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Commercial Supersonic Technology: The Way Ahead 3 New Opportunities for Research on Critical Supersonic Technologies The development of economically viable and environmentally acceptable commercial supersonic aircraft will require continuing advances in many disciplines. This chapter deals with technology areas where important breakthroughs are possible only if current research efforts are augmented by new, focused research (or significant expansions of existing research). The committee identified five areas of critical importance: approaches to sonic boom reduction, new aerodynamic concepts to improve efficiency and reduce environmental impacts, methods for dealing with highly coupled aircraft dynamics, strategies for the design of complex systems, and the continued development of variable cycle engines. CONFIGURATIONS FOR REDUCED SONIC BOOM Background Shock waves, and thus sonic booms, are fundamental to supersonic flight and can be minimized, but not eliminated, on aircraft having lift. Sonic booms can be startling, cause annoyance, and can even result in structural damage. Current U.S. regulations prohibit commercial aircraft from producing sonic booms that can be detected on the ground. If an approach to vehicle aerodynamic design can be found that would result in sonic booms with intensities low enough to gain public acceptance—and if regulations are changed to allow low intensity sonic booms—the economics of supersonic flight would change dramatically. Studies by NASA’s HSR Program identified three key requirements for overland supersonic flight: (1) establishing the criteria for an acceptable “shaped” sonic boom signature, (2) designing a viable aircraft to produce that shaped signature, and (3) quantifying the influence of the atmosphere on such signatures.1 However, no revolutionary aerodynamic approaches to sonic boom elimination seem imminent, and the feasibility of accomplishing step 2 has yet to be established. Nearly 50 years of flight data and experience with sonic booms exist, covering some 20 different supersonic aircraft, including the Concorde and the space shuttle, with over 1,500 flights having produced some 15,000 measured signatures. All of these sonic boom signatures are sawtoothed (N waves), since all the aircraft were so-called N-wave designs—that is, sonic boom minimization was not considered in their basic design. Current signature prediction codes, validated by numerous wind-tunnel model tests, in-flight flow field probes, and ground measurements, work quite well for these N-wave aircraft.2 Fairly substantial efforts by the government, industry, and universities in the mid-1960s and 1970s and, later, during the HSR Program explored boom minimization techniques that decrease overpressure and shape the signature by means of aircraft tailoring and airstream alteration. Vehicle configurations designed for boomless flight were also investigated. However, no flight data have ever been collected on vehicles designed to produce low-boom, shaped signatures. Wind tunnel models can be designed to produce booms with shaped signatures near the aircraft, but it has not been demonstrated, by analysis or experiment, that a shaped signature will persist to the ground during flight in a real atmosphere. Additional research is needed to establish a credible scientific foundation for designing supersonic aircraft with low sonic booms and to develop improved analytical tools. It is well known that atmospheric turbulence in the lower layers of the atmosphere can produce large changes in the overpressure or intensity of N-wave sonic boom signatures. 1 “Signature” refers to a plot of the change in air pressure versus time at a fixed point as a sonic boom passes. 2 An N-wave signature has two rapid increases in pressure, one at the beginning and one at the end of the sonic boom. This produces a loud and particularly startling double-boom for people in the affected region. Shaping the signature so that it has a slower pressure rise would produce a less intense boom.
