational concepts in the air transportation system. The scope of research in this Area includes new NASA R&T Thrusts as well as the expansion of existing technology programs. One Area of particular near-term interest is the incorporation of autonomous and semiautonomous aircraft into the national (and global) air transportation system.

The QFD process described in Chapter 2 was used to prioritize 20 R&T Challenges related to intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications. Table 3-5 and Figure 3-7 show the results. The text that follows describes the 10 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high-and low-priority Challenges, and the R&T Thrusts in this Area. Further details on all Challenges, including the rationale for scoring, are found in Appendix E.

As shown in Table 3-5, many of the Challenges in this Area ranked high because they would enhance the performance of the air transportation system as a whole, bringing about noteworthy improvements related to many of the air transportation system strategic objectives (capacity, safety and reliability, etc.). As shown Figure 3-7, the top 10 Challenges fall into three groups:

  • R&T Challenge E1 stands alone, with a NASA priority score of 936.

  • R&T Challenges E2 and E3 stand together with scores of 780 and 744.

  • R&T Challenges E4 to E8c stand together with scores of 624 to 576.

The difference in scores and rankings of the R&T Challenges in each of the last two groups is not significant.

FIGURE 3-7 NASA and national priorities for Area E: intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

TABLE 3-5 Prioritization of R&T Challenges for Area E: Intelligent and Autonomous Systems, Operations and Decision Making, Human Integrated Systems, and Networking and Communications

 

 

 

Strategic Objective

National Priority

Why NASA?

NASA Priority Score

 

 

 

Capacity

Safety and Reliability

Efficiency and Performance

Energy and the Environment

Synergies with Security

Support to Space

Supporting Infrastructure

Mission Alignment

Lack of Alternative Sponsors

Appropriate Level of Risk

Why NASA Composite Score

R&T Challenge

Weight

5

3

1

1/4 each

E1

Methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems

9

9

9

9

9

3

156

3

9

3

9

6.0

936

E2

New concepts and methods of separating, spacing, and sequencing aircraft

9

9

9

3

3

1

130

3

9

3

9

6.0

780

E3

Appropriate roles of humans and automated systems for separation assurance, including the feasibility and merits of highly automated separation assurance systems

9

9

9

1

3

1

124

3

9

3

9

6.0

744

E4

Affordable new sensors, system technologies, and procedures to improve the prediction and measurement of wake turbulence

9

9

3

1

1

1

104

3

9

3

9

6.0

624

E5

Interfaces that ensure effective information sharing and coordination among ground-based and airborne human and machine agents

3

9

9

1

9

3

102

3

9

3

9

6.0

612

E6

Vulnerability analysis as an integral element in the architecture design and simulations of the air transportation system

3

9

9

1

9

1

100

3

9

3

9

6.0

600

E7

Adaptive ATM techniques to minimize the impact of weather by taking better advantage of improved probabilistic forecasts

9

3

9

3

1

1

98

3

9

3

9

6.0

588

E8a

Transparent and collaborative decision support systems

3

9

9

1

3

3

96

3

9

3

9

6.0

576

E8b

Using operational and maintenance data to assess leading indicators of safety

3

9

9

1

3

3

96

3

9

3

9

6.0

576

E8c

Interfaces and procedures that support human operators in effective task and attention management

3

9

9

1

3

3

96

3

9

3

9

6.0

576

E11

Automated systems and dynamic strategies to facilitate allocation of airspace and airport resources

9

3

9

3

3

1

100

3

9

1

9

5.5

550

E12

Autonomous flight monitoring of manned and unmanned aircraft

3

9

3

1

9

1

82

3

9

3

9

6.0

492

E13

Feasibility of deploying an affordable broad-area, precision-navigation capability compatible with international standards

9

9

3

1

3

1

106

3

3

3

9

4.5

477

E14

Advanced spacecraft weather imagery and aircraft data for more accurate forecasts

