During World War II it became obvious that the National Advisory Committee for Aeronautics (NACA) required new tools to pursue a high-speed/high altitude research program. The National Unitary Wind Tunnel Act of 1949 addressed these needs, providing NACA funds to build three new supersonic wind tunnels at its laboratories, to upgrade other NACA facilities, and to support selected facilities elsewhere.
The NACA effort began in April 1945 with a letter to the committee’s director of research, George W. Lewis, from an engineer at the Aircraft Engine Research Laboratory (AERL) in Cleveland. Bruce Ayer wrote because he believed that the idea had not given “sufficient consideration” to the needs of supersonic flight. Ayer suggested that the advent of jet propulsion ensured that research problems for the foreseeable future would emphasize high-speed flight. He recommended, among other things, building new instruments for researching in this flight regime at existing NACA facilities.
Ayer received a polite and non-committal response from Lewis, but not until the following summer when NACA representatives returned from Germany did the need for new wind tunnels win support at NACA headquarters. When first viewed in 1945, the 100,000-horsepower water-driven supersonic wind tunnel built by the Germans just outside Munich greatly impressed the NACA representatives, as did a planned 500,000-horsepower tunnel designed to produce airspeeds between Machs 7 and 10. NACA leaders concluded that “the Committee should at once take steps to preempt this field of high-speed research and an aggressive and vigorous policy should be adopted in the interest of keeping America first in scientific development along these lines.”
With this decision, the die for the agency’s future had been cast. In essence, in a period of only nine months new supersonic facilities had started as an interesting but essentially unfeasible idea offered by a journeyman research engineer. It had gained support along the review process, and with its adoption through the NACA committee structure new supersonic wind tunnels had become the cornerstone of the agency’s plans for future aeronautical research.
At the same time, the Army Air Forces had also been working quietly on a proposal remarkably similar to that of the NACA. Sensing that the NACA was already on to something important for the future, and seeing firsthand the German research facilities under construction at the end of the war, in June 1945 the USAAF began developing their own proposal to support research for a new generation of jet fighters that would revolutionize aerial combat. The Army Air Forces investigated the need for new supersonic research facilities informally at Wright Field until October 1945, and then established a formal committee to prepare plans for an “air engineering development center.” On December 10, 1945, the USAAF published a formal plan and sent it through Army Air Forces and War Department channels in search of support.
The NACA, not wanting to lose this opportunity to advance supersonic flight technology—in the same way that it had with the jet propulsion revolution of the early 1940s—pursued the effort with diligence. The Army Air Forces, concerned that the NACA might be unable to make the rapid advances the military desired and at a fundamental level wanting a “piece of the action” for itself, was equally tireless. Both started as rivals in the unitary wind tunnel plan, only to be forced into cooperation through an intense political process.
Convergence of these two initiatives became essential for the effort to have much chance in Congress. At the April 25, 1946, meeting of the NACA, the Committee appointed Arthur E. Raymond of Douglas Aircraft Corporation to merge the two proposals into a single package acceptable to all concerned. In June 1946 he recommended a unitary wind tunnel plan incorporating the main features of the rival proposals, a national supersonic research effort for the NACA, and an air engineering development center for the Army Air Forces. The principal addition recommended by the Raymond panel was a provision for wind tunnels at universities, both to allow independent testing and research and to serve as training tools for engineers of the future.
The estimated $2 billion effort recommended by Raymond, which most believed was still not enough to do everything, appeared to many advocates as a “poison pill” for the whole effort. Always a voice of reason, Hugh L. Dryden of the National Bureau of Standards recommended an approach to supersonic facilities less aggressive than those advocated by others.
The National Unitary Wind Tunnel Act of 1949 as implemented by the NACA and by the Air Force included five wind tunnel complexes, one each at the three NACA Laboratories and two wind tunnels plus an engine test facility at what would eventually become known as the Arnold Engineering Development Center (AEDC). These ground test facilities were built and operated to meet the needs of industry, the military services, and other government agencies. Primarily, these organizations needed large, or as near to flight as possible, Reynolds Number (Rn) testing of supersonic aircraft and missiles and high-speed/high altitude testing of engines.
The NACA committed to the construction of five supersonic wind tunnels located at its various research laboratories. At the Langley Memorial Aeronautical Laboratory in Hampton, Virginia, a 9 in. supersonic tunnel was operating, in which much of the pioneering research on swept wing drag reduction was performed. Langley also committed to designing and building a 4 X 4 ft. supersonic research wind tunnel. This tunnel would become operational in 1948 following installation of 45,000 horsepower drive system.
At the Ames Aeronautical Laboratory, in the Bay area of California, two supersonic research wind tunnels were constructed. These included the 1 X 3 ft. SWT that operated to a maximum test section airspeed of Mach 2.2. A larger 6 X 6 ft. supersonic research tunnel was also constructed at Ames. This tunnel is notable as it was the first large supersonic tunnel that made use of the asymmetric supersonic nozzle that would be successfully used in several of the yet to be designed Unitary Plan Wind Tunnels. It also contained for purposes of flow visualization a 50 in. Schlieren window system. Tests performed in the Ames tunnels included research on wing shapes, dynamic stability, aircraft control, panel flutter and air inlet design.
Finally, at the Lewis Flight Propulsion Laboratory in Cleveland, Ohio, a large 8 X 6 ft. transonic wind tunnel with the capability to operate at test section airspeeds from Mach 0.4 to 2.0, was built for testing aircraft power plants and was operational by 1949. This wind tunnel was an open-circuit tunnel where the air was vented to the atmosphere in order to dispose of the engine exhaust.
Through the design of these supersonic wind tunnels, NACA engineers perfected their understanding of the differences between supersonic and subsonic wind tunnels. Lessons learned by NACA engineers in the operation of these five supersonic research wind tunnels at the three NACA sites laid the groundwork for that organization’s future successes in designing and building modern aircraft.