3

Supersonic Facilities

INTRODUCTION

The supersonic flight regime, which is generally considered to include velocities up to Mach 5, is relevant to military aircraft, missiles, and supersonic transports. Although the current demand for supersonic testing is relatively small compared with the demand for subsonic and transonic testing, supersonic test capabilities are an important part of the overall ground test infrastructure. The reduced level of demand, however, creates special problems, because scarce resources tend to migrate to more active facilities. Nonetheless, 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.

PRIOR STUDIES

Many studies of aeronautical ground test facilities during the last few years have included supersonic facilities within their scope. The results of five major efforts are summarized in Tables 3-1a and 3-1b. They indicate that there are essentially two areas of special interest regarding the future of supersonic facilities: (1) maintaining and upgrading supersonic testing at the 16S facility at Arnold Engineering Development Center, and (2) developing a new low-disturbance (or “quiet”) facility to investigate supersonic boundary layer transition, mixing, and turbulent boundary layers, with the goal of achieving supersonic laminar flow control. Laminar flow technology would greatly reduce the drag and surface temperature of new supersonic aircraft designs and enhance the economic performance of a high speed civil transport. However, the five studies are divided over which option to pursue. The 1992 report Future Aerospace Ground Test Facility Requirements for the Arnold Engineering Development Center recommends investing in the center's existing 16S facility (NRC, 1992b). It does not advocate building a new low-disturbance facility, though it does recommend that the 16S upgrade plan consider the need for improved flow quality and laminar flow testing. Three of the reports focus primarily or exclusively on NASA facilities, and they all recommend building a new quiet supersonic wind tunnel.

THE CHALLENGE

The 16S facility at Arnold Engineering Development Center has proven its capability to support the development of advanced supersonic military aircraft and missiles. A high speed civil transport, because of its larger size, will have a higher Reynolds number requirement. Nonetheless, NASA's High Speed Civil Transport Program plans to develop a first generation aircraft by using a combination of ground testing at existing facilities (primarily the 16S supersonic tunnel at Arnold Engineering Development Center) and flight testing. Thus, the challenge faced by the 16S facility concerns primarily (1) counteracting the effects of age to maintain its capabilities and (2) using available new technology to update selected performance capabilities. The most



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Assessing the National Plan for Aeronautical Ground Test Facilities 3 Supersonic Facilities INTRODUCTION The supersonic flight regime, which is generally considered to include velocities up to Mach 5, is relevant to military aircraft, missiles, and supersonic transports. Although the current demand for supersonic testing is relatively small compared with the demand for subsonic and transonic testing, supersonic test capabilities are an important part of the overall ground test infrastructure. The reduced level of demand, however, creates special problems, because scarce resources tend to migrate to more active facilities. Nonetheless, 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. PRIOR STUDIES Many studies of aeronautical ground test facilities during the last few years have included supersonic facilities within their scope. The results of five major efforts are summarized in Tables 3-1a and 3-1b. They indicate that there are essentially two areas of special interest regarding the future of supersonic facilities: (1) maintaining and upgrading supersonic testing at the 16S facility at Arnold Engineering Development Center, and (2) developing a new low-disturbance (or “quiet”) facility to investigate supersonic boundary layer transition, mixing, and turbulent boundary layers, with the goal of achieving supersonic laminar flow control. Laminar flow technology would greatly reduce the drag and surface temperature of new supersonic aircraft designs and enhance the economic performance of a high speed civil transport. However, the five studies are divided over which option to pursue. The 1992 report Future Aerospace Ground Test Facility Requirements for the Arnold Engineering Development Center recommends investing in the center's existing 16S facility (NRC, 1992b). It does not advocate building a new low-disturbance facility, though it does recommend that the 16S upgrade plan consider the need for improved flow quality and laminar flow testing. Three of the reports focus primarily or exclusively on NASA facilities, and they all recommend building a new quiet supersonic wind tunnel. THE CHALLENGE The 16S facility at Arnold Engineering Development Center has proven its capability to support the development of advanced supersonic military aircraft and missiles. A high speed civil transport, because of its larger size, will have a higher Reynolds number requirement. Nonetheless, NASA's High Speed Civil Transport Program plans to develop a first generation aircraft by using a combination of ground testing at existing facilities (primarily the 16S supersonic tunnel at Arnold Engineering Development Center) and flight testing. Thus, the challenge faced by the 16S facility concerns primarily (1) counteracting the effects of age to maintain its capabilities and (2) using available new technology to update selected performance capabilities. The most

