7
AERODYNAMICS

INTRODUCTION

Future progress in aeronautics will be based on the coupling of advanced tools with new understandings of fluid mechanics and interactions between the various aeronautical disciplines. New interdisciplinary computational tools and new experimental capabilities will play increasingly important roles in aeronautical technology progress. These new methods will profoundly affect the cost and speed of aircraft design processes as well as the efficiency and utility of future aircraft. This vision for the future of U.S. aeronautical technology can be realized only through investment in computational and experimental infrastructure and in the development and precompetitive validation of selected technologies. For the United States to compete more effectively in technology development in the future, science and engineering efforts that support industry must be reinvigorated. This reinvigoration will require an increased level of investment as well as careful assessment of investment strategy. A strong need exists to strengthen the weakest link in the technology development chain: namely, technology validation for risk minimization.

In recent years, the National Aeronautics and Space Administration (NASA) has recognized the importance of technology development needs for high-speed civil transports (HSCTs), advanced subsonic transports, and hypersonic vehicles of all types, as well as the need to maintain its facility capabilities through the Wind Tunnel Revitalization Program. In each of these areas, however, overall constraints have limited NASA's ability to establish the kind of aggressive research efforts needed to maintain U.S. competitiveness in aeronautics. This chapter charts a course for NASA, industry, and academia to pursue toward the specific aerodynamics goals needed to achieve competitiveness. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in order of priority, and the benefits that can be gained through aerodynamics research and development.

Although discussion of individual technical disciplines is facilitated by categorization, progress in technical fields is enhanced by an interdisciplinary approach rather than by the more traditional sequential application of various technical skills. In the past, the disciplines of



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Aeronautical Technologies for the Twenty-First Century 7 AERODYNAMICS INTRODUCTION Future progress in aeronautics will be based on the coupling of advanced tools with new understandings of fluid mechanics and interactions between the various aeronautical disciplines. New interdisciplinary computational tools and new experimental capabilities will play increasingly important roles in aeronautical technology progress. These new methods will profoundly affect the cost and speed of aircraft design processes as well as the efficiency and utility of future aircraft. This vision for the future of U.S. aeronautical technology can be realized only through investment in computational and experimental infrastructure and in the development and precompetitive validation of selected technologies. For the United States to compete more effectively in technology development in the future, science and engineering efforts that support industry must be reinvigorated. This reinvigoration will require an increased level of investment as well as careful assessment of investment strategy. A strong need exists to strengthen the weakest link in the technology development chain: namely, technology validation for risk minimization. In recent years, the National Aeronautics and Space Administration (NASA) has recognized the importance of technology development needs for high-speed civil transports (HSCTs), advanced subsonic transports, and hypersonic vehicles of all types, as well as the need to maintain its facility capabilities through the Wind Tunnel Revitalization Program. In each of these areas, however, overall constraints have limited NASA's ability to establish the kind of aggressive research efforts needed to maintain U.S. competitiveness in aeronautics. This chapter charts a course for NASA, industry, and academia to pursue toward the specific aerodynamics goals needed to achieve competitiveness. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in order of priority, and the benefits that can be gained through aerodynamics research and development. Although discussion of individual technical disciplines is facilitated by categorization, progress in technical fields is enhanced by an interdisciplinary approach rather than by the more traditional sequential application of various technical skills. In the past, the disciplines of

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Aeronautical Technologies for the Twenty-First Century Recommendations General NASA must continue to provide the necessary resources for aerodynamics research and validation, including resources focused on specific key technologies, resources to maintain and enhance ground and flight test facilities, and resources for enhanced analytical and design capabilities. Specific The following research topics should serve as the focus of NASA's research effort in aerodynamics: aerodynamic cruise performance, including subsonic and supersonic laminar flow control technology; aircraft propulsion/airframe integration for both subsonic and supersonic aircraft; low speed and high lift for subsonic configurations, including wake mechanics, wake vortex, and measurement technology; computational fluid dynamics for aircraft design, including validation of codes; low speed and high lift for supersonic configurations; and aerodynamics of rotorcraft. The following should be the focus of a program to enhance NASA's ground-based experimental facilities: revitalization of existing facilities on an expedited basis; establishment of an intensive program to develop high-resolution nonintrusive instrumentation; fitting of the 40' × 80' tunnel at the Ames Research Center with acoustic lining; development of a low-speed (Mach 0.1–0.5), low-disturbance test capability to operate at chord Reynolds numbers in excess of 50 million; and research to examine the feasibility of a supersonic (Mach 2–6) low-disturbance test capability to operate at full-chord Reynolds numbers of 400 million to 500 million. The following should be the focus of a program to enhance NASA's experimental flight facilities: revitalization of flight research capability and in-flight technology validation efforts in all speed regimes; and development of advanced measurement technologies for flight research. aerodynamics, structures, and controls were largely exercised sequentially in the design process. Propulsion system analysis proceeded independently and was integrated later into the design. Such a process requires several cycles of iteration to converge on a suitable design, and

