Improving Aircraft Performance
Commercial jet aircraft produced in the United States are highly competitive, but they are also the result of technology investments made a long time ago. Technologies used to support the launch of the Boeing 777, the most recent model of U.S. widebody aircraft, were developed over 20 years ago. The effects of a diminished or misdirected aeronautics research program will not be significant in the near term, but eventually the result will be a diminished U.S. aeronautics industry.
Improvements in aircraft performance are critical to achieving necessary improvements in almost every aspect of the overall performance of the air transportation system (see Chapter 1). For airlines, operational cost (i.e., cost per seat-mile) is a key measure of aircraft performance. However, estimating the ultimate effect that long-term research and advanced technology may have on operational cost is often difficult at best.
A basic measure of aircraft productivity can be computed by multiplying payload by block speed (the average gate-to-gate speed for a given mission leg). Design efficiency is then indicated by the ratio of productivity to maximum takeoff weight (MTOW); high design efficiency is reflected in lower MTOW. A more complete understanding of aircraft productivity and efficiency should include additional factors, such as availability (the average number of hours per day, week, etc., that an aircraft can be operated, taking into account servicing and maintenance requirements), utilization (the actual number of hours per day, week, etc., that a particular aircraft is operated), operational range (which ideally should be matched to the routes on which a particular aircraft is employed), and fuel consumption. For the same aircraft, utilization rates and block speeds vary by airline and route, so these factors are beyond the direct control of manufacturers and design engineers.
However aircraft productivity and efficiency are measured, they can be improved through advances in aircraft aerodynamics, materials, structures, and other disciplines that improve performance parameters such as lift-to-drag ratio (L/D), ratio of empty weight to MTOW, and specific fuel consumption. Technological approaches to the above goals include the use of boundary layer control to reduce profile drag and parasite drag and the use of new materials, such as modern carbon-based or metal matrix composites, to reduce structural weight fraction. Additional technical areas that merit focused research include composite structures with the following characteristics:
high damage tolerance
high stiffness (because a lot of airline structures are sized for stability)
active controls (which, if sufficiently reliable, may reduce the need for high stiffness)
low-cost raw materials and fabrication methods
low density (with high strength-to-weight ratios)
means to assure that material properties are satisfactory following repairs and have not degraded unexpectedly over the life of aircraft in which composites have been incorporated
resistance to lightning strikes (an area where Boeing, for one, is investing millions of dollars)
Improvements in performance parameters not directly related to aircraft productivity and efficiency are also important, because they would improve the performance of the overall air transportation system. For example, reduced landing and takeoff distances enable aircraft to use more runways (and access more airports). In the extreme, rotorcraft are able to operate without runways. In addition, reducing runway occupancy time during landing and takeoff increases
runway throughput. Minimizing the ground footprint of aircraft relative to their capacity also allows for more efficient use of limited airport space.
Although aircraft performance is important, systemwide performance is the overriding concern. Without a systemwide perspective, research and development runs the risk of suboptimization—for example, by improving the performance of a vehicle system in a way that degrades overall performance of the air transportation system. The above discussion of aircraft performance should, therefore, be understood in the larger context of air transportation system performance.
In the discussion that follows, improvements in aircraft performance will be discussed in terms of (1) environmental considerations, (2) advanced airframe concepts, (3) advanced propulsion concepts, and (4) the potential of a cross-cutting technology of particular interest: nanotechnology.
The air transportation system already expends considerable resources to deal with public concerns and government regulations related to the effects of aviation on local and regional air quality, climate change, and community noise. All of these environmental problems will be aggravated by growth in air traffic. Problems related to emissions are abated by propulsion and airframe concepts and technologies that improve aircraft efficiency. However, the rapid growth of demand for air transportation and the growth in capacity have exceeded the rate of improvement of specific fuel consumption, so that over time aviation consumes larger amounts of fuel. The amount of carbon dioxide (CO2) released in the atmosphere is roughly proportional to fuel consumption, so more CO2 is being released. The amount of other emissions, such as oxides of nitrogen (NOx) and particulates, is also increasing even though engines are becoming more efficient and cleaner, producing fewer emissions per pound of fuel burned. Higher engine combustion temperatures tend to improve the efficiency of the propulsion system, but higher temperatures also increase NOx emissions. The production of specific emissions can be minimized by changes to the combustion cycle and other aspects of engine design, although changes in engine design to reduce one emission might increase the production of other emissions.
Noise can also be reduced by improvements in the design of the integrated aircraft as well as specific changes to the engine and propulsion system. In some cases, noise reduction technologies reduce overall aircraft efficiency because, for example, they increase aircraft weight.
Breakthroughs could be achieved through use of an alternative fuel such as liquid hydrogen or revolutionary technologies such as fuel cell-electric propulsion. However, breakthrough technologies such as these are likely to take several decades, at least, to become operational. Accordingly, research in environmental technologies should focus on conventional jet propulsion systems, while continuing to explore promising longer-term technologies. Environmental considerations are discussed in more detail below and in a recent report by the National Research Council (NRC, 2002a).
Local and Regional Air Quality
The principal concerns regarding local air quality are high levels of NOx and particulate matter. At a regional level, NOx and unburned hydrocarbon emissions from aircraft engines also contribute to the formation of ozone and are currently regulated in accordance with standards established by the International Civil Aviation Organization. A standard for measuring particulate matter from aircraft engines is currently being developed. The contribution of aircraft to regional emissions of NOx and particulate matter is currently on the order of 1 percent of all anthropogenic emissions. The aircraft contribution is increasing, however, as air traffic increases, while emissions from other sources are decreasing as a result of more stringent emissions standards and improved emissions reduction technologies.
Limits on total NOx emissions established by local authorities are already threatening to limit capacity at some airports in Europe, while the imposition of landing fees proportional to NOx emissions by each aircraft have been implemented at other European airports. Stringent emissions standards and the threat of emissions caps have led to modest emissions reductions through optimization of current gas turbine emissions technology. However, these reductions have been largely offset by higher engine pressure ratios (for improved fuel efficiency), which tend to increase emissions of NOx and particulate matter. Emissions of NOx by aircraft have not been reduced as much as emissions by surface sources because alternative fuels and exhaust gas cleanup technologies used in other transportation and industrial sources require large, heavy devices that are not practical in aircraft applications. Design improvements that reduce aircraft weight or improve aircraft and engine aerodynamics tend to reduce NOx emissions because less fuel is consumed.