3

3

9

3

1

1

68

3

9

3

9

6.0

408

E15

Technologies to enable refuse-to-crash and emergency autoland systems

1

9

1

1

3

1

60

3

9

3

9

6.0

360

E16

Appropriate metrics to facilitate analysis and design of the current and future air transportation system and operating concepts

3

3

9

3

3

1

70

3

9

3

3

4.5

315

E17

Change management techniques applicable to the U.S. air transportation system

9

9

9

1

3

1

124

1

3

3

3

2.5

310

E18

Certifiable information-sharing protocols that enable exchange of contextual information and coordination of intent and activity among automated systems

3

1

9

1

9

1

60

3

9

3

3

4.5

270

E19

Provably correct protocols for fault-tolerant aviation communications systems

3

9

3

1

3

1

76

3

3

1

1

2.0

152

E20

Comprehensive models and standards for designing and certifying aviation networking and communications systems

3

9

3

1

1

1

74

3

3

1

1

2.0

148

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

Top 10 R&T Challenges

E1 Methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems

The U.S. air transportation system is a complex interactive system whose behavior is difficult to simulate with currently available models. Methodologies, tools, and simulation and modeling capabilities suited for the design and integration of complex interactive systems are needed to understand the air transportation system as an integrated, adaptive, distributed system that includes aircraft, ATM facilities, and airports, each with its own complex systems, all of which interact with one another, the environment, and human operators. Simulations and models for complex interactive systems are needed to accurately estimate system performance, to properly allocate resources, and to select appropriate design parameters. Additionally, the large number of possible future system designs requires models that can be reconfigured to model a wide range of design parameters.

E2 New concepts and methods of separating, spacing, and sequencing aircraft

Expected growth in the demand for air transportation will require efficient, denser en route and terminal area operations. This necessitates procedures that reduce minimum spacing requirements during all phases of flight and in all weather conditions, through an integrated approach that leverages a suite of emerging technologies such as required navigation performance and automatic dependent surveillance broadcast (ADS-B). The objective of this Challenge is to efficiently accommodate a large number and wide range of aircraft, including UAVs, through spacing and sequencing based on aircraft type and equipment rather than a common worst-case standard. Several concepts of operation should be systematically compared in terms of their technological, business, and human factors issues as well as their impact on capacity, safety, and the environment. This Challenge will study reduced separation operations within the context of existing ATM protocols and revolutionary paradigms that could significantly increase capacity, although the latter would involve a much more complicated transition process.

Integration of UAVs into the air transportation system will require procedures that can safely manage aircraft with diverse performance characteristics and highly automated onboard flight management systems (Sabatini, 2006). Safe, high-capacity operations in a complex future airspace environment will require fundamental research into alternative ATM paradigms such as simultaneous noninterfering operations (Xue and Atkins, 2006) in which general aviation, rotorcraft, and UAV traffic are threaded through airspace unused by commercial air traffic. As onboard automation and cooperative control algorithms are matured (McLain and Beard, 2005), UAV traffic might also be efficiently managed using formations of UAVs that are coordinated locally but treated as a single entity by air traffic controllers and pilots of nearby aircraft.

E3 Appropriate roles of humans and automated systems for separation assurance, including the feasibility and merits of highly automated separation assurance systems

Air traffic control is currently a labor-intensive process. FAA controllers—aided by radar, weather displays, and procedures—maintain traffic flow and assure separation by communicating instructions to aircraft in their sector of responsibility. Limitations to this traditional paradigm are, in some areas, constraining the capacity of the air transportation system. For example, the FAA required airlines serving the Chicago O’Hare airport to reduce some of their flights during 2005 because of congestion-related delays. A recent study of en route sector congestion suggested that capacity could be increased by a factor of two or more while maintaining existing spacing, by developing new systems that merge human and computer decision making and automate time-critical separation assurance tasks (Andrews et al., 2005).