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Aeronautical Technologies for the Twenty-First Century Benefits of Research and Technology Development in Aerodynamics Aircraft Operations Reduced fuel consumption Decreased cruise drag Increased climb and descent lift-to-drag ratios Reduced takeoff and landing noise Aircraft Design and Development Shortened development cycle Improved computational capabilities Improved testing facilities Technology validation consumed considerable time and money. Also, not only does this procedure not guarantee an optimum configuration satisfying multiple design constraints, it almost precludes such a configuration because the efforts devoted to the initial steps in the design process become progressively harder to change. In some cases, cost and time constraints preclude more than one or two iterations. The increasing maturity of computational approaches in the various design disciplines provides new opportunities to couple the disciplines more tightly earlier in the design process. Routinely treating aerodynamics, structures, propulsion, and controls virtually simultaneously and continuously throughout each step of design has very large payoffs that fall into several categories. First of all, the resulting design is truly optimized and is, therefore, superior to those derived through the older, sequential approach. A second major payoff is a significant reduction in the time required to evolve a final design. Attainment of this goal is critical to the success of the transport aircraft industry. NASA has recognized the potential advantages of multidisciplinary analyses and plans to focus considerable attention on this in the Computational Aerosciences portion of the national High Performance Computing and Communications program, which was initiated in fiscal year 1991. The goal for the 1990s is to develop the capability to computationally couple aerodynamics, structural response, propulsion system effects, and active controls into a single computation and to structure the resulting codes to take advantage of massively parallel computational system architectures that are expected to be in widespread use after the mid-1990s. These NASA efforts are highly endorsed by the Committee and complement similar industry activities that will include manufacturing considerations in the trade-off decision-making process. The impact of rapid design processes that allow time for examining the trade-offs between various disciplines was evident in the design of the folding wing tip of the Boeing 777.

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Aeronautical Technologies for the Twenty-First Century Three configuration options were examined: an external hinge, an internal hinge retaining the original wing surface contours, and an internal hinge with a local thickening of the wing. An early wind tunnel test provided some baseline aerodynamic information, but the subsequent design iterations and final design decisions were made rapidly by using computational fluid dynamics (CFD). A final verification wind tunnel test could be performed only after the design was frozen. This use of computational aids is being extended into multidisciplinary analyses coupling aeroelastic effects with structural analyses. The use of CFD for preliminary aerodynamic load predictions early in an airplane development program will significantly shorten the development cycle. Use of CFD to permit flutter prediction early in the design process will reduce reliance on traditional after-the-fact remedies such as mass balancing. Early in this study the Committee decided not to address the hypersonic flight regime and to set the upper limit of vehicle speeds to encompass the HSCT. This section of the report, therefore, does not address the aerodynamics of hypersonic flight. Before moving to the areas considered, however, some comments on the state of hypersonic research in the United States are appropriate. Current U.S. capabilities in hypersonic research have their roots in research efforts that were concluded more than 20 years ago with the culmination of the X-15 program. Today, the nation's hypersonic capability is largely applied to the National Aerospace Plane (NASP) program, which is focused on the specific objective of the technologies necessary to design and build the X-30 aerospace plane capable of single stage to orbit. As a consequence, few resources are available for generic hypersonic research that does not support NASP. The key aerodynamics technologies have been divided into the following categories, each of which is discussed in corresponding sections of this chapter: Low speed and high lift for subsonic configurations: This portion of the chapter examines the approach and takeoff flight phases for subsonic aircraft, encompassing high lift system performance in detail and overall takeoff and landing performance in general. Noise is included because this is the flight phase in which it is most troublesome. Subsonic aircraft propulsion/airframe integration: Aerodynamic interaction effects between a subsonic airframe and its propulsion system has significant effects on the economics of aircraft cruise performance. In addition, the integration must also recognize the need for low noise emission during takeoff and landing. Aerodynamic cruise performance: Efforts to maximize lift-to-drag ratio (L/D) and minimize cruise drag are critical to the economic success of most commercial aircraft. Lower drag relates directly to lower fuel costs and higher profits for the air carrier. Low speed and high lift for supersonic configurations: Supersonic aircraft shapes are strongly influenced by the need for economical supersonic cruise performance. Virtually all the design decisions to improve performance in this

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Aeronautical Technologies for the Twenty-First Century flight phase have deleterious effects on the approach and takeoff flight performance. The requirements for improving the technology base for the design of supersonic aircraft are discussed in this section. Supersonic aircraft propulsion/airframe integration: The integration of the propulsion system with the airframe is a very important factor for all categories of aircraft. For supersonic designs in particular, aerodynamic interaction between the airframe and the propulsion system is a critical task, made more difficult by the need to minimize takeoff engine noise. Aerodynamics of rotorcraft: Aerodynamics of rotorcraft and tiltwing aircraft are especially complex, because they require investigations of exceptionally wide speed ranges and varying angles of airflow. Test facilities: The numerous new or updated flight and ground test facilities that will be required to accomplish various technical goals set forth in the report are discussed in this section. The use of computational tools and associated computers that can predict complex flow fields around aircraft in all speed ranges is also of growing importance. Specific applications of CFD are discussed in detail in the preceding categories, but this section of the chapter contains a more general summary. A generally recognized and acceptable measure of aerodynamic efficiency is the lift-to-drag ratio. Table 7-1, prepared by the McDonnell Douglas Aircraft Company, lists subsonic L/D improvements achievable for the time period of this study if the recommendations in this chapter are implemented. TABLE 7-1 Potential L/D Improvements Application Potential L/D Increase Aspect ratio increase (11–17.5%) 15% Laminar flow control (upper wing/tail surface) 10–12% Airfoil development 2–3% Turbulence control (fuselage/lower wing) 2–3% Induced drag 3–4% Total ~35%   Source: McDonnell Douglas Aircraft Company