Better dispersion models will lead to a better understanding of the impact of aircraft emissions and will be a better guide to technology development. Modeling the movement of emissions released by aircraft in flight is significantly more difficult than modeling emissions from a static point source, such as an industrial facility. Another emerging need is the development of a standardized method for measuring emissions of particulate matter; current data on aircraft emissions of particulate matter are sparse and of questionable quality.
Research is needed to develop combustor technologies to reduce emissions of NOx and particulate matter in engines that operate at high pressure ratios with current jet fuels. If
hydrogen fuel becomes widely used in the longer term, particulate matter will no longer be an issue, but low NOx combustion technology tailored for hydrogen fuel engines will have to be developed.
The aircraft emissions with the strongest effects on climate are CO2, NOx, and water vapor (through the formation of contrails and clouds). CO2 is the most prevalent and best understood greenhouse gas, and the warming effect of CO2 emitted by aircraft in flight is indistinguishable from that of CO2 emitted at ground level. NOx emitted at altitudes normally used by subsonic aircraft forms ozone that can lead to additional warming. The magnitude of this effect is uncertain, but some researchers estimate the warming effect to be two to three times that of CO2 emissions from aircraft. There is even more uncertainty surrounding the effect of water vapor emitted by current engines; it may be more important—or less important—than CO2 emissions. Particulate matter emissions can also contribute to warming, though not as much as CO2 (IPCC, 1999).
Current efforts by the International Civil Aviation Organization to control emissions of CO2 are focusing on developing a system of market-based options, such as (1) charges based on aircraft efficiency or (2) emissions trading systems. These measures are intended to increase incentives to improve fuel efficiency. However, minimizing fuel consumption might not be the best approach. For subsonic aircraft, global warming effects could be reduced by designing engines to operate at a lower pressure ratio and by designing aircraft to fly at a lower altitude. Both approaches would significantly increase fuel consumption, but based on current understanding of the atmosphere, NOx emissions and contrail/cloud formation would be reduced enough to offset the increased CO2 and provide a net reduction in the aircraft’s adverse effect on climate. Better understanding of aviation’s effect on climate is needed to have confidence that climate change can really be minimized with this kind of strategy (Green, 2002).
Development of combustors that produce ultra-low levels of NOx at cruise conditions would allow NOx to be reduced from current levels without increasing fuel consumption (and other types of emissions). On balance, it might turn out that the most beneficial approach would be to optimize the aircraft engine combustors for low NOx at cruise conditions, even if this increased NOx emissions in the vicinity of airports. Better understanding of local, regional, and global effects is needed to select the approach that provides the best net environmental benefit.
The use of hydrogen as an aircraft fuel could eliminate emissions of particulate matter and open up new approaches for reducing NOx emissions, but emissions of water would be increased greatly.
Even if all emissions are reduced to insignificant levels, community noise could limit airport capacity. Many airports already have noise quota systems that limit the number of operations at certain times of each day. The certification process for new aircraft consists of measuring system noise at three points (takeoff, side line, and approach) and is the only universally accepted noise standard for aircraft noise (because it is used for certification). In order to keep objectionable noise within airport boundaries, more advanced engine and aircraft noise reduction technologies are essential. New operational procedures (for example, curved approaches) would also be beneficial at many airports. The goal is to reduce noise below objectionable levels even as traffic increases. Achieving this goal requires an accurate measure of what noise level is objectionable. The FAA currently uses 65 dB DNL1 as the level that justifies corrective action, but some environmental groups and national governments maintain that the standard should be 55 dB DNL, and complaints about aircraft noise are unlikely to disappear altogether until someone discovers a silent propulsion system. Efforts to reduce objectionable noise are also complicated by the perception (if not the reality) that some noise complaints are motivated less by the level of noise per se than by the fear that aircraft noise causes in those who dread the thought of aircraft routinely flying over their homes. Noise limits may become more stringent in Europe than in the United States. Since U.S. manufacturers compete for sales worldwide, they would need to meet the more stringent noise requirements.
The Vehicle Systems Program of NASA’s Aerospace Technology Enterprise has established five vehicle classes to facilitate trade studies of candidate technologies and assessments of technology integration issues.
personal air vehicles
uninhabited air vehicles (UAVs)
runway-independent air vehicles
other airframe concepts
The committee considered how research and technology development applicable to each vehicle class might address the key long-term challenge facing the air transportation system, which is how to accommodate increased demand for air travel while still meeting public expectations related to safety, security, capacity, environmental compatibility, and consumer satisfaction. Technologies related to personal air vehicles, UAVs, and supersonic aircraft were examined by focused NRC studies (2002b, 2000, and 2001, respectively). As described below, personal air vehicles and UAVs are unlikely to contribute significantly to the effort to meet increased demand, although research in both areas would help achieve other goals. For example, improved personal air vehicles could expand opportunities for air transportation to small communities, and UAVs are already fulfilling important military missions.
The ability of supersonic aircraft to help meet increased demand is also problematic, especially in the case of supersonic business jets, which are likely to be the next class of supersonic aircraft to be developed. The development of supersonic business jets may be justifiable in terms of economics and their ability to make service more convenient. However, they are not likely to capture an appreciable fraction of the commercial passenger market, even if technology is available to reduce the sonic boom to acceptable levels for overland travel.
Runway-independent air vehicles may be able to help meet increased demand at capacity-limited airports, and in any case they execute unique, important missions that other types of aircraft cannot perform. However, as in the case of supersonic transports, runway independent air vehicles are unlikely to capture an appreciable fraction of the market for commercial air transportation.
The potential of subsonic transports to meet increased demand far exceeds that of the other vehicle classes. This is not surprising, because the primary purpose of subsonic transports is the efficient mass movement of passengers and cargo. Subsonic transports benefit from design efficiencies unavailable to small aircraft, and they avoid design penalties associated with specialized capabilities such as supersonic cruise speed and vertical flight.