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Commercial Supersonic Technology: The Way Ahead Although considerable progress has been made toward establishing a prediction capability to describe the statistical variations of N-wave signatures due to the atmosphere, a suitable prediction code does not exist. No analytical or experimental database exists for quantifying atmospheric effects on low-boom, shaped signatures. Model experiments using a large ballistic range, small projectiles, and a rectangular jet flow nozzle to generate scaled turbulence have provided valuable information. Similar tests using shaped projectiles designed to produce shaped booms could generate most of the required database to determine the influence of the lower layer of atmosphere on shaped boom signatures. An important milestone is scheduled for September 2002: Northrop Grumman, as part of DARPA’s QSP Program, plans to flight test an F5E aircraft modified to produce a shaped signature below the aircraft when flown at Mach 1.4 and 30,000 ft. Experiments using this vehicle will help determine the extent to which analytical tools used to predict the propagation of N-wave signatures can be used for shaped signatures. Most of the data on the response of people and buildings to sonic booms are for N-wave signatures and boom overpressures greater than 1.0 lb/ft2. There is no database on the response of people and buildings to shaped signatures of less than 1.0 lb/ft2. Human response data are also needed for nighttime booms, both to guide the development of new technology and to support necessary changes in federal regulations that prohibit sonic booms over land (see Chapter 2). A significant finding from past sonic boom studies is that startle, rattle, and building vibrations (which can cause damage) are key elements in determining the response of people to sonic booms. Shaped signatures of less than 1.0 lb/ft2 will produce less startle, rattle, and building vibrations. Potential Developments Key requirements for designing a viable supersonic aircraft having a low-boom, shaped signature are low weight, high L/D, long length, and innovative propulsion integration. Focused research on vehicle configurations to produce shaped waves, combined with incremental improvements in existing technologies, may lead to vehicle configurations that produce shaped signatures with a maximum amplitude of less than 1.0 lb/ft2 throughout the total sonic boom ground footprint (not just below the aircraft). Studying the human response to shaped waves will also be necessary, both to assist vehicle design research and to validate new regulatory standards, which may require reducing sonic boom amplitude significantly below 1.0 lb/ft. Research Opportunities Research on methods for sonic boom prediction and techniques for designing aircraft with specified or constrained signatures has been ongoing for decades, but new approaches that permit higher fidelity modeling and more efficient design are required. Research should focus on the continuing development of (1) multidisciplinary design tools and (2) one or more flight technology testbeds to characterize the booms produced by shaped vehicles and measure the persistence of shaped signatures in a real atmosphere over large distances. Laboratory and wind tunnel testing on vehicle shaping and atmospheric effects should also be conducted, including some testing on airstream alteration (e.g., heat/energy addition and dynamic flow modifications) for a variety of vehicle configurations. Finally, community studies are essential to determine how shaped sonic boom signatures with overpressures less than 1.0 lb/ft2 effect buildings and people, including the propensity of people to be roused from sleep by booms at night. ADVANCED AERODYNAMIC CONCEPTS AND CONFIGURATIONS Background The character of flow over a vehicle in supersonic flight is dramatically different from that of a subsonic aircraft. This is one of the fundamental reasons that an economically viable, environmentally acceptable supersonic aircraft has not been achieved after more than a half century of work in aeronautical design. Breakthrough technologies that could address the root causes of the difference will therefore be associated with the vehicle aerodynamics. This section deals with aerodynamic challenges and opportunities for new research that may lead to dramatic improvements in vehicle performance and efficiency. Aerodynamic cruise efficiency is extremely important because it directly and indirectly impacts most of the challenges faced by the development of a viable commercial supersonic aircraft. This is demonstrated clearly in the case of the Concorde, which has a cruise L/D of 7.5 (modern subsonic transports have a cruise L/D of about 18 to 20). The low L/D of the Concorde increases fuel consumption, limits its range, increases the design takeoff weight, and requires a larger propulsion system to provide the higher thrust required at takeoff, which in turn makes it more difficult to meet community noise standards. With engines that are as efficient as those of modern subsonic aircraft, the Concorde would still have to carry about three times the weight and generate more than three times the takeoff thrust of a 737-600, which can carry the same number of passengers the same distance. The low supersonic efficiency of the Concorde, which has a cruise speed of Mach 2, is not a result of poor design but a fundamental consequence of supersonic aerodynamics. For a supersonic aircraft, the maximum achievable L/D depends on several aircraft characteristics and drops as Mach