Initiatives to reduce aircraft separation by providing automated advisories to air traffic controllers and flight crews have not lived up to expectations, because of controller workload concerns, institutional resistance, and other factors. The advent of UAVs has caused additional concern because it may not be feasible for UAVs with human-in-the-loop collision avoidance schemes to act in time to prevent midair collisions. This has led to interest in determining whether automating aircraft separation, whereby the controller is neither in the loop nor responsible for separation, is feasible and desirable. However, changing the role of the controller from tactical separation to traffic flow management and trusting automated systems to manage the tactical separation of aircraft would require resolution of major human factors, safety, and institutional issues (Wickens et al., 1998; Woods and Hollnagel, 2006). Collisions could occur if a UAV fails to respond or the automated traffic separation system fails and if human intervention is not effective. This Challenge would determine the appropriate roles of humans and automated systems to assure separation in high-density airspace during nominal and off-nominal operations. As part of this challenge, NASA should assess the feasibility and merits of highly automated separation assurance systems.

E4 Affordable new sensors, system technologies, and procedures to improve the prediction and measurement of wake turbulence

Existing wake vortex separation standards reduce system capacity during takeoff and landing operations and instru-

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

ment approaches. Encounters with a wake vortex are also a growing concern in en route Reduced Vertical Separation Minima (RVSM) airspace (Reynolds and Hansman, 2001).12

Current research by the FAA and NASA is focused on procedural enhancements that take advantage of wake transport by winds (Mundra, 2001). For example, the capacity of San Francisco International Airport is expected to improve by using this approach to enable arrivals on both closely spaced parallel runways during low-visibility weather. However, the relaxation of in-trail wake separation standards awaits improved measurement and prediction of wake behavior.

Existing sensors and models do not adequately characterize wake decay phenomena, especially at typical final approach altitudes. Improved sensors, including coherent pulsed lidars, capable of directly measuring wake rotational momentum, are needed to support phenomenological studies and enable more accurate predictions of wake magnitude and decay in various atmospheric conditions. Those predictions, combined with models of aircraft upset risk, should allow reduced wake separation standards without degrading safety.

R&T Challenge A10 will conduct research to improve techniques for predicting and measuring the formation, trajectory, and decay of vortices, including methods to accurately predict wingtip vortex formation and define changes in aircraft design to mitigate the strength of the vortices. This Challenge would complement that work by developing affordable new sensors, system technologies, and procedures to improve prediction and measurement of wake strength, location, motion, and aircraft upset risk in terminal and en route airspace. Together, Challenges A10 and E4 will enable safe flight with reduced in-trail wake separation.

E5 Interfaces that ensure effective information sharing and coordination among ground-based and airborne human and machine agents

The potential for sharing a wide range of information within the air transportation system raises additional questions about how multiple agents (pilots, controllers, other system users, and automated system elements) can coordinate and share information given their disparate viewpoints and contexts. For information sharing to be effective, information must be provided to the right agents, at the right time, and in a fashion that facilitates accurate interpretation regardless of the source of the information. Some of the shared information may be factual (e.g., aircraft position, speed, heading, altitude, and flight plan), while some of it may be less tangible (e.g., potential responses to disruptions). The information elements will also likely vary in their timeliness and accuracy, and access to some information will be restricted for security and business reasons. Developing appropriate interfaces (in terms of information-sharing protocols, as well as display and visualization technology) is a nontrivial challenge, because agents can be easily overwhelmed by too much information or by the need to translate and analyze the information relative to their own situation and goals (Woods et al., 2002). Interfaces for human agents, in particular, will need to include methods for visualizing and interpreting operational situations to facilitate effective judgments and decisions. In addition, information sharing and decision-making processes will often be conducted collaboratively by multiple agents. Therefore, they will require knowledge of both individual human cognition and of collaborative work among agents with potentially conflicting goals and different representations of the immediate situation (Brennan, 1998; Olson et al., 2001). Information-sharing protocols become exceptionally critical during crises, such as 9/11, when control of the national airspace was transferred to the military. Communications and decision-making protocols were fragmented. Research related to this Challenge must be coordinated with DoD and DHS to avoid a recurrence of such problems. The Challenge should also capitalize on technologies pioneered in the telecommunications industry that would facilitate the transfer of diverse information through dynamically reconfigured networks using thousands of disparate nodes.