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Aeronautical Technologies for the Twenty-First Century LOW SPEED AND HIGH LIFT FOR SUBSONIC CONFIGURATIONS The efficiency and effectiveness of the low-speed, high-lift systems employed by subsonic jet transports for takeoff and landing have a major impact on overall economic performance. The need is to develop design technology that will produce high-lift systems that provide high L/D in climb-out, high lift coefficient during landing approach, acceptable performance in icing conditions, and acceptable levels of airframe noise, and that are also amendable to low-cost manufacture and maintenance. The payoff from continued improvements in high-lift systems is large. For an airplane the size of the forthcoming Boeing 777, a small increase in lift coefficient, 0.10, at a fixed angle of attack of 8.0 degrees, allows a 14.0-inch reduction in landing gear length and a 1,400-pound reduction in empty weight. A 0.035 (approximately 1 percent) improvement in maximum lift coefficient equates to a 1-knot change in approach speed, a 6,600-pound increase in payload (at constant approach speed), or a 60-foot reduction in landing distance. Simplifying the geometry of the flap system while maintaining aerodynamic performance yields large benefits in terms of cost and maintainability. One of the stated goals for subsonic transport design is the reduction of acquisition and maintenance costs by 25 percent relative to current production airplanes. Future advances in high-lift system technology can contribute significantly to the achievement of this goal. Because of the complex physics and complex geometries associated with low-speed flight, CFD has not had a major impact on the technology of high-lift system design. The primary design tool has remained the wind tunnel, with some design guidance from CFD. Work over the past decade has verified the strong influence of Reynolds number on aerodynamic performance of high-lift systems—aerodynamic performance of high-lift systems in many cases does not scale predictably with Reynolds number. Figure 7-1 shows the experimentally measured maximum lift coefficient of a simple swept wing over a range of Reynolds numbers, which displays large and unpredictable variations in maximum lift with Reynolds number. Such experiments demonstrate that the best aerodynamic performance and lowest risk can be achieved only by carrying out the aerodynamic design and validation at flight Reynolds numbers. Because of this, airframe companies do their development work in high Reynolds number wind tunnels—which, for the design of the new Boeing 777, meant extensive development work in European wind tunnels. The following developments in the technology for advanced high-lift system designs are needed: Improved understanding and measurement of the detailed flow physics at wind tunnel and flight Reynolds numbers: A better understanding is needed of turbulence and its modeling; of boundary layer transition, laminar bubbles, turbulent reattachment and relaminarization phenomena; and of merging boundary layers and wakes, including the detailed behavior of the viscous layers and wakes in the high adverse pressure gradient region above the training edge flaps. Some progress is being made in the United States in this area of research. In contrast, the Europeans have greatly exceeded U.S. technology in high-lift systems,

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Aeronautical Technologies for the Twenty-First Century FIGURE 7-1 Variation of CLmax with Reynolds number for a simple swept wing. principally through their concerted efforts over several years to understand the detailed flow physics associated with high lift. For the United States to compete in this area, greater investment is needed in the experimental capabilities and flow physics studies that lead to breakthroughs in high-lift capabilities. Improved CFD: The challenge is formidable in view of the complex physics, complex geometries, and numerous length scales that must be resolved numerically. Pacing items are the flow models (modeling of turbulence, transition, bubbles and reattachment, merging sheer layers, and relaminarization); the sheer size of the computational problem, including proper numerical resolution of all the important length scales of the physics; and code validation, particularly at flight Reynolds numbers where few data exists. What is needed is a CFD development program that is closely coordinated with other key elements of flow physics and high Reynolds numbers testing, and which is constrained to be economically viable and run able in a timely manner when hosted on today's computers. Although the United States leads in certain respects in CFD, at current investment levels, U.S. research efforts will not provide for the experimental validation of CFD capabilities necessary to maintain this lead. The need for such validation has been recognized for some time as critical to the maturation of such computational capabilities for ultimate use in design method applications.

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Aeronautical Technologies for the Twenty-First Century Experimental research and developmental testing at the highest achievable Reynolds numbers: There is a major need for a low-speed, low-disturbance testing facility in the United States, one that produces the highest reasonable Reynolds numbers at the right Mach numbers and closely simulates the freestream environment of flight with levels of productivity that are needed to support developmental testing. It is no longer acceptable to develop a candidate design at low Reynolds numbers and validate it at high Reynolds numbers. There is also a strong need for obtaining aerodynamics measurements at flight Reynolds numbers on full-scale flight vehicles. Nonintrusive, highly accurate, and responsive instrumentation is urgently needed to make detailed measurements of various flow conditions on and away from the surfaces of a test article. The current level of U.S. high-lift research efforts will not provide the advanced measurement technologies needed for rapid, competitive progress in this research area. In 1989, NASA developed a broad plan for research in advanced aeronautics measurement technology; unfortunately, the plan was never implemented. In comparison, the European competitors, in their BRITE EURAM program, have committed to significant levels of investment in measurement technology. Other phenomena associated with high-lift systems are airframe noise and wake vortex prediction and alleviation. Continuing advances in engine noise reduction will mean that the airframe is responsible for a growing portion of the overall noise profile of an airplane. Reducing airframe noise requires an understanding of the detailed sources and mechanisms of its production. The design challenge is to develop solutions that reduce airframe noise but retain high-lift aerodynamic performance. Airplanes that meet the Federal Aviation Regulation (FAR-36) Stage 3 noise limits have provided substantial noise reductions relative to older airplanes. Nevertheless, many airports impose additional noise restrictions that penalize payload or range by requiring operations at reduced takeoff weight or that prohibit night operations. It is clear that pressure to further reduce noise will continue to increase. To advance the state of the art to significantly reduce noise, concerted and continuing research and development efforts are required. NASA should lead the basic research to substantially improve the sophistication and technical strength of noise analysis/design computational tools. The agency should also lead the development of novel noise reduction concepts. The Committee recommends that NASA: conduct and sponsor fundamental research to understand and reduce high-speed jet and turbomachinery noise; conduct and sponsor fundamental research to understand human response to noise and vibration in aircraft cabins, flight decks, and crew rest areas; develop the technology for predicting and measuring long-range ground-to-ground and in flight-to-ground sound propagation for airport and en route noise assessment; develop technology for the prediction and control of vibroacoustic responses of aircraft structures and materials; develop prediction and suppression technology for near- and far-field noise;

