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Assessing the National Plan for Aeronautical Ground Test Facilities Executive Summary At the request of the National Aeronautics and Space Administration and Department of Defense, the Aeronautics and Space Engineering Board (ASEB) of the National Research Council independently reviewed the findings of the interagency National Facilities Study (NFS). In order to make the ASEB report available shortly after the NFS report, the NFS Task Group on Aeronautical R&D Facilities briefed the ASEB periodically during its study. After release of the NFS report, the ASEB held a far-ranging workshop to critique the NFS results. The workshop involved 49 experts in aeronautical technology development; ground test facilities; and, especially, the use and operation of wind tunnels. The purpose of this report is to document and explain the ASEB's assessment of the NFS report, including recommendations for future action. The conclusions and recommendations of the NFS seem to be supported by factual material wherever it was available, although in some cases they are based on the best judgement of the study participants. The following nine items summarize the ASEB's findings and recommendations. The first five items reinforce key thrusts of the National Facilities Study. The ASEB concurs with each of these items. The last four are recommendations for additional action that go beyond the recommendations of the National Facilities Study. RECOMMENDATIONS REINFORCING THE KEY THRUSTS OF THE NATIONAL FACILITIES STUDY 1. The ASEB agrees with the NFS report that significant aerodynamic performance improvements are achievable, and the nation that excels in the development of these improvements has the opportunity to lead in the global market for commercial and military aircraft.1 The highest priority facilities for achieving these performance improvements are new high-productivity, high-Reynolds-number subsonic and transonic development wind tunnels.2 The NFS report estimates that cruise and take-off/landing performance could be improved by at least 10 percent each. Performance improvements are essential for the U.S. aeronautics industry to maintain or increase market share. Based on the information available to it, the ASEB considers these projected increases in performance to be potentially attainable and believes that the proposed facilities could substantially facilitate such improvements. These forecast advantages do not include the probable operating and development cost reductions that would accrue to future U.S. military aircraft programs. In addition to direct cost reductions, access to improved ground test facilities would make advanced military aircraft more competitive in the world market, thereby further reducing the defense burden carried by U.S. taxpayers. Foreign sales of U.S. military aircraft result in lower unit costs for U.S. government and foreign purchasers. 2. The ASEB agrees with the NFS report that new high Reynolds number ground test facilities are needed for development testing in both the low speed and transonic regimes to assure the competitiveness of future commercial and military aircraft produced in the United States. The NFS report documents that Reynolds and Mach number performance of the best subsonic and transonic development wind tunnels in the United States and Europe 1 The National Research Council report Aeronautical Technologies for the 21st Century (NRC, 1992) documents historical trends and projects future gains in aircraft performance as a result of technological advances. 2 Overall priorities are discussed in more detail in Chapter 6 starting on page 44.
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Assessing the National Plan for Aeronautical Ground Test Facilities are close to parity.3 However, the average age of major U.S. tunnels is about 38 years, and many of the older U.S. wind tunnels are subject to costly maintenance and breakdown. Furthermore, there are no adequate domestic alternatives for many older U.S. facilities. For example, during the past several years U.S. manufacturers have conducted a large amount of their low speed testing in European facilities during refurbishment of the Ames Research Center 12-foot subsonic wind tunnel, which is 48 years old. Table ES-1 Proposed Capabilities of New Low Speed and Transonic Wind Tunnels TUNNEL PARAMETER LOW SPEED TUNNEL TRANSONIC TUNNEL Reynolds Number 20 million at Mach 0.3 (full span model) 35 million at Mach 0.3 (semi-span model) 28.2 million at Mach 1 (full span model) Mach Number 0.05-0.6 0.05-1.5 Productivity 5 polars per occupancy hour* 8 polars per occupancy hour Operating Cost <$1,000/polar <$2,000/polar Operating Pressure 5 atmospheres 5 atmospheres Total Temperature 110°F 110°F at Mach 1 Maximum Power 45 MW 300 MW Test Section Size 20 ft × 24 ft 11 ft × 15.5 ft Flow Quality Low turbulence Low turbulence Acoustic Test Capability Acoustic test chamber Not applicable * A polar is a single test run consisting of 25 data points (see Appendix D). Source: NFS, 1994 In contrast, European industry has a new government-funded transonic facility coming on-line during 1994 that is expected to significantly outperform any transonic development facilities in the United States in terms of Reynolds number capability. 