E6 Vulnerability analysis as an integral element in the architecture design and simulations of the air transportation system

More than three-fourths of air transportation system delays are weather related (Meyer, 2005). Snow or thunder-storms at major hub airports often significantly reduce overall system capacity and efficiency. Abnormal en route winds cause unexpected peaking and depeaking at arrival gateways. En route convective weather causes disruptive and unpredictable rerouting, precipitating en route delays and reducing capacity and efficiency. Disruptions can also be caused by natural disasters (such as volcanoes, hurricanes, tornadoes, and wildfires), electronic attacks (such as power outages, hurricanes, GPS spoofing, spurious communication messages, and hacking into navigation aids), and physical attacks (such as destruction of control facilities and radars). The effects of these disruptions may be local, regional, or national. In all cases, system capacity and efficiency are directly affected, and, more important, the safety of the air transportation system may be compromised by an inadequate response.

Airlines use a variety of techniques to respond to such disruptions. Some reduce schedule to preposition aircraft for the recovery, when the weather abates; others try to fly their full schedule, hoping that the recovery will take care of itself.

12

Reduced Vertical Separation Minima apply to the airspace from flight levels 290 to 410 (which is equivalent to altitudes of approximately 29,000 feet to 41,000 feet) and create twice as many usable flight levels, decreasing the vertical separation between aircraft from 2,000 feet to 1,000 feet. While increasing capacity, this also could exacerbate the effects of wake turbulence.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

Assessing vulnerabilities and risks should be the first step in reducing the likelihood and consequences of unplanned system disruptions (Volpe, 2003, p. 4). System safety impacts of disruptions should be evaluated early in the development cycle of new ATM system architectures, operating concepts, and system components. An agile ATM design should include provisions to counter or recover from system disruptions, and the design of the overall air transportation system should be evaluated by research and simulation to develop both system design concepts and/or operational procedures. In addition, quantitative analyses should be used to assess the safety impact of system architecture options. This Challenge would introduce vulnerability analyses as an integral element in the architecture design and simulations of the air transportation system to reduce the likelihood that the system will experience major system disruptions, to mitigate the severity of specific system disruptions, and to facilitate recovery from system disruptions. The result would be an air transportation system that is self-diagnosing and self-healing.

E7 Adaptive ATM techniques to minimize the impact of weather by taking better advantage of improved probabilistic forecasts

Adaptive traffic flow management methods are needed to take advantage of recent improvements in automated aviation weather forecasts. About 70 percent of aviation delay is due to operationally significant weather, including thunder-storms, low ceilings and visibilities, high winds, and turbulence. Exploitation of weather data collected from ground sensors and satellites using advanced image processing and machine intelligence has enabled significant improvements in aviation weather forecasts. One- to two-hour storm motion products are now being routinely displayed in key airport and en route air traffic facilities and in airline dispatch centers. Included are automatically updated estimates of the forecast accuracy, expressed as a probability (Robinson et al., 2004). This information is beginning to be used by air traffic managers and dispatchers, but only manually (Wolfson et al., 2004).

Algorithms are needed that automatically translate the weather forecasts into actionable traffic flow recommendations, with the goal of fully incorporating the weather data into air traffic automation designs. A few examples of automation that translate probabilistic weather forecasts into traffic flow recommendations have been developed, and FAA air traffic managers have shown they can reduce delays. For example, the LaGuardia Airport traffic flow managers are using storm motion forecast tools, such as the Route Advisory Planning Tool, to automatically identify safe departure routes (Evans, 2006). However, many automation systems are not incorporating the new weather information into their designs. This Challenge would demonstrate the use of automated weather forecasts in making traffic flow decisions and determine where this capability is cost beneficial.