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Aeronautical Technologies for the Twenty-First Century develop experimental techniques and facilities to permit acquisition of accurate acoustic, aerodynamic/structural dynamic, and psychoacoustic data in support of aircraft acoustic technology development; and develop signal processing technology to improve analysis and utilization of data from acoustic wind tunnel testing. Unless the United States mounts concerted efforts to augment the development of advanced computational and experimental capabilities, it will not be possible to achieve technology parity with similar European efforts. In particular, U.S. acoustic research facilities are not as capable as those in Europe (particularly DNW in The Netherlands). In the absence of these acoustic research facility capabilities in the United States, our industry and government research and development efforts typically utilize foreign facilities. In order to compete, it is necessary for the United States to support the development of appropriate facilities. Many airports around the world are reaching capacity operations, limited in part by wake vortex separation requirements on landing and takeoff. Continued growth of the air transport system will require increases in airport capacities. Finding ways to reduce airplane separation requirements would contribute to that goal. Studies of wake vortices, prediction methods, means of vortex detection and avoidance, and means for promoting rapid dissipation of wake vortices should be continued and expanded. Wake vortex research program plans have been developed jointly by NASA and the Federal Aviation Administration (FAA). Unfortunately, work has not begun on carrying out this plan. NASA should play a leading role in the development of the enabling technologies in these areas. There should be a combined program of flow physics and CFD, which together will increase understanding of the complex flow physics and impart an ability to predict and compute such flows. Comprehensive programs that address the issues and opportunities in noise research, wake vortices, and airplane separation requirements are needed. NASA is the appropriate organization to develop the technology and means for testing at the highest Reynolds numbers, which includes advanced flow diagnostic instrumentation development, innovative wind tunnel circuit components, the possible use of heavy gas, moderate cryogenics, and other options for high Reynolds numbers, half-model testing techniques, and wind tunnel wall interference minimization. The resources required in particular for the development of a very high Reynolds number wind tunnel are large, but investment for that purpose is essential if the U.S. competitive edge is not to be further eroded. Investments by NASA in the technology areas described above will benefit all segments of the industry, not just large commercial transports. Commuter and short-haul aircraft will benefit from advances in the enabling technologies of flow physics, CFD, and experimental research.

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Aeronautical Technologies for the Twenty-First Century SUBSONIC AIRCRAFT PROPULSION/AIRFRAME INTEGRATION The technology of integrating propulsion systems and airframes involves the ability to assess and control the development of wave drag, induced drag, and profile drag. Advances in CFD over the past decade have contributed greatly to this technology. It is anticipated that ongoing CFD developments will lead to even further refinements. Two areas remain in which technology improvements are needed. One is the development of wind tunnel test techniques and powered propulsion simulators to better represent installed power effects of the forthcoming generations of very high bypass ratio engines in wind tunnel testing. The other is the need to predict the installed characteristics of thrust reversers, both computationally and with wind tunnel testing techniques. These are areas in which NASA can make important contributions. AERODYNAMIC CRUISE PERFORMANCE Although the fundamental physical principles of subsonic and supersonic airflow around aircraft are the same, design approaches to minimizing drag are greatly affected by the cruise speed. This section of the report discusses cruise performance in the two speed ranges separately. Subsonic Aircraft Cruise Performance Long-haul subsonic transports are now, and will be for the foreseeable future, the major product of the civilian aviation industry and infrastructure. As noted in Chapter 2, from 1975 to 1995, aerodynamic efficiency will have increased by approximately 10 percent, and if the current rate of improvement is maintained, another 5–10 percent is projected by the year 2020. However, ordinary development or evolution alone will not keep the United States at the forefront in the world market. Although continued evolutionary advances in methods and processes (experimental, theoretical, and computational) are needed to provide continued improvement of aerodynamic design technologies, demonstrated innovative technologies are necessary in the longer term to provide opportunities for significant improvements in performance Laminar Flow Control The flow on most of the surfaces of an aircraft is turbulent. Laminar flow control (LFC), hybrid laminar flow control, and natural laminar flow are promising sources of skin-friction drag reduction on aerodynamic surfaces. Laminar flow nacelles are also being studied by NASA. Laminar/turbulent transition of the airflow next to the aircraft surface is delayed through a combination of pressure gradient tailoring of the wing and control such as suction through the skin. If full-chord laminar flow can be maintained in this fashion, fuel savings of up to 25 percent could be realized.

