[507] The NACA used and was used by its facilities. For many years the NACA had the best aeronautical research facilities in the world, and in many ways these facilities determined what the NACA would choose to do and be required to do. Having the world's only full-scale wind tunnel enabled the Committee to perform unique experiments, but it also dictated that the research program make full use of the full-scale tunnel. The same was true of the NACA's other research facilities, so that the agency waged an unending campaign to coordinate the needs of aeronautical research with full exploitation of the equipment on hand, retirement of old equipment, and development of new.
Headquarters was always a paper mill. It never conducted original research, nor did it maintain any research facilities other than its technical library. Editing, publishing, and distributing reports was as close as headquarters came to actually doing research; even here, the Langley laboratory performed many of the paperwork functions such as printing, photography, and artwork. The NACA headquarters thus consisted merely of its offices and library, located at the following sites in Washington, D.C.:
1915
State, War, and Navy Building,
Constitution Avenue, N.W.
1916-1918
Munsey Building, 1329 E Street,
N.W.
1918-1920
Bureau of Aircraft Production,
Building D, 4th Street and Missouri Avenue, N.W.
1920-1941
Main Navy Building, Constitution
Avenue
1941-1947
Leiter Mansion, Dupont Circle, 1500
New Hampshire Avenue, N.W.
1947-1954
1724 F Street, N.W.
1954-1958
Wilkens Building, 1512 H Street,
N.W.
1958
Dolley Madison Building, 1520 H
Street, N.W., acquired for NASA expansion
The NACA's research was conducted at its laboratories and their subsidiary stations. In order of their establishment and with their various titles, these were:
Langley Aeronautical
Laboratory
1920-1958
Langley Memorial Aeronautical
Laboratory (1920-1948)
Ames Aeronautical Laboratory
1940-1958
Lewis Flight Propulsion
Laboratory
1942-1958
Aircraft Engine Research Laboratory
(1942-1947)
Flight Propulsion Research Laboratory
(1947-1948)
Pilotless Aircraft Research
Station
1945-1958
Auxiliary Flight Research Station
(1945-1946)
[507] High Speed
Flight Station
1946-1958
NACA Muroc Flight Test Unit
(1946-1949)
High Speed Flight Research Station
(1949-1954)
Plum Brook Station
1956-1958
A fundamental law of fluid dynamics is that a body immersed in a moving fluid experiences the same forces as if the body were moving and the fluid stationary, given that the relative speed of the fluid and the solid object is the same in both cases. This means that the conditions surrounding an airplane in flight can be replicated by holding the plane stationary and moving the air past it at a velocity comparable to flight speeds. Thus, wind tunnels.
Advantages of wind tunnels over flight-testing are economy, safety, and research versatility. A model airplane can be tested in a wind tunnel at a fraction of the cost of building and operating a full-scale prototype, and the airworthiness of new and experimental designs can be tested without risking a pilot's life. Wind-tunnel testing can simulate flight under conditions more controlled and measurable than would be possible in flight test. Even before man first flew, the wind tunnel was the principal tool of the aeronautical engineer.
All wind tunnels have common features that circumscribe their characteristics and capabilities. All have a test section in which an airplane model or component-or even a complete airplane-can be fixed or suspended. The cross section may be round, oval, rectangular, or polygonal. Test sections may vary in size from a few inches up to the 40- by 80-foot dimensions of the Ames full-scale tunnel, still the largest in the world.* The test section may be open, closed, or ventilated.
Wind tunnels may be either return or no return. No return tunnels draw air from the atmosphere, pass it through a tube that includes a test section, and discharge it into the atmosphere. Such tunnels are simple and inexpensive to build, but are inefficient and limited in the types of flow they can generate. Most sophisticated tunnels use a return-type circuit in one of three basic variations. The single-return tunnel passes the same air around a closed loop. Many such tunnels are designed so that the laboratory building encompasses the test section, with the rest of the tunnel winding a circuitous path outside like an overgrown appendage. The double-return tunnel is shaped like a squared figure-eight with the corners rounded and the test section located at the juncture of the two loops. Annular-return tunnels are doughnut-shaped in cross section. Longitudinally, they look like a tube within a capsule; air pushed around the inner shell of the capsule is channeled down the tube in the center, which contains the test section. Annular-return tunnels are generally small and entirely con-tained within their research building.
A major advantage of closed tunnels is that they can be pressurized, a technique that remains one of the NACA's greatest contributions to wind-tunnel technology. Comparability between conditions of wind-tunnel tests on models and conditions experienced by full-scale aircraft in flight depends on a dimensionless mathematical quantity known as Reynolds number (named for the 19th-century British engineer Osborne Reynolds). The Reynolds number is a flow-similarity parameter that describes forces acting on a body in motion with respect to the fluid in which it is immersed. The number is directly proportional to the size of the body and the density and relative.....