Technologies specifically related to personal air vehicles, UAVs, supersonic aircraft, or runway-independent air vehicles do have the potential to improve the performance of the air transportation system, especially in niche areas. However, research in these areas will not be able to resolve the overall capacity problems that are the primary challenge to the continued success of the air transportation system over the long term. Accordingly, the committee did not examine technologies related to these vehicle classes and makes no recommendations concerning the future direction of research in these areas. Nonetheless, implementation of the process for change recommended by the committee (see Recommendation 5-1) would facilitate planning of research for all vehicle types.
Personal Air Vehicles
NASA’s Small Aircraft Transportation System (SATS) project is a key part of NASA’s efforts to develop improved personal air vehicles and related airspace technologies and systems. NASA envisioned that the SATS project would relieve some of the capacity problems at the nation’s major airports. The NRC determined, however, that it would be very difficult for the proposed SATS concept to address capacity problems. Rather, improved personal air vehicles should be viewed as a complement to commercial carriers that could enhance mobility, especially in regions not well served by scheduled air service. The National Research Council recently completed an in-depth assessment of the SATS project (NRC, 2002b), and another report evaluates research supported by SATS as part of a larger assessment of NASA’s aeronautics research (NRC, 2003).
Uninhabited Air Vehicles
The state of the art of UAVs is rapidly advancing, with the Department of Defense investing heavily in UAV research with military applications. UAVs also have potential commercial applications as, for example, “suborbital satellites”—long-endurance aircraft operating at high altitudes (on the order of 100,000 ft) over a fixed location to provide services now provided by satellites. Lighter-than-air vehicles cannot work at these altitudes because available solar energy is insufficient to overcome wind forces. UAVs also have potential as cargo carriers, but this application requires continued research on operational certification requirements for UAVs and changes to air traffic control regulations and procedures. Performance requirements for surveillance aircraft (a key UAV mission for military applications) are very different than for station keepers, so NASA research in this area seems worthwhile. Nonetheless, UAVs, like personal air vehicles, are unlikely to significantly enhance the ability of the U.S. air transportation system to accommodate increased demand.
Deployment of an environmentally acceptable, economically viable commercial aircraft capable of sustained supersonic flight, including flight over land, would be a remarkable achievement requiring remarkable technological advances. One approach to the ultimate goal of developing a large commercial supersonic transport would be to first develop a business jet certificated for supersonic flight over land. Development of a supersonic business jet would help address many of the technical challenges involved in developing larger commercial supersonic aircraft and would resolve current uncertainties about the regulatory standards for emissions and noise (including sonic boom) that future commercial supersonic aircraft would be required to meet. Also,
some of the technical issues associated with commercial supersonic flight would be easier to address with a smaller aircraft, and economic viability would be easier to achieve for a business jet, which faces different economic drivers than commercial passenger jets. Speed sells, especially in the business jet market, and a commercial supersonic business jet that is able, in terms of technical performance as well as regulatory authorization, to cruise at Mach 1.6 to 1.8 over land would probably be a commercial success even at twice the price of a comparably sized conventional business jet (NRC, 2001).
Even though the effort required to overcome the technological barriers to a supersonic business jet is less than that for a large commercial transport, it would still be sizeable. The most significant challenge involves defining acceptable levels of sonic boom and demonstrating that those levels can be met, to support regulatory changes to permit supersonic flight over land. This will probably require building a full-scale demonstrator, which could cost $1 billion.
The NRC issued reports assessing NASA’s commercial supersonic research in 1997 and 2001 (NRC, 1997, 2001). The more recent report (1) identifies key technology challenges for supersonic business jets and two classes of larger commercial supersonic transports and (2) makes specific recommendations for research. A properly directed and adequately funded research and technology effort could probably enable operational deployment of environmentally acceptable, economically viable commercial supersonic aircraft in 25 years or less—perhaps a lot less if there is an aggressive technology development program for aircraft with cruise speeds less than approximately Mach 2 (NRC, 2001).
For a given payload, range, and MTOW, a high-speed subsonic aircraft will have higher productivity and efficiency than a slower aircraft. However, the ability to cruise at supersonic speeds is not without cost: Supersonic flight increases specific fuel consumption and requires a more robust airframe design. As a result, a supersonic aircraft has a higher fuel weight fraction and a shorter range and/or higher MTOW than a subsonic aircraft with a comparable payload capacity. It is far from certain whether a commercial supersonic aircraft would be more efficient or have higher productivity than subsonic aircraft, and the committee is not aware of any research that characterizes commercial supersonic aircraft as a solution to increased demand for air transportation. More commonly, support for commercial supersonic aircraft is based on other important factors: their ability to provide better service (by reducing travel time), the national economic benefits from being first to market with a commercially successful supersonic aircraft, and the economic damage from a foreign aerospace company being first to market.
Runway-Independent Air Vehicles
Rotorcraft are an essential part of the air transportation system. They provide access to disaster scenes, oil rigs, hospitals, maritime vessels, building rooftops, construction sites, and other locations that other forms of aviation cannot service. With regard to the broad challenge of meeting the general public’s increased demand for air transportation, one of the great potential payoffs offered by rotorcraft—or other commercial aircraft with vertical takeoff or landing (VTOL) capabilities—is at airports that are operating near or at their capacity limits. VTOL aircraft have the ability to provide passenger service without increasing demand for runway usage. Currently, the operating cost per seat-mile for VTOL aircraft is 4 to 10 times higher than for conventional aircraft. In the near term, the commercial success of a greatly expanded network of VTOL aircraft would require economic incentives or subsidies to offset their higher cost. In the long term, technology could help reduce the cost differential and address other issues, such as operation in adverse weather (especially icing conditions) and the noise of rotorcraft operations.
One way for NASA to stay involved in research related to rotorcraft and other runway-independent air vehicles would be through partnerships with the Department of Defense, which has invested heavily in this area.
Nontraditional concepts for new classes of commercial aircraft have the potential to greatly improve the performance of both small and large subsonic transports. Concepts of particular interest include (1) the strut-braced or joined wing and (2) the blended-wing-body (BWB) (see Figure 4-1).