4 The NFS report examines this situation in detail with regard to the development of new commercial air transports, which have very high flight Reynolds numbers. More-capable wind tunnels will facilitate improvements in aircraft performance and producibility. However, as documented by the NFS, no wind tunnel in the world meets or can be affordably modified to meet the goals defined by the NFS for development of future transport and military aircraft (see Table ES-1).5 The ASEB agrees with the NFS that building the two tunnels as proposed is likely to enable subscale development testing for more than half of the new commercial transport aircraft projected for the next twenty years or so at flight Reynolds and Mach numbers. However, the flight Reynolds numbers of (1) very large commercial transports, (2)high speed civil transports, (3) high performance military aircraft, and 3 Mach and Reynolds numbers are defined in Appendix D. 4 The U.S. National Transonic Facility has a Reynolds number capability of 119 million, but its productivity is an order of magnitude less than other large transonic facilities. Thus, even though it has a limited (design-verification) role to play in the development of new aircraft, it is not a “development” wind tunnel. Its primary role is as a research facility. 5 The NFS initially established a Reynolds number test capability of approximately 30 million as a goal for both the low speed and transonic wind tunnels. After assessing the impact of performance goals on facility design and cost, the NFS recommended accomplishing this goal in the low speed regime using semi-span models. Semi-span models include only the left or right half of an airplane. This increases the Reynolds number capability of a given facility relative to tests using full-span models.
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Assessing the National Plan for Aeronautical Ground Test Facilities (4) some revolutionary design concepts that might emerge in the future would exceed the capabilities of the proposed tunnels. Thus, the test results for these aircraft would have to be extrapolated to analyze their performance at flight Reynolds number. Nonetheless, this process would generally be more accurate than extrapolations based on data obtained from the less capable tunnels now available. In particular, the new wind tunnels would allow testing models of existing aircraft such as the B-737 and MD-90 at flight Reynolds number. Comparison of wind tunnel and flight data for these aircraft is likely to significantly improve the correlation of wind tunnel and flight data for future designs of conventional aircraft that have flight Reynolds numbers beyond the test limit of the proposed tunnels. The NFS report recommends taking immediate action to reduce the projected cost ($2.55 billion) and schedule (eight years) of acquiring the proposed low speed and transonic wind tunnels.6 The ASEB agrees that reducing cost and schedule is an important goal, but it cautions against using management-directed cost and schedule estimates to provide the illusion of achieving this goal. 3. Along with the procurement of new facilities, the ASEB agrees with the NFS that selected upgrades to existing facilities are also essential to adequately support future research and development programs. These upgraded facilities will be important during the interim before new tunnels are operational and, afterwards, to round out the United States' test capabilities matrix. However, facility upgrades cannot alone satisfy future ground test requirements. In particular, the ASEB endorses the NFS's proposed upgrade to the common 16S/16T drive system at Arnold Engineering Development Center and urges further consideration of additional activities to improve the reliability of the drive-system motors and compressors. In case of failure, major motor repairs could take from four months (to rewind a motor stator) to over three years (for complete motor replacement). Although Arnold Engineering Development Center estimates that motor problems requiring complete replacement are very unlikely, credible accidents such as an electrical arc-over with severe internal motor damage could reduce the operational capability of 16S (and 16T) for up to a year.7 This would have a severe impact if it occurred at a critical point in an aircraft development program. Additional improvements to the drive system should be carefully considered to reduce the probability of such an occurrence. 4. The ASEB agrees with the NFS that the United States should acquire premier development wind tunnels rather than rely on continued use of European facilities. Over the past 25 years, as European aeronautics technology has risen to equal U.S. technology, the United States ' market share in transport aircraft has declined 30 percent. Although market share is a function of many factors, if other nations achieve a higher level of aeronautical technology, erosion of the U.S. market share may accelerate, with accompanying reductions in balance of trade and jobs.8 Continued advances in aerodynamic technology are necessary to avoid this situation. The proposed facilities represent an investment that is only a small fraction of the potential future gain and will provide an opportunity to enhance U.S. technology development. Acquisition of advanced high-productivity wind tunnels in the United States— where U.S. designers can efficiently coordinate their wind tunnel testing, model building, and computational activities—will improve the effectiveness and 6 The National Facilities Study included a very detailed costing effort, which is documented in Volume II-A of its final report. 7 Laster, M.L. June 17, 1994. National Aeronautical Test Facilities Study Information Memorandum. Directorate for Plans and Requirements, Arnold Engineering Development Center. Arnold Air Force Base. Tennessee. 8 For a more thorough discussion of the factors affecting the eroding U.S. position in aeronautics, the necessary but insufficient role that advances in technology play, and specific technology advances that are possible and desirable, see Aeronautical Technologies for the Twenty-First Century (NRC, 1992), pages 26–34 and the discussions of current industry status, market forecast, and barriers for each of the major speed regimes.
