AppendixWind Tunnels—The Tools of the Trade |
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NASA Langley Research Center’s world-class stature in aeronautics is a direct result of the combination of the expertise and dedication of its staff and the unique test capabilities provided by its facilities. Throughout its long history, Langley has strived to anticipate requirements for new testing facilities, to conduct pilot testing of evolving facility concepts, and to provide updated test capabilities during new aircraft development programs. As a result of these efforts, Langley offers high quality, unique test capabilities in the areas of fundamental aerodynamics, aerodynamic performance, flight dynamics, aeroelasticity and flutter, spinning, structures and materials, impact dynamics, aircraft landing dynamics, and piloted simulators. The discussion of Langley contributions in the text of this document illustrates the critical roles and responsibilities that were assigned to the Langley facilities during the development of these specific military aircraft. As might be expected, wind-tunnel facilities played a key role in the development process. Today, Langley operates wind tunnels that provide critical aerodynamic, flight dynamic, and aerothermodynamic data at speeds ranging from low subsonic conditions to hypersonic conditions. Donald Baal’s and William Corliss’ excellent review of NASA wind tunnels will provide the reader with extensive information on these wind-tunnel facilities (ref. 5). The discussion of the military aircraft in this document reveals that certain Langley wind tunnels provided most of the data for the development of these military aircraft. These wind tunnels included the
New wind-tunnel test capabilities, such as those provided by the National Transonic Facility (NTF) at the Langley Research Center, and reductions in the NASA aeronautics operating budgets in the 1990’s have resulted in the closure or transfer of the first four Langley wind-tunnel facilities on this list. The remaining wind tunnels on the list are still operating and providing valuable data for another generation of aircraft. Brief descriptions of the listed wind-tunnel facilities follow. |
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Langley 4- by 4-Foot Supersonic Pressure Tunnel |
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Design work on the first large supersonic wind tunnel at Langley began in February 1945, with an effort to develop a 4- by 4-ft wind tunnel capable of providing test speeds up to a Mach number of 2. Delayed by labor strikes and other difficulties, the tunnel did not begin operations until May 1948. Initially powered by 6,000-hp motors (because of limited electrical power at the time), the facility was repowered in 1950 with 45,000-hp motors. Many historically significant military aircraft configurations and missiles were tested in the facility, including the Century Series fighters (F-100, F-102, F-104, and F-105), the TFX (precursor to the F-111), and the B-58 Hustler supersonic bomber. The 4- by 4-Foot Supersonic Pressure Tunnel was dismantled in 1977, and its drive motors, cooling towers, and support facilities were used in the construction of the National Transonic Facility (NTF), which was built on the same site. NASA saved an estimated $20 million in construction costs by utilizing the components of the old tunnel to provide key elements for the new cryogenic NTF facility. | |
Langley 7- by 10-Foot High-Speed Tunnel |
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In 1945, Langley initiated operations in two 7- by 10-ft wind tunnels, which were built side by side in the same building. One of these tunnels, known as the Langley 300-MPH 7- by 10-Foot Tunnel, was used for basic and applied aerodynamic research at speeds up to 300 mph. The second tunnel, known as the Langley 7- by 10-Foot High-Speed Tunnel, was used for research at transonic test conditions. The 7- by 10-Foot High-Speed Tunnel was the site of some of the most important Langley aerodynamic research for military aircraft. Researchers utilized the facility to conceive and develop breakthrough technical concepts such as the variable-sweep wing and vortex-lift strakes. It was also used to assess and support the development of most of the high-performance U.S. military fighters. Regarded as one of the most productive research wind tunnels ever operated at Langley, the 7- by 10-Foot High-Speed Tunnel was closed and dismantled in 1993 because of limitations in operating budgets and workforce. | |
Langley 8-Foot Transonic Pressure Tunnel |
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Put into operation in 1953, the Langley 8-Foot Transonic Pressure Tunnel was used to produce some of the most important aeronautical breakthroughs for civil and military aircraft. Located on the original NACA property (East Area) at the Langley Air Force Base, the facility was a follow-on to the 8-Foot High-Speed Tunnel and incorporated the breakthrough slotted-wall concept for transonic testing. The new slotted tunnel was used to develop supercritical wing technology, winglets, and other innovative concepts that are now routinely used by military aircraft. Special test techniques, including the fluorescent oil flow visualization concept, were also developed and matured in the facility. The tunnel was utilized extensively for the support of military aircraft programs, including the assessment of competing designs, analysis of the performance capabilities of new configurations, and development of improvements and problem-solving concepts for evolving aircraft. Following a multiyear research investigation of hybrid laminar flow control, the 8-Foot Transonic Pressure Tunnel was closed in 1995. The National Transonic Facility at the Langley Research Center and the Langley 16-Foot Transonic Tunnel have now taken on the transonic test workload. | |