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Commercial Supersonic Technology: The Way Ahead FIGURE 3-1 Typical variation in L/D with Mach number. SOURCE: Boeing (1989). number is increased. Figure 3-1 shows the typical variation in L/D with Mach number.3 The requirement for low cruise drag generally leads to supersonic vehicles that are much longer than subsonic aircraft with a comparable payload capacity. This, in turn, leads to penalties in the form of degraded vehicle performance at low speed and increased structural weight. As the cruise Mach number is increased, the optimal slenderness of the vehicle increases as well, creating even further disparity between the best cruise design and the best low-speed design. Another critical design conflict arises because the optimum engine bypass ratio decreases as Mach increases. Design compromises result in degraded subsonic performance and lower usable maximum lift during low-speed flight. For example, the Concorde achieves an L/D of only about 5 to 9 in low-speed flight (Rech and Leyman, 1980). As a result, almost 40 percent of the fuel carried at takeoff is devoted to low-speed flight and reserve fuel. Offsetting the difficulties associated with higher Mach numbers is the increased utilization (distance flown per year) that higher speeds make possible. Also, the range achievable in long-range cruise at constant speed is proportional to where V is the cruise speed, C is the specific fuel consumption, and W1/W2 is the ratio of the initial weight (at takeoff) to the final weight (at landing). Compared with more aerodynamically efficient subsonic aircraft, the aerodynamic penalty (in terms of lower L/D) for flying at Mach 2.0 to 2.4 is about 10 percent greater than the penalty at Mach 1.6 to 3 The minimum drag of a supersonic aircraft may be expressed as follows (Jones and Cohen, 1960): (1) where b is the wingspan, is the parasite drag at zero lift (i.e., skin-friction drag and all other drag except induced drag), l is the effective length of the vehicle, M is the Mach number, q is the dynamic pressure, S is the reference wing area, Vol is the total volume, and W is the total weight of the aircraft. If the altitude is chosen to be optimal at the given Mach number, the L/D can be written as follows: (2) where AR, the aspect ratio, is given by b2/S; ARl, the length aspect ratio, is l2/S; and , the zero lift wave drag, is a function of aircraft length, volume, and wing area. Equation 2 is plotted in Figure 3-1, assuming that and have values of 0.01 and 0.006, respectively, and that AR and ARl have values of 2.7 and 10, respectively. (The wave drag terms, , and the term containing ARl are set to 0 below Mach 1.)
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Commercial Supersonic Technology: The Way Ahead 1.8 (see Figure 3-1). As the cruise speed of a supersonic aircraft increases, specific fuel consumption also increases. However, in principle, the parameter V/C actually increases with speed by a factor of about V¼4 over the range of interest here. An increase of this magnitude would largely offset the drop in L/D that occurs at higher speeds. However, this increase has not been demonstrated in operational engines. Furthermore, parametric design studies of supersonic airplanes show that designers have much less freedom to make necessary compromises than with subsonic aircraft. Thus, a net reduction of a few percent in [V/C × L/D] may have a serious cascading effect on the ability to meet other design goals at higher speeds. Regardless of the cruise speed of interest, increases in L/D are vital to enabling an economically viable supersonic aircraft, and meeting L/D goals (see Table 2-1) should remain the focus of aerodynamic research for commercial supersonic aircraft. Most work to date has focused on the higher speeds to increase the benefit in terms of utilization. Based on more recent estimates of environmental constraints, however, a range of 1.4 to 2.0 seems more reasonable and would open the possibility for new benefits from advances in vehicle configuration and aerodynamics. As described in the preceding section, improved vehicle configuration designs are also likely to be a key part of reducing sonic boom to levels that might permit overland supersonic flight. State of the Art Research in supersonic aircraft aerodynamics, which has been ongoing for almost 50 years, has been marked by intermittent efforts—first in the late 1960s and early 1970s when the Concorde was developed and a U.S. SST was worked on, then again in the 1990s as part of NASA’s HSR Program and concurrent work by U.S. industry to develop designs for an HSCT. Currently attention is being given to smaller supersonic aircraft by the DARPA QSP Program, and industry (e.g., Gulfstream and Dassault) is exploring the development of an SBJ (George, 2000). One view of the progress that has been achieved is shown in Figure 3-1, which compares the L/D of a recent Boeing HSCT design with that of the Concorde. Although the estimates for the HSCT are somewhat optimistic, the improvements in cruise performance and particularly in subsonic performance are significant. Although a change in cruise L/ D from 7.5 to 9 represents only a 20 percent improvement in drag at a given weight, supersonic aircraft are more sensitive to such changes than are conventional subsonic aircraft. For a design range of 5,000 NM and with an assumed engine efficiency of 45 percent and an empty weight fraction of 0.25, a 20 percent increase in L/D corresponds to a 40 percent reduction in the required takeoff weight for a 100-passenger aircraft. Several promising concepts, while immature, may become key features of a successful future commercial supersonic aircraft. Related Promising Technologies Supersonic aerodynamics could be revolutionized