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Assessing the National Plan for Aeronautical Ground Test Facilities efficiency of the aircraft design and development process. When aircraft designers introduce a new product, they must determine how far to push available technology before selecting the final design. The nation with the most efficient design-test-redesign process can achieve either (1) a given level of performance sooner or (2) better performance within a given period of time. Inferior, inefficient design or test processes, on the other hand, allow the competition to produce an equal or better product sooner. Slow design and test methodologies also extend the period that manufacturers must fund product development, increasing the costs of bringing new products to market. Although U.S. designers have access to European facilities, the ASEB believes that the scheduling constraints faced by U.S. users and the inefficiency of conducting transatlantic design and development efforts inevitably delay the introduction of new products. Conversely, European competitors have greater access to better test facilities and, potentially, to the data generated when U.S. aircraft manufacturers use their wind tunnels. In combination with other improvements that industry is making in its design and manufacturing process, the ASEB believes that the construction of advanced development wind tunnels will be an important contribution to the productivity of the U.S. aeronautics industry. Because of national security concerns, foreign facilities are especially inappropriate for development of military aircraft. The U.S. defense industry is generally limited to U.S. facilities, even if more-capable facilities are available elsewhere. The NFS report identifies three options for funding the construction of the proposed subsonic and transonic wind tunnels: industry only; a government/industry consortium; and government only. After assessing these options, the NFS “envisioned that the facilities will be constructed primarily with government funding,” and it concluded that “funding by industry alone is not a viable source of capitalization.” However, it also determined that the possibility of obtaining funding jointly from government and industry “could not be ruled out” and it recommended conducting “further studies to look at innovative funding approaches and government/industry consortia arrangements.” The ASEB understands that these studies are underway. 5. The ASEB agrees with the NFS that additional action is necessary to address future requirements for supersonic, hypersonic, and aero-propulsion test facilities. It is not appropriate to immediately proceed with the construction of new supersonic, hypersonic, or aeropropulsion development facilities. Each of these areas, however, will be important to the aeronautics industry of the future. Thus, appropriate action should be taken to ensure that required facilities will be available when necessary. Supersonic Facilities. The Department of Defense will have continuing needs for supersonic ground testing of new and upgraded military flight vehicles and systems, and NASA's High Speed Civil Transport Program will create additional demands for access to supersonic wind tunnels. Incorporating supersonic laminar flow characteristics into military and commercial aircraft would significantly reduce drag and surface heating and increase fuel efficiency. However, designing a cost-effective supersonic laminar flow facility to conduct development testing is beyond the current state of the art. Solution of the complex problems involved will require a continued program of theoretical and experimental investigation. In order to partially address shortfalls in U.S. supersonic facilities regarding productivity, reliability, maintainability, and laminar flow test capabilities, the 16S facility at Arnold Engineering Development Center, which would be used to support development of a first-generation high speed civil transport, should be upgraded. In addition, research should continue on supersonic laminar flow technology and facility concepts. Hypersonic Facilities. More-capable hypersonic ground test facilities are needed to provide the option for future development of hypersonic vehicles. State-of-the-art technology, however, is not adequate to build major new hypersonic facilities that will have