Langley 30- by 60-Foot Tunnel |
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The design of the Langley 30- by 60-Foot (Full-Scale) Tunnel was initiated in 1929, and the tunnel was put into operation in 1931 for the intended purpose of obtaining full-scale aerodynamic data for the biplane configurations of the day. The tunnel contributed to military, commercial, and general aviation aircraft technology for over 64 years with a wide variety of test techniques including free-flight tests of remotely controlled, dynamically scaled models. In 1985, the Langley Full-Scale Tunnel was named a National Historical Landmark. NASA closed this historic wind tunnel in 1995 as a cost-saving action. It was subsequently transferred to the Old Dominion University, which now operates the facility for a diverse customer base, including race car enthusiasts. | |
Langley 16-Foot Transonic Tunnel |
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Since becoming operational in 1941, the Langley 16-Foot Transonic Tunnel has undergone a series of improvements and upgrades. Initially capable of speeds up to a Mach number of 0.7, the tunnel can now test over a speed range of up to a Mach number of 1.29. The 16-Foot Transonic Tunnel is an atmospheric, closed-circuit tunnel with an octagonal test section that measures 15.5 ft across the flats. Twin 34-ft-diameter drive fans provide power, and the tunnel has an exceptionally low disturbance level through the transonic Mach number range. From its earliest operations, the facility has specialized in propulsion-airframe integration issues, ranging from propeller research and engine cooling research during World War II to the design and integration of today’s advanced multiaxis thrust-vectoring and reversing propulsion concepts. The breakthrough nonaxisymmetric (2-D) nozzle and thrust-vectoring nozzle technologies were primarily developed in this facility. The availability of high-pressure airlines, water cooling lines, and hydraulic lines permit this unique facility to conduct in-depth studies of powered advanced military configurations. Because of the large test section, which results in low blockage values for typical models, this facility has also been extensively utilized for transonic aerodynamic studies on a wide range of military aircraft. The 16-Foot Transonic Tunnel is operational and continues to contribute to military aircraft development programs. This tunnel operates in conjunction with an auxiliary test facility, known as the Jet Exit Test Facility, to provide design information for the development and maturation of innovative nozzle-propulsion system concepts and to support specific NASA and Department of Defense aircraft development programs.
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Langley 20-Foot Vertical Spin Tunnel |
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Following the initial operations of a 15-ft-diameter spin tunnel in 1935, Langley designed and developed the Langley 20-Foot Vertical Spin Tunnel (Spin Tunnel) and initiated operations in 1941. The Langley Spin Tunnel is a closed-throat, annular return wind tunnel that operates at atmospheric conditions. The 12-sided test section is 20-ft across by 25-ft high. The test section velocity can be varied up to approximately 85 ft/sec. A 3-bladed, fixed-pitch fan powered by a 400-hp direct-current motor that is located above the test section produces test section airflow. This motor is equipped with a control system designed to allow rapid changes in fan speed, which results in rapid flow accelerations in the test section. Dynamically scaled, free-flying models are used to investigate the spin and spin-recovery characteristics of aircraft configurations. To study spin characteristics, the model is hand launched with prerotation into the vertically rising air stream. The tunnel operator varies the tunnel speed so that the spinning model remains in equilibrium in front of video cameras for documentation of results. Direct observation of the test article is possible during tunnel operations via panoramic control room windows. The spin-recovery characteristics of aircraft are studied by using remote actuation of the model aerodynamic control surfaces. The size of emergency spin-recovery parachutes systems for flight test aircraft is also determined with specialized tests of scaled parachutes. Free-spin data have been acquired in the Spin Tunnel and used by nearly all U.S. military fighter programs during and since World War II. The tunnel has now logged over 500 different aircraft studies. Nearly all U.S. attack and jet trainer programs have used Langley free-spin test data. In addition to free-spin tests, the facility permits the measurement of aerodynamic forces and moments during spin conditions with a unique rotary-balance apparatus. In addition to providing aerodynamic input data for analyses and theoretical studies of spins, the rig has been used to provide electronically scanned pressures on models during simulated spin motions.