by successful technologies in any of four areas: supersonic laminar flow, other methods for modifying the flow field around the aircraft, unconventional vehicle configurations, and detailed, computational systems for high-fidelity analysis. Supersonic laminar flow has long been recognized as a potential breakthrough that might reduce skin friction drag by as much as 90 percent. But achieving extensive laminar flow has been an elusive goal. Substantial efforts are being made to achieve laminar flow for subsonic aircraft, but results have not been particularly encouraging. Research on suppressing the transition from laminar to turbulent flow using active flow control (via suction, blowing, or time-dependent boundary-layer manipulation) continues in many laboratories, but the prospect of developing an economically viable system of this sort remains remote. Perhaps more intriguing is the possibility that laminar flow may be more easily maintained at supersonic speeds than at lower speeds. A few related approaches involve the careful design of wing surfaces to achieve favorable streamwise pressure gradients and minimize cross-flow transition. These approaches range from those described by Tracy et al. (1995), in which wings with low leading-edge sweep and favorable chordwise pressure gradients are integrated into the aircraft concept, to recent work at the National Aerospace Laboratory of Japan, which emphasizes more highly swept wings with low upper-surface cross-flow achieved with rather flat streamwise pressures (Yoshida et al., 2000). The latter concept, while more sensitive to disturbances and aimed at achieving laminar flow over 25 percent of the wing surface, permits the use of substantial wing sweep. The more mildly swept natural laminar flow concept has demonstrated much larger extents of laminar flow in recent flight tests but may incur structural penalties because of the need for very thin wings. In addition, the short lifting length of the reference concept is difficult to reconcile with the requirement for shaped sonic boom signatures. Research on each of these concepts is in its infancy but, if successful, may have a dramatic effect on achievable supersonic aircraft performance. It appears feasible to extend these ideas with additional measures for cross-flow suppression. Active cooling or passive techniques for suppressing the initial cross-flow instability (White and Saric, 2000) may permit additional sweep on the supersonic leading-edge natural laminar flow concept or more extensive laminarity for the subsonic concept. The second area of general interest for dramatic improvements in supersonic aerodynamics involves much more speculative approaches to the modification of the flow field. Active flow control, virtual shaping, and energy addition in various forms have been proposed for many years as a possible means for reducing wave drag or sonic boom amplitude. The committee does not believe that any of these approaches promise near-term breakthroughs in supersonic
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Commercial Supersonic Technology: The Way Ahead performance or boom reduction. Indeed, some of them appear to have no reasonable physical basis, while others are either too complex to evaluate at this time or feature concepts whose practical implementation is hard to imagine (see, for example, Rethorst and Kantner, 1996; Rising and Vadyak, 1999; and Soloviev et al., 1999). Future research programs should consider investigating such concepts, with the understanding that success, while unlikely, would be important. A third alternative is to investigate unconventional designs that attempt to address some of the fundamental problems encountered with supersonic aerodynamics. These range from nonplanar and multiple-surface configurations to asymmetric, oblique wings. Several of these ideas are based on fundamentally sound aerodynamics, but integrating them into practical aircraft designs has been difficult. In some cases, this is due to a basic limitation of the concept. For example, the oblique all-wing concept that accommodates passengers inside the wing structure appears to offer spectacular aerodynamic performance and great potential for reducing sonic boom, but it is difficult to configure as a passenger aircraft unless it is scaled up to accommodate 500 passengers (Jones, 1991; Seebass, 1994). In other cases, the complexity of the configuration may limit the applicability of simple analyses, and the associated risk and large amount of work required to develop appropriate analysis methodologies cannot be accommodated within the time and resource constraints of ongoing supersonic research programs. Immature vehicle configurations also have a hard time competing against vehicle configurations that have long histories of wind tunnel testing and computational design analyses. The solution to this dilemma may lie in the development of analytical methods that permit higher-fidelity analysis of new concepts early in the design cycle. High-fidelity analysis of unconventional vehicle configurations is just now becoming feasible and represents a true opportunity for breakthrough technology. Advances in computational algorithms for aerodynamic analysis and shape optimization, together with a revolution in computer hardware capabilities, now make it possible to consider a much wider range of design possibilities at a level of detail formerly restricted to a single baseline design. More mature flow solvers, improved representations of boundary layer turbulence, and methods for efficient calculation of flow field sensitivities to design changes make the evaluation of alternative design concepts feasible. Coupled with