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Aeronautical Technologies for the Twenty-First Century Transition is extremely sensitive to freestream conditions (e.g., freestream turbulence and acoustics) and surface roughness (e.g., rain and ice crystals, insect debris, surface finish, and fasteners); lack of confidence in these issues has hindered the use of this concept on vehicles. Also, of perhaps greater significance have been the questions of fabrication cost and operational cost and maintainability. Engineering and optimization tools have outpaced the state of the art in transition prediction theory.1 Thus, the design of LFC, hybrid laminar flow control, and natural laminar flow systems depends on empirical bases to determine transition. This method is also limited because it cannot account for the effects of surface roughness and freestream disturbances. Knowledge of transition—so very important to the success of LFC techniques—is, in general, limited to the simplest of geometries. Efforts to better understand the transition flow physics are under way to provide valuable guidance for the surface roughness and freestream disturbance problems. Only a limited number of flight tests have been flown since the original and successful X-21 program of the 1960s;2 these are the JetStar (NASA/Langley) and Boeing 757 (NASA/Boeing). In both cases, extensive laminar flow was successfully achieved on the upper surface of the swept wing through the use of suction. Very low suction levels were required, with power penalties of the order of 1 percent. Studies with engine noise indicated no effect. The use of a Krueger nose flap eliminated a potential buildup of insect debris on the leading edge. The remaining challenges to the implementation of laminar flow technology in large subsonic transport designs include validation of the technology in actual airline service operating environments and exploration of the technical issues associated with making laminar flow operate effectively on the inboard portion of the wings of very large aircraft. Recognizing the challenges, during 1990 NASA and the industry developed a cooperative research plan; however, these efforts have been delayed by overall program constraints. Meanwhile, the Europeans have rapidly advanced their laminar flow efforts. Airbus plans for laminar flow technology validation include extensive large-scale testing, targeting technology validation as early as 1993. Turbulent Drag Reduction The most promising technique demonstrated thus far has been passive control by riblets, tiny streamwise grooves on the aircraft surface. This device is useful for surfaces on which laminar flow is very difficult to achieve (e.g., the fuselage). The approach was used 1   Bushnell, D.M., M.R. Malik, and W.D. Harvey. 1989. Transition Prediction in External Flows via Linear Stability Theory. Symposium Transsonicum III, J. Zierap and H. Oertel, eds. Berlin, Heidelberg: Springer Verlag. 2   Bushnell, D.M., and M.G. Tuttle. 1979. Survey and Bibliography on Attainment of Laminar Flow Control in Air Using Pressure Gradient and Suction.

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Aeronautical Technologies for the Twenty-First Century The largest wind tunnels are subsonic: NASA Ames' 40' × 80' and 80' × 120'; NASA Langley's 30' × 60'; Canada's 30-foot; U.K. Royal Air Establishment 5 meter; Japan's 6 meter; and the Dutch/German DNW with three interchangeable test sections, of which the largest is 9.5 meters; transonic: NASA Ames' 14 foot and 11 foot; NASA Langley's 16 foot and its Transonic Dynamic Tunnel (also 16 foot); the U.S. Air Force's Arnold Engineering Development Center's 16T (16 foot); and France's S-1 (26 foot); supersonic: NASA Ames' 9' × 7' and 8' × 7'; NASA Lewis' 10' × 10' and 8' × 6'; U.S. Air Force Arnold Engineering Development Center's 16S (16 foot) and Aerodynamic and Propulsion Test Unit (16-foot dia.); Rockwell's 7 foot; and France's S-2 (6 foot). Wind tunnels with the largest Reynolds number (given in parentheses in millions), based on effective chord length, are subsonic: NASA Langley's 7.5' × 3' Low Turbulence Pressure Tunnel (30); NASA Ames' 40' × 80' (17) and 80' × 120' (10); U.K. RAE's 5 meter (8); Germany's DLR cryogenic tunnel (8); and France's F-1 (7.5); transonic: NASA Langley's National Transonic Facility (120) and Transonic Dynamic Tunnel (TDT) (14); NASA Ames' 11-foot tunnel (10); and France's ONERA S-1 (10); supersonic: McDonnell Douglas' 4-foot Polysonic (200) and 4-foot Trisonic (120); Vought's 4-foot High-Speed (150); Rockwell's 7 foot (130); Lockheed's 4-foot Trisonic (120); Canada's NAE 3-D (120); the Netherlands' 4-foot SST (120); and India's 4 foot (100). The McDonnell Douglas, Lockheed, and Vought wind tunnels have a speed range up to Mach 5. NASA Langley and the Department of Defense Wright Laboratory both have high Reynolds number Mach 6 wind tunnels. Because much larger model sizes relative to test section dimensions can be tested at supersonic speeds, the Reynolds numbers noted are significantly higher than for subsonic and transonic wind tunnels;13 however, the Reynolds numbers for complete supersonic aircraft configurations are at least twice as high as those noted above, which are based on root chord. A comparison of the Reynolds number capabilities of major subsonic, transonic, and supersonic wind tunnels is shown in Figures 7-5 through 7-7. 13   NASA, op. cit.