The Langley 20-Foot Vertical Spin Tunnel is the only facility in the United States currently configured for free-spin tests and determination of emergency spin-recovery parachute requirements for military aircraft. The facility continues its service to the nation for both military and civil spin studies. The majority of work, however, supports high priority military aircraft programs. | |
Langley 16-Foot Transonic Dynamics Tunnel |
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The NASA Langley 16-Foot Transonic Dynamics Tunnel (TDT) became fully operational in 1960. It is now internationally regarded as the leading wind tunnel for performing flutter tests of large aeroelastically scaled full-span models at transonic speeds. The TDT is the only facility in the world capable of studying a full range of aeroelastic phenomena at transonic speeds. The tunnel is used by the aircraft industry to clear new designs for safety from flutter, to evaluate solutions to aeroelastic problems, and to study aeroelastic phenomena at transonic speeds. The TDT is used to perform flutter clearance investigations and to investigate flutter trends and aeroelastic characteristics of fixed-wing and rotorcraft configurations. The TDT is also used to perform a variety of active controls tests, to determine the effect of ground-wind loads on launch vehicles, and to make steady and unsteady aerodynamic pressure measurements to support computational fluid dynamic (CFD) code development. The TDT is a closed-circuit, continuous-flow wind tunnel capable of tests at stagnation pressures from near zero to atmospheric and over a Mach number range up to 1.2. The test section of the TDT is 16-ft square with cropped corners. One feature of the TDT that is particularly useful for aeroelastic testing is a group of bypass valves that connect the test section area to the opposite leg of the wind-tunnel circuit downstream of the drive fan motor. In the event of model instability, such as flutter, these quick-actuating valves are opened, which causes a rapid reduction in the test section Mach number and dynamic pressure, which may result in stabilizing the model. Other features that make the TDT uniquely suited for aeroelastic testing include high visibility of the model from the control room, a highly sophisticated data acquisition system, flow oscillation vanes upstream of the test section that can be used to generate sinusoidal gusts, a variety of model mounting and suspension systems such as cantilever sidewall mounts for component models and a 2-cable-suspension system for full-span free-flying models, safety screens that protect the tunnel fan blades from debris in case of a model failure, and state-of-the-art instrumentation and test equipment.
Tests can be performed in the TDT with air as the test medium; however, the most distinguishing feature of the tunnel is the use of a heavy gas, presently R-134a refrigerant, as the primary test medium. R-134a is about four times as dense as air, and yet sound travels half the speed in R-134a as it does in air. These properties of higher density and lower sonic speed have beneficial effects on the design, fabrication, and testing of aeroelastically scaled wind-tunnel models that must accurately represent full-scale counterparts. Physically larger models may be built, thereby simplifying the model fabrication process. The scaled natural frequencies of these larger models are lower, resulting in lower flutter frequencies, thereby reducing the risk of model destruction during flutter. Other advantages resulting from the use of a heavy gas are a nearly three-fold increase in Reynolds number and a lower tunnel-drive horsepower requirement. The TDT has played a key role in the development process for virtually all military aircraft in the U.S. inventory, including transports and high-performance fighters. The facility is extremely active and supports civil and military programs. | |
Langley Unitary Plan Wind Tunnel |
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The Langley Unitary Plan Wind Tunnel has been in continuous operation since construction was completed in 1955. Congressional approval for the tunnel was provided by the Unitary Wind Tunnel Plan Act of 1949, which stated that the objective was to “Promote the national defense by authorizing a unitary plan for construction of transonic and supersonic wind-tunnel facilities…” Developmental tests of virtually every supersonic military aircraft, missile, and spacecraft in the current U.S. inventory were performed in the Unitary Tunnel. In addition, methods for predicting supersonic aerodynamic performance have been developed through basic experimental fluid mechanics research conducted in the Unitary Tunnel. The Unitary Tunnel is a closed-circuit pressure tunnel with two 4-ft by 4-ft by 7-ft test sections, and a Mach number capability ranging from 1.47 to 4.63. The Langley Unitary Plan Wind Tunnel participates in advanced military aircraft and missile research.
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