....speed of the fluid, and inversely proportional to the viscosity of the fluid. Other things being equal, a model "moving" with respect to an airstream would have a smaller Reynolds number than a full-scale plane in flight. The easiest way to equalize the Reynolds numbers-and thus to obtain comparable flow conditions for the plane and the model-is to increase the speed or density of the airstream in which the model is immersed. To increase airspeed within a wind tunnel is a complicated and expensive undertaking that would violate equality of the ratio of airspeed to speed of sound, another condition for strict comparability. In a return-type tunnel, however, it is comparatively easy to increase air density by increasing air pressure. The NACA's first pressurized tunnel-Max Munk's variable-density tunnel of 1923-could pressurize the air to 20 atmospheres, making tunnel results on a 1/2oth-scale model comparable to those of a full-size plane in the atmosphere.
The speed of a wind tunnel is the velocity of the airflow measured at the test section. Tunnels are customarily classified in the following speed ranges:
Class
.
Low-speed
0 to 0.5
0 to 380
High-speed
0.5 to 0.9
380 to 684
Transonic
0.7 to 1.4
532 to 1,064
Supersonic
1.4 to 5.0
1,064 to 3,800
Hypersonic
5.0 to 10.0
3,800 to 7,600
Hypervelocity
10.0 and above
7,600 and up
**Mach no. equals stream velocity/velocity of sound.
As aircraft speeds increased, wind-tunnel speeds had to increase. Above 300 to 400 mph, the compressibility of air begins to affect the results of scale-model tests. Thereafter, not only the Reynolds number but also the actual mach number must be matched between the model and the aircraft. A plane moving through the air at low speed sets up something like a bow wave, a layer of compressed air at the leading edges that moves ahead of the plane at the speed of sound, pushing the approaching air out of the way. When the plane moves near or above the speed of sound, the air has no time to get out of the way, and its collision with the plane produces shock waves-patterns of energy dispersion-with unique aerodynamic effects. High-speed wind tunnels are expensive to build and operate (the power required increases as the cube of the speed) and present major problems in turbulence, heating, and flow condensation, but they are indispensable to accurate testing in high-speed regimes of flight. Some of the NACA's greatest achievements were the development and application of high-speed tunnels, especially in the anomalous transonic region.
In most conventional wind tunnels the air is moved by fans powered by electric motors. Some tunnels, however, produce the airstream differently: Blowdown tunnels use a jet of air from a pressurized reservoir. Induction tunnels use a stream of air flowing into a vacuum chamber. Hypervelocity tunnels may combine these methods, passing air from a pressurized vessel across the test section and into an evacuated vessel at pressure ratios of several hundred. Although blowdown and induction-drive systems can produce extremely high-velocity air, they are severely limited by the brief availability of that air and their limited ability to modulate the velocity. At the extreme end of the spectrum is the counterflow tunnel in which a model is shot from a gun into a high-velocity airstream from a blowdown or induction-drive system. Some wind tunnels in the NACA laboratories shared drive systems, and some blowdown tunnels used compressed air stored in nearby pressure tunnels.
These basic characteristics, common to most wind tunnels, by no means encompass all the features, capabilities, and equipment involved in wind-tunnel research. Almost all wind tunnels employ a complex array of balances and other measuring devices designed specifically for the purpose. Most closed-circuit tunnels use tunnel vanes to guide the airflow smoothly around the corners in the circuit. Most tunnels use complex arrays of settling chambers, screens, and throat contractions to smooth and straighten the airstream as it accelerates into the test section. A variety of model-support systems is used, depending on the configuration of the test object. Some [512] tunnels use smoke to help visualize air flow. Some are rigged for Schlieren photography, a special technique that records shock waves produced at high speeds. Some tunnels are refrigerated to produce ice on the models like that encountered under certain flight conditions.
In fact, wind tunnels have been designed to replicate nearly every condition encountered by airplanes in flight. There are vertical wind tunnels to study aircraft spinning characteristics, gust tunnels to determine the effect of fluctuations in the airstream, and curved-flow tunnels with variable geometry in the test section to determine flight characteristics in turns or maneuvers. There are even free-flight tunnels in which the model floats free and the test section cants to simulate different angles of attack.
The characteristics of a tunnel are not necessarily fixed permanently during construction. Many NACA tunnels saw long and varied service, upgraded to incorporate advances in wind-tunnel technology that adapted them to modern regimes of flight. The most frequent modification was repowering to produce higher velocities in the test section. Improved instrumentation and mountings were less dramatic but equally important.
The complexity of NACA tunnels-the vague distinction between a tunnel's basic equipment and the changing battery of auxiliary equipment that supported it, the shared housings and drive systems that many tunnels employed, and the repeated modifications that some tunnels underwent-makes it difficult to present a uniform picture of tunnel characteristics. Still more difficult to achieve is an accurate estimate of costs. The following lists contain the available data on the test section, circuit, speed,....