E8a Transparent and collaborative decision support systems

Air traffic operations are enhanced by effective decision support systems that assist pilots, controllers, traffic flow managers, and airline personnel in tasks such as routing, flight planning, scheduling, and traffic separation. These decision support systems contribute to safe and efficient operations by using technology to enhance human capabilities and collaborate with the operator, as opposed to fully automated systems, which use technology rather than an operator to perform tasks. Collaborative decision support systems are most effective when the operators understand the basis for and limitations in the system’s reasoning process and can judge the appropriateness of system-generated recommendations. Similarly, the system’s recommendations should take into account operators’ knowledge and intentions as well as the context in which they operate. Support for reciprocal information sharing and mutual understanding of intentions and actions—a process called grounding—is critical to avoid breakdowns in human–machine collaboration and overall system performance (Sorkin et al., 1988; Lee and Moray, 1994; Smith et al., 2001; McGuirl and Sarter, 2006). This Challenge will identify the type of information to be shared between human operators and automated decision support systems and develop candidate designs for these systems.

E8b Using operational and maintenance data to assess leading indicators of safety

Safety analysis is often a reactive, ad hoc process made difficult, in part, by the very high level of safety required of air transportation in the United States. Few unambiguous data points (accidents) are available for analysis, the number of data points continues to decrease because of the success of ongoing safety efforts, and accidents that do occur are increasingly the result of a complex chain of unlikely circumstances, each of them benign (Leiden et al., 2001). While human error is often cited as a major safety concern, successful human performance is also a major (and under-reported) contributor to system safety. Thus, a particular concern for safety analysis is the human contribution to safety, especially when predicting the safety impact of dramatic changes to the role of human operators and increased reliance on automation. Likewise, safety analysis must consider individual aircraft as well as systemwide safety, which involves complex interactions among many agents. Using a common set of safety metrics (see R&T Challenge E16), this Challenge would develop methods both for monitoring the current system through ongoing analysis of operational and

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

maintenance data and for predicting potential safety problems associated with proposed changes to the air transportation system.

E8c Interfaces and procedures that support human operators in effective task and attention management

The expected growth in air transportation demand will likely require operators to perform a wider range of tasks and to collaborate more closely with one another and with modern technologies. Pilots may begin to play a more active role in traffic separation or spacing and will need to coordinate their activities and intentions with other pilots and controllers. They will need to interact and exchange information, often interrupting each other and creating new tasks for one another. In general, more information will need to be distributed in a timely manner, task sets will increase, interruptions will become more likely, and the tolerance for delayed action or intervention will probably be reduced. It will be critical to ensure that operators are supported in properly scheduling and prioritizing their tasks, to improve attention management and avoid errors caused by unnecessary task switching, unnecessary interruptions, or inappropriate dismissals of demands (i.e., the failure to switch attention when appropriate and necessary) (Woods, 1995; McFarlane and Latorella, 2002; Ho et al., 2004).

High-Priority R&T Challenges and Their Associated Thrusts

Only one R&T Challenge in this Area (E1) had major relevance and impact on the energy and environment Strategic Objective. The other nine high-priority R&T Challenges in this Area each had major relevance and impact on at least two of the other three highly weighted strategic objectives (capacity, safety and reliability, and efficiency and performance).

In some cases, the R&T Challenges in this Area would have direct operational impact, for example, by developing new concepts and methods of separating, spacing, and sequencing aircraft to increase capacity and safety in all flight conditions (E2). In other cases, the benefits would be less direct, for example, by developing more capable methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems (E1) and determining appropriate roles of humans and automated systems for separation assurance in high-density airspace during nominal and off-nominal operations (E3). Although the results of research in these areas would take longer to produce operational benefits, the research is essential, it is appropriate for NASA to include the research in a 10-year plan, and NASA involvement in the research is necessary to ensure that this research moves forward and can be readily applied to the air transportation system.