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Aeronautical Technologies for the Twenty-First Century FIGURE 7-5 Comparison of major subsonic tunnels. Air-breathing Propulsion Facilities Major air-breathing propulsion facilities can be divided into three categories: propulsion wind tunnels, altitude engine test facilities, and engine/propulsion component facilities.14 The wind tunnels are large facilities in which engine burn tests and propulsion/airframe integration tests can be run. A drawback of wind tunnels for engine testing is their inability to provide true temperature simulation over wide operating conditions. There are seven such tunnels in the United States including NASA Ames' 40' × 80' and 80' × 120', Lewis' supersonic 10' × 10' and 8' × 6', the Arnold Engineering Development Center (AEDC) supersonic 16T and 16S, and Boeing's 9' × 9'. Foreign tunnels include Canada's 10' × 20', France's transonic S-1, and the Netherlands' DNW. 14   National Aeronautics and Space Administration. 1985. Aeronautical Facilities Catalogue. Volume 2. Air-breathing Propulsion and Flight Simulators (NASA RP-1133). Washington, D.C.

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Aeronautical Technologies for the Twenty-First Century FIGURE 7-6 Comparison of major transonic tunnels. Of some 60 altitude engine test facilities in the world, the one with the greatest capability by far is the AEDC Aeropropulsion Systems Test Facility with two 28-foot diameter cells, 85 feet long, mass flows exceeding 2,000 pounds per second, a broad temperature range, and speeds up to Mach 3.8. There are 46 engine/propulsion component facilities in the United States for testing turbines, compressors, and combustors; the only foreign ones are in Japan and Belgium. These facilities are smaller and simpler than the other two categories.15 NASA owns approximately one-third of the major test facilities in the United States and one-fifth of those in the world. Most of NASA's facilities are more than 20 years old and are heavily utilized. Many are in need of repair, have marginal productivity, and need updated 15   Ibid.

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Aeronautical Technologies for the Twenty-First Century FIGURE 7-7 Comparison of major supersonic tunnels. control and data systems.16 To overcome these deficiencies, NASA has undertaken a major facility revitalization program of which the first phase—returning them to operational status—is approaching completion. It is important that the revitalization program be continued to bring these facilities to the state of the art and then broaden their capabilities. Flight Test Facilities The national flight research assets include the flight testing range and related ground facilities and the aircraft in NASA's research fleets stationed at Ames-Dryden Flight Research Facility, Ames-Moffett, Langley, and Lewis Research Centers. Current research aircraft are listed in Table 7-2. 16   National Research Council. 1988. Review of Aeronautical Wind Tunnel Facilities. Washington, D.C.: National Academy Press.

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Aeronautical Technologies for the Twenty-First Century TABLE 7-2 Current Research Aircraft Center Aircraft Capability NASA Ames-Dryden Research Facility PIK 20E Sailplane General Dynamics F-16 Lockheed F-104 (2) Grumman X-29 McDonnell Douglas F-15 Eagle Boeing B-52 McDonnell Douglas F-18 Hornet Convair 990 Glider Supersonic Supersonic Supersonic high alpha Supersonic Transonic Supersonic Transonic NASA Ames-Moffet Research Center Bell XV-15 British Aerospace YAV-8B Harrier Boeing/DeHavilland QSRA Sikorsky UH-60 Blackhawk (2) Bell TH-1S Tiltrotorcraft VTOL STOL Rotorcraft Rotorcraft NASA Langley Research Center Piper PA-28R Arrow Boeing 737-100 Learjet 28/29" Rockwell OV-10 Bronco Subsonic Transonic Transonic Subsonic NASA Lewis Research Center DeHavilland DHC-6 Twin Otter Rockwell OV-10A Bronco Subsonic Subsonic Test Requirements for Future Systems Common to all speed regimes is the requirement for full-scale (high) Reynolds number facilities and flight capability, not only for high-lift devices and high-angle-of-attack configurations at low speeds, which are highly sensitive to Reynolds numbers, but also for conventional aircraft because of the emphasis on drag reduction. There is also a need for very accurate, nonintrusive instrumentation with high resolution for spatial and temporal measurements. At low speeds the requirements are for flight and ground facilities to test takeoff and landing of conventional aircraft; high-lift devices (short-takeoff-and-landing vehicles), terminal area noise (engine, control surfaces, airframe); rotorcraft; and vehicles with expanded flight envelopes (e.g., high angle of attack). There is particularly strong emphasis on reducing the noise of flight systems, necessitating test facilities with very low background noise of their own. In the transonic and supersonic regimes, large facilities are needed for propulsion/airframe integration and aeroacoustic studies. Also, low-disturbance supersonic