[514] ....and drive systems of all major NACA tunnels. Cost information is not sufficiently reliable to merit inclusion, but one example will suggest the range of expenses involved. The first NACA wind tunnel (the 5-foot atmospheric tunnel built at Langley in 1920) cost about $45,000. The 10-by 10-foot supersonic tunnel built at Lewis in the early 1950s cost $35,000,000.
The wind tunnel that dominated NACA research could not provide all the answers the Committee needed to solve the problems of flight. Over the years the NACA constructed other research laboratories, buildings, and equipment to answer questions not aerodynamic in nature. These facilities are especially hard to trace because they frequently had no building of their own but occupied space in office buildings that housed a number of research functions. No attempt has been made to inventory these facilities in the same detail as the NACA wind tunnels, but a list of major nontunnel facilities at Langley may indicate the great variety of NACA equipment. Langley had more of these facilities than any other laboratory or station.
Facility
.
NACA [Towing] Tank No. 1
1931
Aircraft Engine Research
Laboratory
1934
Structures Laboratory
1940
Seaplane Impact Basin
1942
NACA Tank No. 2
1942
Helicopter Apparatus
1944
Aircraft Loads Building
1945
Aircraft Loads Calibration
Laboratory
1945
Physical Research Laboratory
1945
Instrument Research Laboratory
1946
Pilotless Aircraft Research
Laboratory
1946
Landing Loads Track
1955
High-Speed Hydrodynamics
Facility
1956
Note the rapid tempo of expansion of facilities during World War II, a measure of the NACA's concentration on wind-tunnel research in the 1920s and 1930s. And compare the small number of these with the extensive family of tunnels described in the following section.
Langley Aeronautical Laboratory
[515] Variable-Density Tunnel
Propeller-Research Tunnel
5-Foot Vertical Wind Tunnel
Atmospheric Wind Tunnel (AWT) (7- by 10-Foot Wind Tunnel)
Full-Scale Tunnel
11-inch High-Speed Tunnel
24-Inch High-Speed Tunnel
[517] 15-Foot Spin Tunnel (15-Foot Free-Spinning Tunnel)
8-Foot High-Speed Wind Tunnel
[518] 5-Foot Free-Flight Tunnel
Two-Dimensional Low-Turbulence Tunnel (Ice Research Tunnel)
19-Foot Pressure Tunnel
12-Foot Free-Flight Tunnel
Low-Turbulence Pressure Tunnel
20-Foot Spin Tunnel (20-Foot Atmospheric Free-Spinning Tunnel)
16-Foot High-Speed Tunnel
Stability Tunnel
[521] 9-Inch Supersonic Tunnel
Gust Tunnel
Flutter Tunnel
300-Mph 7- by 10-Foot Tunnel
High-Speed 7- by 10-Foot Tunnel
11-Inch Hypersonic Tunnel
4-by 4-Foot Supersonic Tunnel
26-Inch4 Transonic Blow down Tunnel
Gas-Dynamics Laboratory (Hypersonic Aero thermal-Dynamics Facility)
8-Foot Transonic Pressure Tunnel
Unitary 4- by 4-Foot Supersonic Tunnel
9-by 6-Foot Thermal Structures Tunnel
20-Inch Hypersonic Tunnel
[524] Ames Aeronautical Laboratory
16-Foot High-Speed Tunnel
40- by 80-Foot Wind Tunnel
1- by 3.5-Foot High-Speed Tunnel
1- by 3-Foot Supersonic Tunnel
12-Foot Pressure Tunnel
[526] 6-by 6-Foot Supersonic Tunnel
Supersonic Free-Flight Tunnel
2- by 2-Foot Transonic Tunnel
Unitary 11- by 11-Foot Transonic Tunnel
Unitary 9-by 7-Foot Supersonic Tunnel
[527] Unitary 8- by 7-Foot Supersonic Tunnel
10- by 14-Inch Hypersonic Tunnel
14-Foot Transonic Tunnel
1-Foot Hypervelocity Tunnel
Lewis Flight Propulsion Laboratory
Altitude Wind Tunnel
Icing Research Tunnel
8- by 6-Foot Supersonic Tunnel
Unitary 10- by 10-Foot Supersonic Tunnel
*At the time of
this writing, the tunnel was being modified to provide an 80- by
120-foot test section.
Unless otherwise indicated, all data on the NACA wind tunnels was derived from Donald D. Baals and William R. Corliss, Wind Tunnels of the National Aeronautics and Space Administration (SP-440; Washington: NASA, 1981).
1. Research and Development Board, Committee on Aeronautics. "U.S. and Foreign Wind Tunnels in Operation, under Construction, or Authorized," AR 26/11.5, 4 Feb. 1948.
2. The Working Committee of the Aeronautical Board, "Survey of Wind Tunnels," preliminary copy, 1 Jan. 1947.
3. Alan Pope, Wind Tunnel Testing (New York: John Wiley and Sons, Inc., 1947), pp. 16-29.
4. Bernard A. Goethart, Transonic Wind Tunnel Testing, ed. by Wilbur C. Nelson (New York: Pergamon Press, 1961), pp. 383-89.