Strut-Braced or Joined Wing
The joined wing configuration is similar in concept to a biplane whose lower wing has positive dihedral (the tip of the wing is higher than the point of attachment to the aircraft centerbody) and is swept back from where it is attached to the fuselage. The upper wing is attached to the vertical tail, sweeps forward with negative dihedral, and is joined to the lower wing. A plan view shows the wing as a rhombus, which gives the wing a rigid structure and allows it to have a greater span.
A strut-braced wing uses a strut to support the wing, allowing increased aspect ratio, reduced wing thickness, lower weight, reduced wing sweep, larger wing areas with laminar flow, reduced drag, higher L/D, smaller engines, reduced fuel consumption, and reduced noise and emissions. Pfenninger (1987) has estimated that a strut-braced wing configuration could provide an L/D of 40 if laminar flow
boundary control is applied. A NASA-sponsored study compared the performance of various strut-braced wing designs with the performance of a traditional design with a cantilevered wing. The study concluded that, for an aircraft with a capacity of 325 passengers and a service entry date of 2010, strut-braced wing designs would have a lower takeoff gross weight (by 9 to 17 percent) and lower fuel consumption (by 14 to 22 percent) (Gundlach et al., 1999).
BWB aircraft are a form of flying wings, a configuration that has been investigated by many different aircraft designers, manufacturers, and countries for over 50 years. During the 1940s, the United States developed flying-wing bomber prototypes (the propeller-powered YB-35 and a jet-powered variant, the YB-49); the Germans supported development of flying-wing bombers and fighter-bombers; and the United Kingdom developed a flying-wing fighter prototype, the Armstrong Whitworth AW-52. The Hawker Vulcan is a successful flying-wing-type aircraft that had the same payload range characteristics as the B-47. Currently, the most notable flying-wing aircraft is the U.S. Air Force B-2 bomber, which first flew in 1989.
BWB aircraft integrate the fuselage into the wing structure to reduce fuselage drag. This concept was suggested for commercial applications by Boeing about 10 years ago and has been the subject of studies by Boeing, NASA, and universities in the United States and Europe. The thick center section distinguishes the concept rather fundamentally from pure flying wing designs and leads to structural efficiencies that improve the overall performance of BWB aircraft.
A BWB aircraft with an optimized design could have an L/D as much as 50 percent greater than that of current aircraft. Performance could be further improved by the ability to use turboprop propulsion and the option of hydrogen fuel (because of the large size of super cargo aircraft, the low density of hydrogen fuel becomes less of an issue than it is with conventional aircraft). Existing commercial cargo aircraft are derivatives of passenger aircraft or military cargo aircraft. Technological advances would allow a modern cargo aircraft to have much better performance than multiple-use aircraft. Laminar flow over the wings would further increase the efficiency of BWB aircraft—and of every other aircraft type discussed above.
Two major airlines have indicated interest in purchasing BWB aircraft if they are ever developed. However, such expressions of interest—no matter how vigorous—do not always result in a commercially successful product. For example, the GE 36 was an unducted fan engine that completed flight demonstrations in the late 1980s (see Figure 4-2). The engine consumed 35 percent less fuel than conventional engines with comparable performance, and during initial development airline representatives responded very favorably. When it came time to make a binding commitment to purchase the engines, however, airlines decided that greatly improved fuel economy was insufficient to overcome concerns about life-cycle costs, noise, blade loss, and the possibility that airline passengers might be put off by the appearance of the engine’s external blades. Concerns that BWB aircraft will have to overcome include passenger acceptance of cabins with few, if any, windows; how to handle emergency evacuation of large passenger cabins; and how to fit such a large aircraft (in terms of passenger capacity and size) into existing airport environments.
Other Airframe Concepts
The committee considered other vehicle concepts, as well, but does not recommend focused technology research related to these concepts.
Wing-in-ground-effect aircraft were pioneered in the former Soviet Union. In 1967, the Defense Intelligence Agency detected a Soviet wing-in-ground-effect aircraft with a MTOW of more than 1 million pounds operating in the Caspian Sea (Losi, 1995).
Ground-effect aircraft fly at an altitude equal to a fraction of the wingspan in an aerodynamic regime called “ground effect,” which increases aircraft efficiency. According to some mathematical models, the aerodynamic efficiency of a wing-in-ground-effect aircraft continually increases as the altitude of flight decreases. Operational and safety considerations, however, will always define a minimum flight altitude. Because of their low flight altitude, wing-in-ground-effect aircraft are generally unsuitable for operation over land, though they may be feasible in arctic or desert areas. To operate over open ocean, wing-in-ground-effect aircraft have very large wingspans to avoid collisions with waves. Even with a MTOW on the order of several million pounds, the cruising altitude would be so low that a wing-in-ground-effect aircraft might be at risk from rogue waves.2 The vast size of such an aircraft means it would take a tremendous financial investment to produce an operational product. Also, the power required during takeoff is much greater than the power needed for cruise, thereby increasing the mass of the engines and reducing both payload and aircraft efficiency (Losi, 1995).
Extremely large lighter-than-air craft are another possibility for improving the productivity and efficiency of commercial aviation. Diesel propulsion becomes a possibility for lighter-than-air craft, but water vapor in the exhaust would need to be captured to control the buoyancy of long-distance transports.
The feasibility of seaplanes is limited; they require more propulsion power than a similarly sized conventional aircraft to take off, and there are relatively few landing sites. Protected harbors with ready access to cargo facilities that could accommodate a seaplane facility generally are already
tied up for other uses. The integrity of takeoff and landing areas is also an issue; floating logs and other debris are a major hazard.
Airframe Research Needs
Additional airframe research and technology development are needed to improve the performance of aircraft, particularly with regard to the feasibility of nontraditional subsonic transport concepts. For example, improved materials, especially composite materials, have the potential for tremendous payoffs. The economic viability of many nontraditional concepts, especially the BWB, would be enhanced by composite materials that weigh less, are more damage tolerant, are easier to fabricate, are more suitable for modular construction techniques, and are compatible with effective means for joining assemblies.