advances in techniques for multidisciplinary optimization, such capabilities hold out the promise that unconventional concepts can be transformed into practical breakthrough technologies. As an example, consider the concept of natural laminar flow. Until recently, the ability to predict transition of a three-dimensional boundary layer and use this prediction to design a wing with extensive laminar flow was a remote possibility. Indeed, tests on an F-104 in the late 1950s showed that limited laminar flow could be achieved, but tools were not available to analyze the results, let alone use them to design a wing. More recently, a specified wing design was analyzed to assess its potential for extensive natural laminar flow (Agrawal and Powell, 1991). The conclusion was that despite the small sweep, little laminar flow would occur. Current computations including nonlinear computational fluid dynamics, three-dimensional boundary layer analysis, and stability calculations have made it possible to successfully optimize a wing for extensive laminar flow. Combining this capability with structural analysis and more comprehensive aircraft performance calculations would greatly advance the prospects for using natural laminar flow to significantly improve the performance of a commercial supersonic aircraft. Basic and Applied Aeronautics Research Future research programs that would support the development of the technologies described above should include the following: techniques to predict and control the transition from laminar to turbulent flow, including supersonic natural laminar flow, passive control techniques, cooling, periodic roughness, and active control methods improved, design-oriented computational fluid dynamics for improved multipoint performance and sonic boom reduction, including adaptive, unstructured methods that achieve the efficiency of current multigrid structured methods and low dissipation methods that can be used in combination with boom propagation codes multidisciplinary design methods compatible with high-fidelity modeling, as well as optimization, decomposition, and methods that exploit parallel computing architectures flight R&T demonstrations to help investigate technologies of interest that remain difficult to validate computationally, or even in existing wind tunnel facilities (NRC, 1994) Flight experiments will play an important role in the development of new concepts, and methods for more efficient flight tests, including the development of sensors for flow diagnostics, will be especially important. New supersonic wind tunnel capability may also be needed. VEHICLE DYNAMICS AND CONTROL Problem Description To achieve the required aerodynamic performance, next-generation supersonic transport aircraft will likely exhibit greater aerodynamic instabilities than any other existing or planned transport aircraft. These instabilities must be stabilized using high-authority, flight-critical feedback control
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Commercial Supersonic Technology: The Way Ahead systems. Furthermore, owing to their size and shape, large commercial supersonic aircraft will also exhibit unusually low structural-vibration modal frequencies. For example, the 1- to 1.5-Hz first-fuselage modes predicted for the baseline HSCT designs considered by the HSR Program would be significantly lower than those for any manned aircraft, commercial or military (NRC, 1997). Either aerodynamic instability or structural flexibility considered separately would present significant technical challenges. Taken together, they create severe frequency coalescence (between the rigid-body and structural modes) that must be carefully examined before developing the flight-and structural-mode control systems. The combination of aerodynamic instability and structural flexibility also creates two forms of multidisciplinary feedback phenomena involving the elastic airframe, flight-control systems, and either the pilot or the aircraft’s propulsion systems. Hence, both phenomena are encompassed by the acronym APSE, which can refer to either aircraft-pilot servo-elastic or aero-propulsive servo-elastic phenomena. Although both APSE phenomena fundamentally depend on structural deformations, APSE is not an aeroelastic problem per se and it cannot be solved by methods used to counter classical aeroelastic flutter. The aero-propulsive servo-elastic phenomenon was discussed in some detail in a previous NRC report that reviewed NASA’s HSR program (NRC, 1997). That report concluded that this phenomenon was “completely outside industry’s experience base,” and the HSR program had not found a solution to the problems created by the phenomenon. Regarding the aircraft-pilot servo-elastic phenomenon, the excitation of the elastic modes of aircraft with structural modal frequencies below 2 Hz (which is a natural consequence of turbulence and pilot control inputs) will be orders of magnitudes greater than that encountered in other transport aircraft. If left unmitigated, these modal excitations would create unacceptable handling quality and ride quality. In particular, pilot excitation of low-frequency structural modes, coupled with the pilot’s biodynamic response to these excitations, has been recently demonstrated to lead to aircraft-pilot coupling instabilities (see Box 3-1) (Raney et al., 2001). Configuration Design Implications Both forms of APSE phenomena are exacerbated by vehicle configurations with long and slender fuselages and thin or highly swept wings, by lightweight (hence low stiffness) structural design, and by increased aerodynamic instability, all of which are key factors in achieving high-performance, low-boom commercial