The ultimate objective of NASA’s aeronautics research as it relates to intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications is to provide the fundamental capabilities required for an adaptive and robust air transportation system that meets the nation’s goals for economic growth, public well-being, and national security. Because the air transportation system is a complex interactive system, the linkages among its component systems are just as important as the component systems themselves. The committee identified four R&T Thrusts that describe threads of commonality among the Challenges in the intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications Area:

  • Decision making, negotiation, collaboration, information sharing, and allocation of airspace resources.

  • Aircraft separation, spacing, and sequencing.

  • Simulation, modeling, and analysis of complex, adaptive distributed systems.

  • Wake and weather sensing, modeling and prediction, and other enabling air transportation technologies.

Each of these thrusts is discussed below. As shown in Figure 3-8, the first two Thrusts would lead directly to improvements in air transportation system operations, and the last two Thrusts would provide enabling technologies and capabilities that support the first two.

Decision making, negotiation, collaboration, information sharing, and allocation of airspace resources

Mechanisms must be constructed to facilitate and structure the interactions of all air transportation system components—businesses, organizations, individual humans, technologies—such that the emergent system performance is adequate. To do so requires foundational research into several topics, including the appropriate roles of automation; methods of supporting effective decision making and task management by individual humans, by automated systems, and by humans and automation working together; and information sharing, negotiation, and coordination within and between organizations. One outcome of these functions of particular importance to the design of the Next Generation Air Transportation System is allocation of airspace and airport resources, insofar as the method used to allocate these resources can determine how demand relates to capacity and whether airports and flights experience outcomes such as delays or denial of service. The following R&T Challenges are most closely related to this Thrust:

E5, Interfaces that ensure effective information sharing and coordination among ground-based and airborne human and machine agents.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

FIGURE 3-8 R&T Thrusts related to Area E: intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications.

E8a, Transparent and collaborative decision support systems.

E8c, Interfaces and procedures that support human operators in effective task and attention management.

E11, Automated systems and dynamic strategies to facilitate allocation of airspace and airport resources.

E18, Certifiable information-sharing protocols that enable exchange of contextual information and coordination of intent and activity among automated systems.

Aircraft separation, spacing, and sequencing

The high-density airspace of the future will require effective management of aircraft separation, spacing, and sequencing in all flight conditions. The future air transportation system must develop improved models and novel operational concepts to support capacity growth and accommodate scheduled and unscheduled operations in airspace shared by manned and unmanned aircraft without compromising safety. Of particular importance to this research are separation assurance methods and understanding the appropriate roles of humans and automation in high-capacity air-space. Research in several R&T Challenges is needed to provide critical inputs, including accurate wake and weather forecasting as well as flight monitoring capabilities. The following Challenges are most closely related to this Thrust:

E2, New concepts and methods of separating, spacing, and sequencing aircraft.

E3, Appropriate roles of humans and automated systems for separation assurance, including the feasibility and merits of highly automated separation assurance systems.

Simulation, modeling, and analysis of complex, adaptive distributed systems

Design of the Next Generation Air Transportation System is a tremendous engineering challenge. This network of safety-critical, complex interactive systems will be vast in scope and involve multiple disparate organizations with separate objectives and capabilities. Examining the challenges facing this development, the committee found that individual technologies and systems will contribute to system performance only indirectly through their influence on how the larger system is collectively operated by the many user organizations; thus, system operations must also be a focus of research and development. Understanding the complexities of these operations requires new design tools and methodologies. Metrics are also important, because system performance metrics have a direct impact on the design of the system: Parameters that are not measured—or are measured incorrectly or incompletely—will not be fully considered or accounted for in the final design. Thus, it is critically important that the appropriate metrics be identified and incorporated into system analysis and design tools and processes. However, there is no comprehensive, widely held set of metrics to analyze and design the current and future air transportation system. Because many issues (e.g., economic, efficiency, safety, environment) must be addressed simultaneously, the problem cannot be decomposed into isolated examinations of capacity, safety, technology, human factors, etc. New simulation tools are needed with extensive predictive capabilities. Likewise, new analysis methods are required to integrate safety and vulnerability assessments into

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
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the design process. Coordinating all of these insights requires understanding of complex, adaptive distributed systems with the unique characteristics of air transportation, including the need to simulate and predict the behavior of radically different system configurations.