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Assessing the National Plan for Aeronautical Ground Test Facilities significant maintenance action anticipated concerns the common 16S/16T drive system, which would take over three years and several hundred million dollars to completely replace. Past studies have also recommended considering technology upgrades for the 16S facility to, for example, increase productivity by installing advanced computer systems, reduce airflow turbulence and noise, and provide for supersonic laminar flow testing (NRC, 1992b). Table 3-1a Recent Studies of Supersonic Ground Test Facilities Future Aerospace Ground Test Facility Requirements for the Arnold Engineering Development Center (NRC, 1992b) Focus: Military requirements for new and improved facilities for ground testing and computational modelling at Arnold Engineering Development Center Recommendation (relevant to supersonic facilities): Arnold Engineering Development Center should plan for long-term maintenance and upgrading of the 16S tunnel, its premier supersonic test facility, which is now over 40 years old. Cost: No estimate provided Industry/DoD/NASA Workshop on Aerodynamics/Aerothermodynamics/Acoustics Ground and Flight Testing Requirements (NASA, 1992) Ad hoc group of about 75 individuals from government, industry, and academia hosted by NASA Langley Research Center, July 23–24, 1992 Focus: Major research and production-oriented ground and flight test facilities for industry, DoD, and NASA Recommendation: (relevant to supersonic ground test facilities): The U.S. should acquire a new quiet (low disturbance) supersonic (Mach number 1.6–2.6) tunnel by 2000. Cost: No estimate provided Building an aircraft with supersonic laminar flow characteristics would significantly reduce its drag and surface heating and increase its fuel efficiency. However, designing a cost-effective supersonic laminar flow facility to conduct development testing is complicated by interactions between the various structures within supersonic wind tunnels. For example, noise created by turbulent boundary layers on wind tunnel walls directly affects the stability of model boundary layers. The solution to this complex problem is likely to require a continued program of theoretical and experimental investigation. Each of the prior studies summarized in Table 3-1a and Table 3-1b recommends acquiring the capability to conduct supersonic laminar flow wind tunnel testing. However, these studies indicate that there is some diversity of opinion regarding how best to achieve that goal. NATIONAL FACILITIES STUDY SUPERSONIC FACILITY REQUIREMENTS The NFS benchmarked the capabilities of U.S. and foreign supersonic wind tunnels, evaluating over 40 different facilities. It focused on facilities with flight-like Reynolds numbers and Mach numbers in the range of 2.0 to 5.0. The study noted that the primary users of supersonic test facilities have been the Department of Defense and military aircraft manufacturers, although NASA's High Speed Civil Transport Program, if successful, will generate a need for the civil aircraft industry to conduct extensive supersonic testing of large aircraft with cruise speeds between Mach numbers 2.0 and 2.4. Having reviewed the requirements of the Department of Defense and the High Speed Civil Transport Program, the NFS Task Group on Aeronautical R&D Facilities concluded that existing test facilities such as 16S were essentially adequate to meet future needs. It recommended investing $42 million in 16S, including $24 million for drive-system upgrades. The balance would fund additional reliability upgrades, productivity