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Aeronautical Technologies for the Twenty-First Century facilities and flight research aircraft are needed for laminar flow control and boundary-layer transition studies required for vehicles such as an HSCT. Large air-breathing propulsion facilities are needed to test full-scale turbojet engines integrated into the airframe up to high supersonic speeds. Adequacy of Existing Test Facilities to Meet Requirements Low Speeds Such wind tunnels as NASA Ames' 80' × 120' and 40' × 80', the Dutch/German DNW, the U.K. 5 meter, and France's (ONERA) S-1 (8 meter) meet many of the low-speed test needs; however, none can meet the range of full-scale Reynolds numbers required. An important high Reynolds number capability will be added when NASA restores the Ames 12-foot Pressure Wind Tunnel to operate at 6 atmospheres and speeds up to Mach 0.6, but it will still be far short of the full-scale Reynolds number required. NASA Ames' 80' × 120' is well-suited for testing VTOL powered lift, full-scale rotorcraft, and high-lift devices for takeoff and landing, and is one of the few facilities suitable for full-scale high-angle-of-attack tests; however, it is limited to a Mach number of 0.15 and utilizes very large (full-scale) and, therefore, relatively expensive models. For aeroacoustic testing, DNW offers the best capability to study noise generated by engines, airframes, and rotors. In the United States, consideration is being given, and an approach defined, toward fitting NASA Ames' 40' × 80' wind tunnel with a thick acoustic lining that will make it the largest facility capable of noise tests on full-scale aircraft and rotorcraft. However, this will not meet the requirements for research on noise reduction at higher climb-out and cruise/climb flight conditions, which are increasingly important challenges for the future. Transonic Regime The only facility with full-scale Reynolds number capability is NASA Langley's cryogenic National Transonic Facility with an 8.2' × 8.2' test section; however, its reliability and productivity to date have been relatively low. It is currently being upgraded. A major facility expected to become operational in the mid-1990s is the European Transonic/Cryogenic Wind Tunnel near Cologne, Germany. It will have a test section of 2.4 m × 2.4 m and cover the speed range from Mach number 0.15 to 1.2, with Reynolds numbers based on effective chord length up to 50 million, which is adequate to meet most future test requirements. Special attention in the development of the European Transonic/Cryogenic Wind Tunnel has been given to high productivity and reliability. The extreme cooling to cryogenic temperatures of models and instrumentation used in these facilities imposes special requirements and handling techniques.

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Aeronautical Technologies for the Twenty-First Century Supersonic Regime Although several supersonic wind tunnels have a Reynolds number capability (based on model size) exceeding 100 million, these are based on wing sections, which are rather large relative to tunnel dimensions, and not the full vehicle. None of these facilities reach the full-scale Reynolds numbers needed for a HSCT or supersonic military systems. In terms of low-disturbance supersonic wind tunnels to study laminar flow control and boundary layer transition, NASA presently has in operation a Mach 3.5 tunnel and is developing a new facility with Mach 3.5 and Mach 6 nozzles. Other low-disturbance wind tunnels are being planned. For supersonic laminar flow research, flight test aircraft play a vital role in technology development and validation by providing access to the real atmospheric disturbance environment. In addition, it is important that large-scale aircraft be available for laminar flow research to provide answers to critical high Reynolds number issues. Air-breathing Propulsion The Aeropropulsion Systems Test Facility at the Arnold Engineering Development Center provides a unique capability for full-scale propulsion/airframe integration and combustion tests up to Mach 3.8. Other engine test facilities are inadequate to meet future full-scale requirements. However, one should bear in mind that, even in the Aeropropulsion Systems Test Facility, only segments of a complete vehicle can be tested but these would include parts of the airframe as well as the engine. Computational Fluid Dynamics Computational fluid dynamics has become an increasingly important complement to wind tunnels as a tool in the design of new aircraft. Each does different things best. Together they provide more detailed and complete information than wind tunnels alone. This vital role for CFD stems from the tremendous advances over the years in speed and memory size of supercomputers, as well as improvements in numerical algorithms. Among current supercomputers, the Cray 2 has an effective speed exceeding 200 million floating point operations per second (MFLOPS). The more recent Cray YMP has four times that speed (i.e., about 1,000 million floating point operations per second MFLOPS). Comparable advances are projected to continue for several years, and it is important that they be channeled toward facilitating the application of CFD to airplane design. Experience gained by the airframe industry in the utilization of CFD in recent years has served to identify areas of future development in computational technology. In the early 1980s NASA undertook the development of a Numerical Aerodynamic Simulation facility as a means of providing a continually updated capability with the latest available high-speed processors, for the purpose of solving the basic flow equations and with the intent of enhancing the application of CFD to aerospace vehicle design. Achievement of these goals requires a refocus of objectives. In the area of hardware—mainframes, communications,

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Aeronautical Technologies for the Twenty-First Century and workstations—NASA's Numerical Aerodynamic Simulation facility has had a pathfinding role and is positioned to continue as such. In the area of algorithm research and code development, the main focus in the past was principally on solving increasingly complex and complete forms of the basic flow equations. In the future, the focus needs to be expanded to include other factors that are essential toward the effective application of CFD to airplane design. In particular, there is a need for CFD capabilities that can handle the flow about arbitrary geometries; resolve all the important physical length scales; provide reliable accuracy, when used by a design engineer, with sufficient credibility to support engineering decisions; can be rapidly set up and executed (in one day or less); and are economically viable (i.e., the real computing costs for the tens or hundreds of runs typically needed to solve a design problem are justifiable and acceptable). It is expected that the next decade will witness the emergence of CFD as the critical technology for aerodynamic design. There should be a dramatic change and shortening of the design process, which will enhance and enable concurrent engineering and the optimization of air vehicle systems in terms of overall economic performance. This will require a significant advance in CFD algorithm research and code development. Such developments are heavily dependent on a close working relationship between NASA and the aeronautical industry to enhance the relevance of NASA research and enable a rapid translation of research advances into application to the design process. Recommendations for Test and Computational Facilities This section of this report defines a course for NASA, industry, and academia to achieve competitiveness in aerodynamics. Specific investment needs are described to strengthen the U.S. science and engineering infrastructure and technology validation efforts. The fact that critical technology initiatives planned by NASA and industry have not been implemented demonstrates that the problem is not a lack of ideas and plans but, rather, the priority given to aeronautics. Experimental facilities and methods, along with computational capabilities, comprise the infrastructure upon which advances in technology can develop and flourish. The future ability of the United States to remain competitive in aeronautics is directly dependent on the quality of its experimental and computational facilities, which, along with flight testing, are the design tools for new aircraft. Economic and technical compromises must be made to determine where each of these tools fits into the design process. The three must serve in a complementary fashion, and the extent of use of any one of them is dependent on its ability to provide certain types of data more effectively than the other two. Finally, precompetitive technology validation through cooperative efforts among government, industry, and academia must be substantially increased if the United States is to remain competitive in aeronautics.