supersonic aircraft. Thus, even though APSE effects are likely to be a major factor in defining the vehicle configuration for next-generation commercial supersonic aircraft, their importance in this regard is not generally recognized and they are rarely incorporated into the early stages of aircraft design. Consequently, research into APSE phenomena and their causes, along with new analysis and synthesis tools, is required. These tools include new active-control concepts, new control-system synthesis techniques, and aeroelastic modeling approaches that may be used in the vehicle configuration-design phase and integrated into multidisciplinary optimization techniques. Additional Research Required Low-frequency structural vibration modes will require active structural mode control systems that are highly integrated with the primary flight-control systems. Success in this effort will be particularly beneficial because solving the control problems associated with low-frequency structural vibration modes is one key to resolving both forms of APSE phenomena. New techniques must be developed and validated for designing affordable, certifiable, highly integrated, high-authority flight- and structural-mode control systems. Research in handling qualities is necessary to develop design criteria for aircraft control systems. Additionally, novel sensors and actuation devices, along with novel distributed control approaches, must be considered.4,5 New structural design approaches are also required, and new tools and technology must be developed and validated for designing affordable, advanced structural systems that can be certificated for use on commercial aircraft. Options include (1) multidisciplinary configuration optimization techniques that capture both types of APSE phenomena, (2) active or smart structures, and (3) viscoelectric or electrorheological materials.4,5 Finally, current practice in the industry creates functionally separate flight controls engineering organizations and structural dynamics engineering organizations. This is not an appropriate organizational structure for handling the issues associated with these APSE phenomena. As previously recommended by the NRC in an assessment of the HSR Program, interdisciplinary teams should be formed to fully address relevant aspects of the APSE problem, and the organizational distance between groups responsible for (1) guidance and control systems and (2) structural-mode control laws should be reduced or eliminated (NRC, 1997). Experimental programs should be initiated as a first step in establishing better handling- and ride-quality requirements for highly unstable and highly flexible aircraft and associated flight control and structural-mode control systems. Real-time, manned simulations of the dynamics of these vehicles, however, would severely tax both fixed-base and inflight simulation facilities, limiting the ability to establish 4 Anna-Maria McGowan, NASA Langley Research Center, personal communication with David Schmidt, 2001. 5 Terrance Weisshaar, Purdue University, personal communication with David Schmidt, 2001.
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Commercial Supersonic Technology: The Way Ahead BOX 3-1 Demonstration of Aircraft-Pilot Coupling Instability in a Ground-Based Simulation The upper graph shows the time histories of lateral cockpit acceleration and pilot lateral stick input for a moving-base piloted simulation of a demanding landing-approach task. These time histories reveal an undamped oscillatory instability beginning at about 19 normalized time units. The cause of this instability is revealed in the power spectra of these two time histories, shown in the three lower graphs, where curves labeled “1” show lateral stick power spectral density and curves labeled “2” show lateral acceleration power spectral density. In the first power-spectra plot, on the left, calculated early in the time history, there is little correlation between the peak-power frequency of the stick input and the lateral accelerations. But in the power-spectra plot on the right, which is calculated from the traces in which the instability is evident, the peak power of the stick input shows a strong correlation with the peak power of the lateral acceleration. This indicates a feedback process is present between the cockpit accelerations and the stick input. That is, the accelerations are driving the stick inputs, through the dynamic responses of the pilot’s body to the stick, and these stick inputs are in turn driving the accelerations. These data further indicate that the instability is inadvertent—the pilot could not avoid the instability despite—or because of—stick inputs intended to maintain stable flight. SOURCE: Raney et al. (2001) handling- and ride-quality requirements and to design and verify the performance of control systems. The ability of national simulation facilities to deliver high-fidelity manned simulations of highly flexible aircraft may need to be upgraded. HIGH-FIDELITY INTEGRATED DESIGN TOOLS Highly integrated designs are required for virtually all new aerospace systems. For aircraft, the importance of integration increases with flight speed. Supersonic aircraft are much more sensitive to how components and disciplines are