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Assessing the National Plan for Aeronautical Ground Test Facilities improvements, and other work consistent with previous studies. The ASEB endorses the proposed investment in upgrading the common 16S/16T drive-system and urges further consideration of additional activities to improve the reliability of the drive system motors and compressors. The actions recommended by the NFS include modifications to existing motors that should significantly enhance their reliability. Nonetheless, 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 an electrical arc-over with severe internal motor damage could reduce the operational capability of 16S (and 16T) for up to a year (Laster, 1994). This would have a severe impact if it occurred at a critical point in an aircraft development program. Additional drive-system improvements should be carefully considered to reduce the probability of such an occurrence. Compressors are also critically important to the health of the 16S/16T facility complex. A 16T compressor failure once took that facility out of service for four years, and a 16S compressor failure took nearly six years to repair. Arnold Engineering Development Center, however, has greatly reduced the expected severity of future compressor failures by replacing steel compressor rotor blades with plastic blades, and motor upgrades now seem to be more urgent than compressor improvements (Laster, 1994). Although the NFS acknowledges the importance of laminar flow technology for supersonic aircraft such as a high speed civil transport, it concludes that (1) upgrading 16S to provide this capability is not practical and (2) improvements in nozzle design and fabrication technologies are needed to build a new facility with levels of tunnel turbulence low enough to conduct desired laminar flow testing. Thus, the NFS recommends addressing this area by (1) using existing ground and flight test facilities to develop a first-generation high speed civil transport, (2) allocating $12 million to conduct Table 3-1b Recent Studies of Supersonic Ground Test Facilities Aeronautical Technologies for the Twenty-First Century (NRC, 1992a) Focus: High-leverage technologies for advancing commercial aeronautics Recommendation (relevant to supersonic wind tunnels): NASA should study the means to develop a supersonic, low-disturbance test capability at full-scale Reynolds numbers of 400–500 million and Mach 2–6. Cost: No estimate provided Review of Aeronautical Wind Tunnel Facilities (NRC, 1988) Focus: Military and civil requirements for wind tunnels and the ability of existing facilities, especially NASA wind tunnels, to meet them. Recommendation (relevant to supersonic wind tunnels): NASA should use existing technology to acquire a small (nozzle size of 20 × 30 inches) low-disturbance supersonic wind tunnel with a Mach number of 3.5 and the ability to accept additional nozzles to generate Mach numbers of 2.5–6 Cost: No estimate provided Assessment of NASA's Major Wind Tunnel Facilities with Respect to Current and Future National Needs (NASA, 1987) Ad hoc group of retired NASA, industry and DoD personnel, August 1987 Focus: Military and civil requirements for wind tunnels and the ability of existing NASA facilities to meet them. Recommendations (relevant to supersonic wind tunnels): Same as the 1988 NRC Review of Aeronautical Wind Tunnel Facilities (see above) Cost: $5 million for single Mach number capability of 3.5 research and development on how to construct a quiet supersonic tunnel operating at Mach numbers between 2.0 and 2.4 (the anticipated cruise velocity for a high speed

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Assessing the National Plan for Aeronautical Ground Test Facilities civil transport) to support the development of subsequent generations of high speed civil transports, and (3) evaluating the use of a flying laboratory such as a Tu-144, the Russian supersonic transport, as a near-term supplement to ground testing. Currently, supersonic laminar flow control is being evaluated on the NASA F-16 XL aircraft. Finding 3-1: U.S. supersonic wind tunnels adequately satisfy most current and potential test requirements. Shortfalls exist in productivity, reliability, maintainability, and laminar flow test capabilities. Recommendation 3-1: The Aeronautics and Space Engineering Board recommends upgrading the 16S facility at Arnold Engineering Development Center to include continued improvements in the 16S/16T motor drive and compressor systems. Research to define test requirements and develop practical facility concepts for supersonic laminar flow technology should be continued. REFERENCES Laster, M.L. 1994 . National Aeronautical Test Facilities Study Information Memorandum . Directorate for Plans and Requirements, Arnold Engineering Development Center . June 17, 1994 . Arnold Air Force Base . Tennessee . NASA (National Aeronautics and Space Administration) . 1992 . Preliminary Findings of Working Group Deliberations, Industry / DoD / NASA Workshop on Aerodynamics / Aero-thermodynamics / Acoustics Ground and Flight Testing Requirements . Held July23, 1992, at NASA Langley Research Center, Hampton, Virginia . NASA. 1987 . Assessment of NASA's Major Wind Tunnel Facilities With Respect to Current and Future National Needs . NASA Wind Tunnel Study Task Team . Washington, D.C. : NASA. NRC (National Research Council) . 1988 . Review of Aeronautical Wind Tunnel Facilities . Aeronautics and Space Engineering Board, NRC . Washington, D.C. : National Academy Press . NRC. 1992a . Aeronautical Technologies for the Twenty-First Century . Aeronautics and Space Engineering Board, NRC . Washington, D.C. : National Academy Press . NRC. 1992b . Future Aerospace Ground Test Facility Requirements for the Arnold Engineering Development Center . Aeronautics and Space Engineering Board, NRC . Washington, D.C. : National Academy Press .