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Aeronautical Technologies for the Twenty-First Century The key aerodynamic technologies that require increased investment for competitiveness include aerodynamic cruise performance, including subsonic and supersonic laminar flow control technology; aircraft propulsion/airframe integration for both subsonic and supersonic aircraft; low speed and high lift for subsonic configurations, including wake mechanics, wake vortex, and measurement technology; computational fluid dynamics for aircraft design, including validation of codes; low speed and high lift for supersonic configurations; and aerodynamics of rotorcraft. Throughout this report, references are made to needs for a variety of improved ground and flight experimental and computational capabilities. It is important that NASA receive the support and direction required to provide the United States with experimental and computational capabilities that will enable the aeronautical community to develop competitive or superior products in all flight regimes. The following are specific ground-based experimental capability recommendations to NASA: To vigorously pursue its facility revitalization program beyond the present phase, toward raising its facilities to the state of the art and then broadening their capabilities. To generate an intensive program to develop high-resolution, nonintrusive instrumentation for spatial and temporal measurements. To proceed with all haste in fitting its Ames' 40' × 80' wind tunnel with an acoustic lining essential for noise testing on full-scale vehicles. To develop a low-speed, low-disturbance test capability that can operate at full-scale Reynolds number (based on chord length of at least 50 million) in the range from Mach number 0.1 to 0.5. Because of dimensional constraints, such a facility would have to be pressurized; also, consideration in its design might be given to the possibility of using heavy gases instead of air for aeroelastic scaling or as a means of boosting Reynolds numbers to the required values. Mixtures of heavy gases in proportions to provide the proper scaling of physical variables have been successfully used in testing turbine and compressor stages at realistic conditions of engine operation. A possible alternative to using a heavy gas as test medium would be to operate with air at moderate cryogenic temperatures—down to -150ºF—based on the experience gained in operating the National Transonic Facility.

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Aeronautical Technologies for the Twenty-First Century To develop a supersonic, low-disturbance test capability that can operate at a full-scale Reynolds number of 400 million to 500 million over the range from Mach 2 to 6 to meet test requirements for an HSCT and military systems. A pressurized test section would be needed to meet the stringent Reynolds number requirement. The first important point regarding facilities in general is to ensure high reliability because it is crucial to the value of their data, but good productivity must also be ensured by adequate staffing and by efficient means of moving test models in and out of tunnels. Furthermore, data acquisition and processing equipment must be upgraded periodically to keep these valuable test capabilities current. Regarding aeronautical flight experimental capabilities, NASA should (1) vitalize the flight research capability and in-flight technology validation efforts in all speed regimes, and develop advanced measurement technologies for flight research. To carry out these recommendations for flight capability, NASA must structure the management of flight efforts to strengthen the integration of flight and ground-based research. NASA must plan for the future requirements for flight to include experimental aircraft, platform test-bed aircraft, and ground and flight infrastructure capabilities. Finally, in the area of computational technology it is essential that NASA, on a continuing basis, acquire and maintain state-of-the-art computing hardware and develop the CFD algorithms and software that are key to the application of computational technology to aerodynamic design. This includes an aggressive pursuit, with the input of industry, of advances in the development and verification of both design and analysis CFD codes. RECOMMENDED READING Epstein, A.H., G.R. Guenette, and R.J.G. Nirton. 1984. The M.I.T. Blowdown Turbine Facility. ASME Paper 84-GT-116. Washington, D.C.: American Society of Mechanical Engineers. Kerrebrock, J.L., A.H. Epstein, D.M. Haines, and W.T. Thompkins. 1974. The M.I.T. Blowdown Compressor Facility. ASME Paper 75-GT-47. Washington, D.C.: American Society of Mechanical Engineers. National Research Council. 1983. The Influence of Computational Fluid Dynamics on Experimental Aerospace Facilities—A Fifteen Year Projection. Washington, D.C.: National Academy Press. Poisson-Quinton, Ph. 1990. EUROMART (European Cooperative Measures for Aeronautical Research and Technology) Survey Results. France: ONERA.

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Aeronautical Technologies for the Twenty-First Century Guenette, G.R., A.H. Epstein, M.B. Giles, R. Haines, and R.J.G. Norton. 1989. Fully Scaled Transonic Turbine Rotor Heat Transfer Measurements. Journal of Turbomachinery 111 (January).