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Commercial Supersonic Technology: The Way Ahead combined than subsonic aircraft, particularly with regard to APSE effects and sonic boom, as already discussed. Stringent requirements for component performance (with attendant development, manufacturing, cost, and operational issues), coupled with the economic and environmental challenges faced by commercial supersonic aircraft, leave little room for inefficiencies in the design of the airframe, engine, flight controls system, or other performance-critical systems. Design integration tools should allow design teams to interact in the design of complex systems where technical and other factors (including cost) can be appropriately traded; to compress the design cycle time by concurrently considering all critical constraints and disciplines; to adapt quickly to changes in design and manufacturing processes; to easily accept new and improved tools; and to provide databases with levels of complexity appropriate to each task. Fortunately, a substantial national investment has been made in tools for integrated design, including system engineering methods, multidisciplinary optimization methods, detailed discipline methods and interfaces, and design-integration frameworks. The aircraft design and manufacturing industry is heavily committed to improving such tools. Universities and the government also have critical roles in advancing the state of the art in many of these areas. NASA and the Department of Defense have both made significant investments in the development of advanced integration environments and tools, although NASA’s flagship program in this area, the Integrated Synthesis Environments Initiative, was recently cancelled. Despite the progress that has been made, important work remains to be done. Existing tools cannot model some key technologies (e.g., active controls), nor do they have sufficient model validity in all important disciplinary areas. At the broad technical scale, it is extremely important to begin with a full understanding of the design objectives and constraints, such as payload, range, takeoff gross weight (TOGW), noise, sonic boom, and cost, and to identify all the critical disciplines. This will prevent suboptimization and the debilitating effects of discovering, too late, that a first-order design driver (such as APSE in the HSR Program) has not been fully appreciated or adequately addressed. Many existing design schemes do not fit well with integration/ optimization algorithms, and user-friendly frameworks that accommodate such schemes are not available. Off-the-shelf software and interface mindsets are needed. Faster mechanisms for geometric modeling are required for improved efficiency at both the conceptual and detailed design levels. For design teams that might be geographically dispersed, mechanisms for sharing the geometric models are lacking. Indeed, the management and sharing of information and data are themselves first-order issues, as is reducing the time for each design cycle. Very large quantities of data must be transferred; presently, both modeling and data management are much too labor-intensive. Analytical design tools, such as computational fluid dynamics and finite element modeling, have been greatly improved, but they often take so long to run that they are impractical in an iterative design context, and they are not robust enough to be fully integrated into a design framework. Other important issues for the development of advanced integrated tools are associated with uncertainties: how they propagate through a design and how to develop calibration and validation processes that quantify them. Too often, even after an optimization calculation has been made, new designs can be evaluated only by comparing them against a previous baseline rather than by making an absolute measurement of expected performance and comparing it against a validation metric. Major benefits could be realized from the development and effective use of advanced design and integration tools, but there are significant barriers to achieving these benefits. On the nontechnical front, NASA could help by creating a new culture of collaboration, which is required for the most effective utilization of university, government, and industry talent in the realm of integration tools. NASA also has the charter and opportunity to provide much of the technology that is needed to enable viable supersonic aircraft designs. First, it will be vital to develop advanced, high-fidelity methodologies and tools for intradiscipline analysis in areas such as computational fluid dynamics and finite element modeling for structures. Interdisciplinary and multidisciplinary tools are also crucial for integrated design of complex systems and entire vehicles. Particular attention should be given to (1) integrating the design of mechanical systems with the design of electrical systems and software development and (2) factors such as computational speed and robustness that will increase the utility of new tools in an integrated design context. Speed should come naturally with advances in computer hardware capability, but algorithms must be tailored to take advantage of the massively parallel computing environment. Second, to realize the potential for design improvements through advanced tools, substantial new efforts must be focused on the automation of integration and validation. Today, design processes can require weeks to a couple of months to set up and compute the aerodynamic, weight, stability and control, aeroelastic, and other performance characteristics resulting from a configuration change. Optimizing in any context takes a number of such cycles. While multidisciplinary optimization techniques can reduce the optimization time dramatically, the setup time for the basic configuration is still counted in weeks and the validation of designs resulting from multidisciplinary optimization techniques, at least in the usual context of experimental verification, is extremely difficult because of the highly integrated nature of the process (it typically involves the use of sophisticated analyses to check design calculations). There is, as a result, a great need for focused research on how to validate highly integrated design capabilities. Clever combinations of analytical, computational, and experimental approaches