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Assessing the National Plan for Aeronautical Ground Test Facilities the needed capabilities in areas such as model size, run time, pressure, temperature, and velocity. Therefore, near-term efforts should focus on a program of research to select, develop, and demonstrate the most promising hypersonic test facility concepts. Long-term efforts to build hypersonic development facilities will be contingent upon successful completion of the near-term facility research effort and concurrent efforts to validate future requirements for hypersonic vehicles. Aeropropulsion Facilities. Aeropropulsion test facilities within the United States have the capability to test current air breathing engines under the operating conditions experienced during take-off, climb, cruise at flight speeds up to Mach 3.8, approach, and landing. Looking to the future over the next 10 to 30 years, air breathing engine test facility requirements will be determined by engine size, type, configuration, and air flow requirements. The Aeropropulsion System Test Facility at Arnold Engineering Development Center, as currently configured, is adequate for altitude testing of the newest generation of high-bypass engines. However, a 40 percent increase in flow capacity might be required to handle the next generation of ultra-high-bypass, gear-driven propulsor engines such as the PW4000 Advanced Ducted Propulsor. These engines could be certified after the year 2000—if the aircraft manufacturers develop new, larger aircraft requiring such engines. Implementation of facility upgrades for these larger subsonic engines would take four to eight years, so there is time to “wait and see” before deciding how to proceed. RECOMMENDATIONS GOING BEYOND THOSE OF THE NATIONAL FACILITIES STUDY As previously indicated, the remaining four items go beyond the recommendations of the National Facilities Study report. These recommendations will (1) reduce risk associated with carrying out the actions recommended by the NFS and (2) facilitate long-term efforts to provide U.S. users with improved aeronautical ground test facilities. 6. The Wind Tunnel Program Office should conduct trade studies to evaluate design options associated with the proposed new low speed and transonic wind tunnels.9 Facility configuration trade-off studies conducted by the NFS on Reynolds number, productivity, and life cycle cost appear to be sound. However, additional configuration studies should be conducted during the design phase of the wind tunnel program. These assessments should take into account the differences in tunnel and model parameters between subsonic and transonic wind tunnel testing. They should evaluate the merits of the following design options: Using a single tunnel to test both the low speed and transonic speed regimes. While a single tunnel would be unlikely to offer the same capabilities as two separate tunnels, the extent to which performance and operational costs would be compromised should be evaluated in terms of savings in acquisition costs. This assessment should verify the accuracy of projected utilization rates to determine if a single facility could meet the expected demand for test hours. Making incremental changes to the tunnel operating pressures (e.g.,from 5 to 5.5 atmospheres). Increasing wind tunnel operating pressure would allow facility size and cost reductions without sacrificing Reynolds number capability. The extent to which higher pressures could be used without unduly jeopardizing the cost, efficiency, and effectiveness of the overall ground test process is unclear, and the interaction between tunnel pressure and model design should be investigated further for both the transonic and subsonic tunnels. This investigation should take into account the considerable differences that exist 9 NASA has established a Wind Tunnel Program Office at Lewis Research Center. This office, which reports to the NASA Administrator, is now working with industry to develop an acquisition strategy and conduct design trade studies for two new low speed and transonic wind tunnels, as recommended by the National Facilities Study. Participants in this effort include veteran wind tunnel designers, operators, and users from government and industry. If federal responsibility for development of these facilities is reassigned, then the designated successor should assume responsibility for actions assigned in this report to the Wind Tunnel Program Office.