Other areas of particular interest include technology transition issues, safety, and security. To better understand the research requirements related to each of the vehicle classes associated with NASA’s Vehicle System Program, NASA could form a team for each class with members from government, industry, and academia. If such teams are established, a method should be established to ensure that cross-functional performance and design issues, such as reduced wake vortices, reduced drag, improved high-lift performance, and reconfigurable wings, also receive adequate attention.
In some cases, NASA is constrained by the administration or the Congress from supporting aeronautics research related to an aircraft concept of particular interest to an aircraft manufacturer; the concern is that the government may inappropriately subsidize industrial research. The danger is that NASA research may be limited to topics of little or no interest to industry, which brings into question the value of conducting the research.
The committee reviewed a range of potential aircraft propulsion cycles and configurations in an effort to assess how the NASA vision and goals might be addressed in the specified time periods. In this process the committee recognized the importance of evaluating the entire propulsion package in terms of the weight, volume, and costs of the prime movers and the related fuel. The committee also recognized the importance of evaluating these parameters in the context of the overall performance of aircraft and the transportation system—not just as a vehicle subsystem. Figure 4-3 compares the conventional heat engine cycles assuming component efficiencies of 100 percent and maximum hydrocarbon fuel combustor temperatures. As shown, the simple Brayton
cycle, while not providing the absolute maximum cycle efficiency is very competitive on this “ideal” basis. Furthermore, the Brayton cycle operates continuously, whereas the Diesel and Otto cycles operate intermittently, so their net performance is not nearly as good as their ideal performance. As a result, the Brayton cycle proves to be superior to the Diesel and Otto cycles for many applications. In aviation, the Brayton cycle has historically been the cycle of choice when the effects of propulsion system weight, volume, and durability are factored into the entire aircraft and air transportation system.
The efficiency of the Brayton cycle is governed by the maximum compressor exit temperature, which is determined by high temperature material limits. The propulsion taxonomy in Appendix D describes a broad range of propulsion concepts and substantiates the conclusion that the conventional gas turbine engine and its variants based on the Brayton cycle will continue to be the primary aircraft propulsion system of choice at least through 2025. One of the variants entails a departure from the concept of isentropic compression and expansion. Here, two alternatives exist. The first is combustion in the turbine (i.e., interturbine burning). This alternative offers the possibility of reducing the temperature drop across the turbine while increasing the work done by the turbine, thus improving overall engine performance even if interturbine burning is active only during peak power (takeoff and climb). A second alternative is the introduction of volume cooling ahead of or in the compressor by introducing a mist of water or other coolant either ahead of or between compressor stages. This alternative has two benefits. The first is the increase in total pressure owing to the volume cooling, and the second is the increase in mass flow. The former has the effect of increasing the compressor efficiency in that the compressor outlet temperature (T3) is reduced for a given pressure ratio. The latter increases the exit momentum flux, which could also be used to increase takeoff and climb performance. Either improvement could reduce the propulsion system weight fraction and improve aircraft efficiency. These modified Brayton cycles warrant research and could be incorporated into operational systems by 2025.
Turbomachinery-Based Propulsion Systems
Propulsion system performance is directly related to the safety, capacity, mobility, noise, and emissions of individual aircraft and the air transportation system as a whole. Figure 4-4 shows the significant advances that have been made in
gas turbine propulsion systems using thrust-specific fuel consumption as a figure of merit. Although the rate of improvement in this commonly used parameter has decreased, incentives remain great to pursue further technology advances in order to meet customer-driven goals. As an example, NASA has successfully worked with industry to develop and verify analytical tools that address design and system evaluation and reduce the number of experiments needed for a successful engine.
Commercial aviation is highly competitive, so minimizing costs is critical to the survival of individual airlines. The propulsion system is not immune to these pressures: Advanced propulsion technologies will rarely be incorporated in operational products unless they reduce costs or are needed to meet some other requirement, such as more stringent noise or emissions standards.
The propulsion systems of commercial aircraft are only a small contributor to the accident rate as a result of tremendous investments and decades of work to improve the reliability of turbomachinery. It is essential that new propulsion systems and components also demonstrate very high levels of safety.
Propulsion research plans should be structured to meet the needs of advanced airframe concepts in the context of the long-term vision for the air transportation system. Concepts such as BWB aircraft, supersonic business jets, and runway-independent aircraft dictate unique requirements and opportunities for advances in propulsion technology. Areas of interaction include extremes in engine size and fan bypass ratio, design for boundary layer ingestion, highly integrated engine-airframes, power extraction for boundary layer manipulation, variable cycle features, and architectures for integration of system controls.
Emerging Propulsion Concepts and Fuels
In the 2025 to 2050 time frame, low-cost hydrogen could become attractive as an aircraft fuel that would reduce the environmental effects of aviation. The key challenge to the use of hydrogen as an aircraft fuel is its low energy density compared with hydrocarbon fuels—unless new (high-density) means of storing hydrogen are developed. Even though the committee is not aware of any particularly promising approaches for overcoming this problem, the high potential payoff warrants continued research. Widespread use of hydrogen as an aircraft fuel would also require an economically and environmentally benign method for producing hydrogen, a challenge that is being addressed by broader efforts to enable hydrogen to replace hydrocarbon fuels in ground-based vehicles and industrial uses.
Advances in electric power systems may eventually allow them to replace internal combustion engines. In particular, methane or hydrogen fuels for fuel cell power systems in various forms offer a potentially significant improvement in energy conversion efficiency over today’s gas turbines, and ongoing research programs are addressing both mobile and stationary fuel cell applications. Even so, tremendous advances in the power density of fuel cells would be required to make them technologically feasible as a source of propulsion power for large commercial aircraft. Other technology issues associated with the development of an electric aircraft propulsion system (such as the development of lightweight electric motors using, for example, room temperature superconductors) would also need to be resolved to make a fuel cell energy conversion system into a successful aircraft propulsion system. It might also be feasible to use the electricity produced by fuel cells to add heat to the gas in a gas turbine engine in place of combustion. Since electricity, lasers, and electromagnetic devices can provide volumetric heating in place of combustion of hydrogen or hydrocarbon fuel, exploratory research is in order to determine the conditions under which these alternatives may be attractive. Other alternative approaches are given in Appendix D.