The following R&T Challenges are most closely related to this Thrust:

E1, Methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems.

E6, Vulnerability analysis as an integral element in the architecture design and simulations of the air transportation system.

E8b, Using operational and maintenance data to assess leading indicators of safety.

E16, Appropriate metrics to facilitate analysis and design of the current and future air transportation system and operating concepts.

E17, Change management techniques applicable to the U.S. air transportation system.

E20, Comprehensive models and standards for designing and certifying aviation networking and communications systems.

Wake and weather sensing, modeling and prediction, and other enabling air transportation technologies

Several critical technologies warrant fundamental research by NASA for their likely value as enablers of many possible operational concepts, and because some are still in a nascent state. Of particular importance are the sensing, modeling, and prediction of aircraft wakes and hazardous weather. Other enabling technologies center on the creation of enhanced automation capabilities for safety (e.g., “refuse-to-crash”) and for capacity (e.g., agents for negotiating resource allocation) beyond those that the research community currently knows how to develop and certify as robust in a wide range of conditions. Additional technologies focus on further enhancements to the communication and navigation needs of air traffic management functions. The following R&T Challenges are most closely related to this Thrust:

E4, Affordable new sensors, system technologies, and procedures to improve the prediction and measurement of wake turbulence.

E7, Adaptive ATM techniques to minimize the impact of weather by taking better advantage of improved probabilistic forecasts.

E12, Autonomous flight monitoring of manned and unmanned aircraft.

E13, Feasibility of deploying an affordable broad-area, precision-navigation capability compatible with international standards.

E14, Advanced spacecraft weather imagery and aircraft data for more accurate forecasts.

E15, Technologies to enable refuse-to-crash and emergency autoland systems.

E19, Provably correct protocols for fault-tolerant aviation communications systems.

Low-Priority R&T Challenges

Seven of the 10 R&T Challenges that did not rank in the top 10 by NASA priority also ranked low in national priority. Which is to say, they had substantial impact on only one of the highly weighted strategic objectives (capacity, safety and reliability, efficiency and performance, or energy and the environment). The other three R&T Challenges (E11, E13, and E17) would have substantial impact on two of the highly weighted strategic objectives, but they ranked low in terms of NASA priority because of low Why NASA? scores. Because these Challenges rank high in national priority, it is important that some part of the national civil aeronautics R&T effort (by NASA, other government agencies, industry, or academia) support work to overcome them.

E17 Change management techniques applicable to the U.S. air transportation system

The current ATM airspace architecture and associated procedures are antiquated and so reliant on interim fixes that there is significant resistance to additional changes. A novel and consistent approach is needed to manage changes to the ATM system and to overcome barriers and organizational inertia within the FAA and other stakeholders. The FAA should support research related to this Challenge, though it may take external pressure (e.g., from the Office of Management and Budget, the Government Accountability Office, and the White House) to prompt action. It may be appropriate for NASA to also conduct research related to this Challenge, but NASA would first need to increase its relevant capabilities.

E11 Automated systems and dynamic strategies to facilitate allocation of airspace and airport resources

The current allocation of airspace and airport resources (e.g., airport departure and arrival slots) at major airports is heavily biased toward airline operations. The competition for airspace and airport resources will be exacerbated by growth in commercial and private air travel, including the introduction of very light jets. Future air transportation systems would benefit from built-in automatic response systems driven by software agents that quickly negotiate and make decisions regarding, for example, real-time allocation of airspace and landing slots among aircraft with diverse size and performance characteristics, while considering the needs of all stakeholders, air transportation system efficiency, and energy conservation (Cramton et al., 2002). This Challenge ranked low in NASA priority because the FAA already is

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
×

funding some related research and is best suited to address this Challenge.