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Assessing the National Plan for Aeronautical Ground Test Facilities between these two flight regimes. In particular, use of higher pressures is likely to be more feasible for subsonic wind tunnels than for transonic wind tunnels because of the differences in dynamic pressures. Including within the baseline design the ability to provide future growth in Reynolds number capability through use of higher operating pressures (up to 8 atmospheres), reduced temperatures (down to about -20°F), and/or a heavy test gas (such as SF6). Incorporating these capabilities into the new facilities would add significant cost. There are also technical concerns regarding wind tunnel tests using high pressure or gases such as SF6. However, it would add only a few percent to the cost of the new facilities to plan ahead for future upgrades that would use one of these capabilities. For example, initially designing the Low Speed Wind Tunnel pressure shell to withstand 8 atmospheres would facilitate subsequent facility upgrades to higher operating pressures. Experience with existing facilities shows that test requirements often evolve beyond the expectations of the original designers. Failure to initially build in growth capability would make future facility upgrades highly unlikely and limit the ability of future facility operators and users to enhance tunnel capabilities. (Appendix D provides more information on how pressure, temperature, and test gas impact wind tunnel performance capabilities.) Improving the robustness of the tunnel designs. Designing selected sub-systems and components of the new wind tunnels with margin for growth relative to pressure and operating power could improve system reliability, increase facility lifetime, and reduce the costs of future upgrades. In addition, the Wind Tunnel Program Office should ensure that the new transonic and low speed facilities will be able to adequately support development of supersonic aircraft. The importance of low speed and transonic wind tunnels extends beyond their application to subsonic and transonic aircraft. They will also be of special importance to supersonic aircraft such as high speed civil transports that must also operate in lower speed regimes during take-off, acceleration, transonic flight over land, and landing. The design of the proposed new wind tunnels should be compatible with the test requirements of higher speed aircraft to the extent that this additional capability is affordable and does not unacceptably degrade the tunnels' ability to execute their primary mission. The detailed design phase of the new wind tunnels should also ensure that features necessary to adequately accommodate development testing of military aircraft, including stores separation testing, are incorporated into the design of the new wind tunnels as appropriate. Ongoing efforts by the U.S. Air Force to more closely define military requirements for future development wind tunnels will assist in this effort. 7. NASA and the Department of Defense should continue support for facility research in the subsonic and transonic regimes. The highest priority need in the area of low speed and transonic facilities is for new development facilities. Related research, which includes both vehicle- and facility-oriented efforts, is also important to long-term competitiveness. For example, the ability to construct practical development test facilities that use heavy gas (such as SF6) and/or very high operating pressures (15 atmospheres or more) would (1) greatly reduce facility size and cost and (2) increase Reynolds number test capability. Continued funding of appropriate research is an essential precursor to the development of future generations of ground test facilities and future upgrades of existing and planned facilities. 8. NASA and the Department of Defense should expand coordinated efforts that involve aerodynamic test facilities, computational methods, and flight test capabilities. Computational methods such as computational fluid dynamics are used during the aircraft design process to analyze and predict aerodynamic characteristics in all speed regimes. However, they must be validated by experimental ground and flight tests before they can be relied upon for design or evaluation in any phase of development.
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Assessing the National Plan for Aeronautical Ground Test Facilities Improved aerodynamic wind tunnel testing will provide a better understanding of aircraft fluid dynamics, including Reynolds number and boundary layer effects. This understanding will permit more-accurate scaling of ground test data to in-flight performance. Nonetheless, for the foreseeable future, computational methods will not eliminate the need for highly capable wind tunnels to support development of advanced aircraft. Continued work to improve computational methods and continued flight exploration (e.g., X-planes) are required adjuncts to the acquisition of new and improved wind tunnels. Better scaling methodologies are needed as soon as possible. They will be useful during the interim before new tunnels are available, and, in the long run, they will extend the utility of new tunnels for the design of very large and unusually configured future aircraft. 9. NASA and the Department of Defense should develop a continuing mechanism for long-term planning of aeronautical test and evaluation facilities. Assigning the responsibility to study future requirements and conduct long-range planning to a permanently established body would provide greater continuity than the current process of relying on intermittent, ad hoc committees. Experience with current facilities indicates that the service life of major new facilities could easily extend to the middle of the next century. The long-term utility of major new facilities will be greatly enhanced if their designs are based on a broad view of future test requirements. An overall assessment of Volume II of the NFS report and a complete list of the ASEB's findings and recommendations appear in Chapter 7.
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