The first application of electric power sources on commercial aircraft is likely to be as auxiliary power units rather than for propulsion. Although fuel cells are larger and heavier than conventional auxiliary power units, they generate water. This would reduce the amount of water that must be carried onboard at takeoff, thereby improving the overall assessment of fuel cells as auxiliary power units, from a systems perspective. Unrelated technology developments, however, may produce aircraft toilets that flush with 90 percent less fluid, reducing the onboard demand for water.
Intermittent combustion concepts, such as pulse jets (a.k.a. pulse detonation engines), have the potential for improved performance relative to traditional turbomachinery systems. In some cases, intermittent concepts may also significantly reduce complexity. However, it seems unlikely that systems based on intermittent concepts will outperform gas turbine aircraft propulsion systems in the foreseeable future. Nonetheless, continued basic research would be worthwhile to better understand the long-term limitations and potential benefits of intermittent combustion concepts.
Nuclear power is unsuitable for aircraft applications for many reasons, including the weight of radiation shielding, radiation exposure during normal operations, and the risk of widespread radioactive contamination in the event of an accident. The committee did not identify any other specific propulsion or fuel concepts of particular interest, although research to explore new concepts would be consistent with NASA’s vision and goals.
Propulsion Research Needs
Future airframe and propulsion research will lead to a better understanding of the synergies and tradeoffs that exist among system and subsystem concepts, technologies, design characteristics, and performance parameters, including environmental performance parameters—for example, specific fuel consumption, noise, and specific engine emissions. Cur-
rently available information indicates that propulsion research should generally support the continued evolution and use of high-bypass turbofan engines burning liquid hydrocarbon fuels. At the same time, a portion of the research should anticipate the possibility of (1) an eventual changeover to hydrogen, (2) the use of an advanced gas turbine engine core, and/or (3) the use of fuel cells to generate electric power for electrically driven engines if and when room temperature superconductivity becomes practical. Research in these areas should start at a low level and proceed at a pace consistent with research focused on nonaerospace applications.
The development of environmentally beneficial propulsion technologies that might eventually be applied to aircraft systems should be tracked to understand their potential environmental benefits and tradeoffs (for example, evaluating the potential advantages and disadvantages of using hydrogen fuel, including the potential to use cryogenic hydrogen as a heat sink for electrical components). Support should be provided for research necessary to take advantage of new technologies, including the design of components (such as low-emission combustors compatible with hydrogen fuel or electrically driven propulsion engines compatible with advanced fuel cells) and the development of new system concepts (such as an environmentally acceptable means of releasing water into the atmosphere to mitigate the effect of greatly increased emission of water vapor that would result from the use of hydrogen fuel).
Avionic systems include computers, communications networks, sensors, controls, operational software, and human-computer interfaces. Avionics plays an increasingly critical role in the safe and efficient operation of commercial aircraft and now accounts for up to 40 percent of the capital cost of new aircraft.
Onboard electronics perform or monitor virtually all critical functions in an aircraft, including engines, control surfaces, stability augmentation systems, active flow controls, flight path, collision avoidance, and interactions with the external air traffic control system.
The federal government, especially the Department of Defense, has supported basic long-term research and applied research and technology development that continue to enhance the capabilities of avionics on both civil and military aircraft. The success of this research has been enabled in large part by smaller, more capable computers and more sophisticated software.
Chapter 2 discussed the importance of research in automation and the ability of automated systems to enhance the performance of human operators and the overall system. Advanced on-board avionics will be necessary to implement any new operational concepts that call for increased automation of cockpit and navigation functions.
Federal agencies should continue research aimed at enhancing airborne avionic systems through evolutionary improvements, while pursuing longer range research that could lead to major breakthroughs. For example, advances in nanotechnology may provide major benefits to avionics in computing, sensors, and active distributed control. Research in avionics that relates to air traffic control and automation should be integrated into the overall research strategy for the air transportation system as a whole. Two examples of ongoing research and development of this type are (1) the Alaska Airlines all-weather approach system and (2) work by NASA’s SATS project to enable safe low-visibility operations at minimally equipped landing facilities through the development of new operational concepts, sensors, pilot interfaces, and procedures.
Nanotechnology is an emerging technology with the potential for broad application to many aspects of aircraft design. Nanotechnology deals with materials and devices having at least one dimension on the order of 1 to 100 nanometers. The design of nanoscale materials deals with molecular-scale structures, whose physical and chemical properties are different from materials at larger scales. At nanoscales, no atom is far from a surface. This changes chemical reactivity, coherent scattering, and other processes. Devices also involve large, but countable, numbers of atoms.
Nanotechnology is still in its infancy and is just starting to move into operational applications. In fact, the term nanotechnology is somewhat misleading, implying that research has generally advanced to the stage of developing useful technology, when in many (or most) cases, nanoscale research is still scientific research (and would more accurately be referred to by the less common term nanoscience).
Global investment in nanotechnologies is about $1.5 billion per year, primarily in the United States, Europe, and Asia. The U.S. federal government appropriated $604 million for nanotechnology research and development in fiscal year 2002. The four agencies most heavily involved in nanotechnology research and development are the National Science Foundation ($199 million), the Department of Defense ($180 million), the Department of Energy ($91 million), and NASA ($46 million) (Roco, 2002).
Areas of Interest
The aeronautical community should maintain an awareness of nanotechnology research in other disciplines that might be used in aeronautical applications, such as flow control, lubrication, structures, and manufacturing. A recent study (NRC, 2002c) on the future role of micro- and nanotechnologies in improving Air Force capabilities identified three scientific frontiers that nanotechnology research
should explore: materials, devices, computer processing requirements, and fabrication.
Research into nanotechnology devices for aeronautics applications should investigate bonding of dissimilar materials, material properties, and scaling. Industry would greatly benefit from any technology that improved the ability to bond dissimilar materials. Microelectromechanical systems (MEMS) research is investigating the ability to create strong bonds between (1) silicon and silicon and (2) silicon and other materials, and the committee is hopeful that nanotechnology research might someday lead to material bonding methodologies for critical aviation applications.