E13 Feasibility of deploying an affordable broad-area, precision-navigation capability compatible with international standards

Many small airports cannot operate when visibility is restricted because they do not have the equipment (e.g., a Category I, Category II, or Category III instrument landing system) necessary for a safe approach and landing.13 The number of ILS frequencies available in large metropolitan areas, where multiple runways require precision approach and landing capabilities, is limited. Increased access to these airports and runways would increase efficiency significantly, particularly for some segments of the aviation industry (e.g., feeder airlines, business aircraft, and air taxis). While satellite navigation systems are currently deployed by the United States and others, they do not provide sufficient coverage, accuracy, and reliability for aviation requirements to replace or substitute for ground-based aids, particularly as regards precision landing guidance provided by ILS (Shively and Hsaio, 2005). The objective here is to design, develop, and deploy a space-based navigation system augmentation that complements GPS, Galileo, and the Global Navigation Satellite System (GLONASS), so that guidance equivalent to Category IIIc ILS is universally available and no additional ground-based capability (e.g., pseudolites) needs to be installed. Deployment of this capability would open up any airport or temporary landing area for all-weather operations with no need for expensive and time-consuming construction; facilitate reconfiguration of approach paths; and improve safety. It would allow many existing runways and small airports not equipped with ILS to operate in low visibility conditions and would support emergency operations and homeland security. It would have the added benefit of allowing the FAA to remove thousands of existing en route and terminal navigation aids, such as very high frequency (VHF) omnidirectional range (VOR) equipment, distance measurement equipment (DME), tactical air navigation (TACAN) systems, ILSs, and nondirectional beacons (NDB). Decommissioning these systems would eliminate associated maintenance and operating costs. This Challenge ranked low in NASA priority primarily because the feasibility of deploying an affordable, broad-area precision navigation capability will be determined by technical, economic, and regulatory issues. The technical feasibility issues are well aligned with NASA’s aeronautics mission, but economic feasibility issues are better handled by industry, and regulatory feasibility issues are better handled by the FAA.

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An ILS is a ground-based precision approach system that provides course and altitude guidance to pilots as they prepare to land. ILS systems are rated according to their capabilities:A Category I system can provide guidance regarding course and glide slope down to an altitude of 200 feet with a runway visual range of not less than 1,800 (or 2,400 feet depending on runway lighting and configuration).A Category II system can provide guidance regarding course and glide slope down to an altitude of 100 feet with a runway visual range of not less than 1,200 feet.A Category IIIa system can provide guidance regarding course and glide slope all the way to touchdown as long as the pilot has some external visual reference during the final phase of landing and the runway visual range is not less than 700 feet.A Category IIIb system can provide guidance regarding course and glide slope all the way to touchdown even without any external visual references, as long as the runway visual range is not less than 150 feet (for taxi operations after landing).A Category IIIc system can provide guidance regarding course and glide slope all the way to touchdown and during taxi operations without any external visual references and with zero visibility.

Suggested Citation:"3 Research and Technology Challenges." National Research Council. 2006. Decadal Survey of Civil Aeronautics: Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/11664.
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Next: 4 Common Themes and Key Barriers »
Decadal Survey of Civil Aeronautics: Foundation for the Future Get This Book
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The U.S. air transportation system is very important for our economic well-being and national security. The nation is also the global leader in civil and military aeronautics, a position that needs to be maintained to help assure a strong future for the domestic and international air transportation system. Strong action is needed, however, to ensure that leadership role continues. To that end, the Congress and NASA requested the NRC to undertake a decadal survey of civil aeronautics research and technology (R&T) priorities that would help NASA fulfill its responsibility to preserve U.S. leadership in aeronautics technology. This report presents a set of strategic objectives for the next decade of R&T. It provides a set of high-priority R&T challenges—-characterized by five common themes—-for both NASA and non-NASA researchers, and an analysis of key barriers that must be overcome to reach the strategic objectives. The report also notes the importance of synergies between civil aeronautics R&T objectives and those of national security.

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