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Commercial Supersonic Technology: The Way Ahead may be necessary. Where experimental approaches are needed, it is very likely that current ground test capabilities are inadequate. In any case, only after successful application of these newly developed validation methods will confidence be high enough to encourage the widespread use of the new tools. Third, design and integration frameworks should be developed to allow teams of analysts and designers at different locations to come together and immerse themselves in a user-friendly design environment. Such frameworks must be able to achieve a wide range of design and integration objectives and accommodate discipline-specific analyses at varying levels of breadth and depth. Having a common framework to carry out the conceptual, preliminary, and detailed design phases is important, and approaches ranging from approximate to high-fidelity must be accommodated. Framework technologies should also facilitate the exchange and management of large databases. Design team members must have access to the same data, new data must be readily transferable and modifiable, and the entire process must take place in a secure environment with user-friendly methods for preventing unauthorized alteration of or access to information. This requires emphasis on system-user interfaces and data-management tools. Frameworks should also allow the inclusion of manufacturing and operations parameters, which are important to affordability. NASA should work closely with engine and airframe manufacturers and other industries, government agencies, and universities engaged in the development of integrated design tools and methods to define the specific characteristics required of advanced technology collaboration infrastructures and to develop a comprehensive plan for meeting those needs. An expanded discussion of the above issues and a comprehensive set of recommendations for action is contained in Design in the New Millennium (NRC, 2000). VARIABLE CYCLE ENGINES A variable cycle engine would, in theory, allow the propulsion system to be optimized for different flight conditions (e.g., takeoff, supersonic cruise, subsonic cruise, and landing). A variable cycle engine is similar to a conventional mixed-flow turbofan, except that it has an additional secondary outer bypass duct to increase the overall bypass ratio and, thus, the air flow handling capability. The second bypass stream improves TSFC and improves fan surge control by allowing the fan to pass a maximum amount of air throughout a broader flight regime. Unlike conventional turbofans, a variable cycle engine varies the bypass ratio to optimize performance for different flight conditions. Reducing the bypass ratio during cruise improves fuel efficiency, whereas increasing it during takeoff and landing reduces community noise. Ejector nozzles would still be needed to mix ambient air with the jet exhaust to reduce noise enough to meet community noise standards. With a variable cycle engine, however, the nozzle can be smaller, which reduces aircraft weight and improves the economic viability of the design. One of the leading design approaches for a variable cycle engine has a core-driven fan stage directly in front of the high-pressure compressor to supercharge both the core and inner bypass flow streams. The term “variable cycle engine” has come to apply narrowly to this type of engine. However, other engine cycles can also adjust their bypass ratio during operations and thus fall within the general class of “variable cycle engines.” These include the fan-on blade (“Flade”) cycle and the turbine bypass engine with an inlet flow valve (TBE/IFV) cycle, both of which were investigated by NASA’s HSR Program (NASA, 2001). Initial studies at NASA indicated that the additional design complexity of variable cycle engines outweighed the benefits (Berton, 1992). Research has continued, but most of it has been proprietary. Continued development of engines with advanced cycles, such as the variable cycle engine, that are compatible with high cruise efficiency, low community noise, and small, lightweight nozzles could lead to important breakthroughs in the realization of commercial supersonic aircraft. REFERENCES Agrawal, S., and A.G. Powell. 1991. “Supersonic Boundary-Layer Stability Analysis of an Aircraft Wing.” Journal of Aircraft 28 (November): 721-727. Berton, J., W. Haller, P. Senick, S. Jones, and J. Seidel. 1992. Comparative Propulsion System Analysis for the High Speed Civil Transport. Cleveland, Ohio: National Aeronautics and Space Administration. Boeing Commercial Airplane Group. 1989. High Speed Civil Transport Study, Summary, NASA Contractor Report 4234, September. George, F. 2000. “The Supersonic Business Jet,” Business and Commercial Aviation (July): 44-48. Jones, R., and D. Cohen. 1960. High Speed Wing Theory. Princeton, N.J.: Princeton University Press. Jones, R., 1991. “The Flying Wing Supersonic Transport.” Aeronautical Journal, March. NASA (National Aeronautics and Space Administration). 2001. Available online at <http://www.lerc.nasa.gov/WWW/HSR/PSI.html>. NRC (National Research Council), Aeronautics and Space Engineering Board. 1994. Aeronautical Facilities: Assessing the National Plan for Aeronautical Ground Test Facilities. Available online at <http://www.nap.edu/catalog/9088.html>. Accessed on September 13, 2001. 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