Nanotechnology may lead to the development of new structural materials with high strength-to-weight ratios and fracture toughness, durable coatings, greater resistance to corrosion, self-healing, and multifunctional characteristics. For example, structural materials might have embedded sensors and actuators; custom-designed properties, such as electrical conductivity, mechanical strength, magnetic behavior, and optical properties; or improved damping properties. Multimode damping could lead to the elimination of swash plates in helicopter rotors, which would be a major design breakthrough, and greatly reduced fatigue failure in turbofan blade applications. Self-healing materials (e.g., materials embedded with small particles of liquid that would be released and fill in cracks to prevent them from propagating) may allow flying aircraft closer to their fatigue limits, but generally the benefits of self-healing are likely to be greatly exceeded by the benefits of increased strength and reduced weight.
The properties that nanomaterials demonstrate at nanoscales do not necessarily predict the properties of macroscale materials that incorporate nanomaterials. For example, nano-microtubes have heat-transfer rates comparable to that of diamonds, but more research is needed to assess the ability of nanotubes to increase the heat transfer capabilities of structural materials. Also, segments of some nanotechnology fibers are on the order of 30 times stronger than glass fibers. The challenge is to demonstrate strength on a macroscale by combining strong nanoscale segments to form suitable matrix composite materials.
Research into nanotechnology devices for aeronautics applications should investigate distributed sensing, electric propulsion, flow control, fuel controls, MEMS materials, photonics, and security.
Nanotechnology can support distributed sensing: adhesive tape with embedded sensors has been developed that can be used on vehicles during flight tests. In the future, distributed sensors may transition from research applications to operational applications, where they would be used as part of the flight control system.
The feasibility of electric propulsion would be enhanced by the development of (1) a fuel cell catalyst that would not spoil (or become poisoned) as current catalysts do during the operating process or (2) a room-temperature superconductor.
An application area with near-term potential is flow control. Long term, nanotechnology has a role to play in reconfigurable wings. (A NASA flight test recently demonstrated wing warping.) Potential benefits might include the elimination of moving control surfaces, resulting in hingeless wings, the ability to adjust wing camber in flight to reduce drag and improve lift, improved handling qualities and maneuverability, and reduction of noise and vibration.
Propulsion efficiency could be enhanced through the development of better fuel controls (e.g., sensing and calculating devices to measure fuel flow, temperature, and pressure).
Nanoscience is the key to developing materials for MEMS devices, particularly with regard to structural stability, surface durability, fabrication, and packaging (NRC, 2002d). For example, an aircraft skin embedded with MEMS devices might greatly reduce turbulent skin friction.
Optics is used for transmission of data over long distances. The Defense Advanced Research Projects Agency (DARPA), among others, is conducting research to use light in processing on an integrated circuit. Light offers great advantages—and creates large challenges. The photons being used are larger-than-nanoscale, on the order of a half-micron. This is much larger than the transistors they would replace, so photon-based integrated circuits would be larger than current devices.
Carbon nanotubes arranged in sensor configurations might contribute to the development of more capable explosives detectors. (Carbon nanotubes are small graphite cylinders with unusual electrical properties. They can act as metals, semiconductors, or insulators, depending on how they are constructed.)
Computer Processing Requirements
Deployment of distributed nanotechnology sensors will require significant advances in computer processing, so that data from hundreds or thousands of sensors can be processed in real time. Similar challenges would be associated with the use of swarms of small autonomous vehicles based on micro- and nanotechnology. Investments in research and development for algorithms, architectures, and software are necessary to maximize the utility of new hardware (NRC, 2002d).
Developing nanomaterials and devices will require research into the assembly of multifunctional nanostructures.
A recent study recommended that the Air Force monitor and selectively invest in self- and directed fabrication and assembly, particularly with regard to processes related to primarily military components, such as sensors and propulsion (NRC, 2002c). Current abilities in the self-assembly of nanoscale materials are rather crude, suitable for growing crystals, for example. However, biological organisms are all self-assembled systems, so the potential obviously exists.
The potential posed by nanoscience and technology is enormous, but how, when, and the extent to which this potential will be realized are impossible to predict, and the specifics of predictions become more uncertain the farther they are extended into the future (NRC, 2002d). It is especially difficult to determine how the application of nanotechnology may improve top-level characteristics such as overall aircraft performance or the safety of the air transportation system. To date, nanotechnology has been very successful in some devices, but not in devices large enough and robust enough to be directly applicable to commercial aviation. Major advances in the application of nanotechnology are likely to depend upon the ability to integrate nanotechnology fibers and features in intelligent ways to create macroscale materials with specific desired properties. The National Science Foundation recently completed a solicitation for research in this area.
To be successful, nanotechnology research and technology must be sustained over the long term. It will take time and money for research in particular areas to bear fruit. Some practical results in the near term would be helpful in sustaining long-term support for nanotechnology research. The National Nanotechnology Initiative is structured to support projects with near-term, mid-term, and long-term applications. With nanotechnology, as with any new technology, industry funding of research and technology will occur only when supported by a solid business case: “Will it make money—or increase market share?” Ultimately, the success of nanotechnology rests upon the development of successful commercial products.
NASA and the aeronautics community should continue their involvement in interdisciplinary nanotechnology research and development to ensure that advances will be applied to applications of interest to aviation. For example, in June 2002 NASA established seven University Research, Engineering and Technology Institutes, including one for bionanotechnology materials and structures for aerospace vehicles at Princeton University and Texas A&M University. Research by the institutes is intended to increase fundamental understanding and lead to the development of basic technology. The institutes will also support the education of university students and training for working engineers and scientists.
Most nanotechnology research and development in the United States is structured using a bottom-up investment strategy, where individual agencies do research in areas of interest to them. To better integrate research plans at a high level, the NRC has already recommended that the Office of Science and Technology Policy establish an independent standing committee to advise the federal interagency coordinating committee for nanotechnology on research investment policy, strategy, program goals, and management processes. Nanotechnology research and development in the United States would also benefit from the formation of a “crisp, compelling, overarching strategic plan” to articulate short-, medium-, and long-term goals and objectives (NRC, 2002d). Nanotechnology research related to commercial aviation would likely benefit from the implementation of these recommendations.
RECOMMENDATIONS TO IMPROVE AIRCRAFT PERFORMANCE
The Integrated High Performance Turbine Engine Technology (IHPTET) program exemplifies one approach for conducting advanced research on application-ready technology. The IHPTET program was a joint Department of Defense-NASA-industry program whose cost was shared 25 percent-25 percent-50 percent, respectively (with the industry money coming from internal research and development funds, some of which is earned on other government contracts). IHPTET produced useful research results that were transitioned into operational products because systems demonstration at technology readiness level (TRL) 6 was included in the program.3 Each phase of the IHPTET program had high-level goals (e.g., thrust-to-weight ratio) as well as component-level goals (e.g., efficiency and cooling flows). The process was successful because it allowed different groups participating in the program to develop different approaches to high-level goals, and the lower-level goals depended on the accepted systems approach. As a consequence, IHPTET, as a program, did not try to pick winners in a technical sense. The program’s flexibility was a key to its success.
Similarly, a research program to improve the performance of commercial aircraft could be structured with several parallel tracks to cover short-term, medium-term, and long-term goals. Each track would focus on concepts with the potential
to reach maturity by a given date. For example, one track might focus on BWB designs with turbine engines and conventional fuels, with the idea of reaching maturity in 15 years; another track might focus on BWB aircraft with laminar flow and conventional fuels, with a maturity date of 2030; and a third track might focus on hydrogen fuel and advanced propulsion concepts, with a maturity date of 2050. This kind of framework would facilitate coordinated research along a wide range of interesting technologies while producing a steady stream of operationally useful technologies.
Finding 4-1. Advanced Aircraft Technology. Improvements in aircraft performance are critical to achieving necessary improvements in almost every aspect of the overall performance of the air transportation system. Innovative long-range research leading to the implementation of new operational concepts is also required for the air transportation system to take full advantage of gains in the performance of commercial aircraft.
Recommendation 4-1. Aircraft Research and Technology. To improve the performance of aircraft through 2025, federal agencies should continue to support research leading to evolutionary improvements in aircraft performance. Looking out to 2050, however, research should support innovative concepts aimed at major advances in performance. In addition, agencies should continue to monitor research in related emerging technologies, such as nanotechnologies, and support research aimed at aircraft applications as emerging technologies mature. The areas listed below are prime candidates for this kind of long-term research.
Analytical tools, advanced technologies, and the fundamental science behind both, to reduce the need for costly hardware testing and to more easily achieve overall research goals (especially in emerging technologies).
Composite materials with improved qualities that would increase their use in airplane structures and reduce aircraft weight.
Environmental consequences of aircraft noise and emissions locally, regionally, and globally, to better understand those consequences and support the establishment of better informed priorities and goals for noise and emissions reduction that (1) reflect the need for integrated approaches (involving advances in airframes, engines, and operational procedures) to meet environmental goals and (2) accurately account for the tradeoffs among different environmental goals and different approaches to achieving those goals.
Low emissions combustor technology, to (1) reduce substantially emissions of NOx and particulate matter at airports (to improve local and regional air quality) and at cruise altitudes (to reduce global climate effects) and (2) reduce emissions produced by engines with high pressure ratios.
Nanotechnology, to explore its long-range potential for dramatically enhancing aircraft performance through the development of advanced avionics (computing, sensors, and active distributed controls) and high-performance materials.
Nontraditional aircraft configurations, including but not limited to (1) the blended-wing-body and (2) the strut-braced or joined wing, to improve aircraft productivity and efficiency and reduce noise and emissions.
Passive and active control of laminar and turbulent flow on aircraft wings (laminar flow to increase cruise efficiency and turbulent flow to increase lift during takeoff).
Nontraditional power and propulsion concepts and technologies, especially concepts and technologies that support the use of alternative fuels, such as fuel cells (which may have application as auxiliary power units in the foreseeable future) and high-density storage of hydrogen to improve the feasibility of using it as a propulsion fuel.
High-temperature engine materials and advanced turbomachinery, including (1) lower speed, highly loaded, fan drive turbines and fan reduction gears; (2) very large fan systems, which require advances in manufacturing and material systems; (3) boundary layer control on turbomachinery airfoils, to improve component efficiency and packaging; (4) aspirated turbomachinery components, which could greatly reduce noise and improve component efficiency; and (5) other innovative concepts, such as interturbine burning or volume cooling ahead of or in the compressor by means of a mist of water or other coolant.
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Intergovernmental Panel on Climate Change (IPCC). 1999. Aviation and the Global Atmosphere. Cambridge, U.K.: Cambridge University Press.
Losi, P. 1995. The Wingship’s Potential for Strategic Lift. Executive Research Project. The Industrial College of the Armed Forces. Fort McNair, Washington, D.C.: National Defense University. Available online at <www.ndu.edu/library/ic6/95-S12.pdf>.
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NRC. 2000. Uninhabited Air Vehicles: Enabling Science for Military Systems. Washington, D.C.: National Academy Press. Available online at <www.nap.edu/catalog/9878.html>.
NRC. 2001. Commercial Supersonic Technology: The Way Ahead. Washington, D.C.: National Academy Press. Available online at <www.nap.edu/catalog/10283.html>.
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NRC. 2002b. Future Flight: A Review of the Small Aircraft Transportation System Concept. Transportation Research Board. Washington, D.C.: The National Academies Press. Available online at <www.nap.edu/catalog/10319.html>.
NRC. 2002c. Implications of Emerging Micro and Nano Technologies. Washington, D.C.: The National Academies Press. Available online at <www.nap.edu/catalog/10582.html>.
NRC. 2002d. Small Wonders, Endless Frontiers: A Review of the National Nanotechnology Initiative. Washington, D.C.: The National Academies Press. Available online at <www.nap.edu/catalog/10395.html>.
NRC. 2003. An Assessment of NASA’s Aeronautics Technology Programs. Washington, D.C.: The National Academies Press. Available online at <www.national-academies.org/aseb>.
Pfenninger, W. 1987. Long-range LFC [laminar flow control] Transport. Research in Natural Laminar Flow and Laminar-flow Control. J. Hefner and F. Sabo, compilers. NASA CP-2487, Part 1, pp. 89-115.
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