Nearly thirty years ago historian Alex Roland published a very fine history of the National Advisory Committee for Aeronautics (NACA) with the main title, Model Research. It was an ironic title, cleverly emphasizing the aeronautical investigations that the NACA undertook with wind tunnels. In working on a new, short history of the NACA and NASA and I have come to appreciate the importance of that title. The NACA did much more than work with wind tunnels, but those instruments/facilities certainly enabled its path-breaking research.
The wind tunnels of the NACA, in which models of aircraft could be tested for their flight characteristics, certainly proved critical to the advance of aviation technology in the pre-World War II era. This had been the case for any pioneering aeronautical research prior to the establishment of the NACA, and such remained the same as the history of the agency progressed.
The NACA built its first wind tunnel at the Langley Memorial Aeronautical Laboratory (LMAL) in 1917. It was not much of a tunnel, copying one previously used at the British National Physical Laboratory. Definitely not a world-class facility, the NACA’s first wind tunnel was obsolete before it was even built. Knowing this, the NACA’s primary purpose for this “Model Tunnel” involved obtaining experience in how to use a wind tunnel.
NACA Wind Tunnel No. 1; it was a low-speed tunnel with no return circuit for the air passing through the test section. A 200-horsepower electric motor generated airspeeds of 90 mph in the 5-foot-diameter circular test section. Operational in the summer of 1920, it was used for testing models but the results were ineffective and non-duplicable elsewhere. This Wind Tunnel No. 1 also was more useful as a learning experience for agency engineers than anything else.
It was the Variable Density Tunnel at Langley that really set the NACA on a firm footing in terms of ground based research. This success led directly to the establishment of several other wind tunnels at Langley by the latter 1920s. Two were built first. The first had a 5 feet test section tilted at 90 degrees to study aircraft spins. The problem of a spinning aircraft was both common and poorly understood; “tail spins” proved fatal for many pilots of the era. And this tunnel helped aeronautics engineers understand the problem and develop countermeasures for it.
A second tunnel at Langley, a 7 x 10-foot Atmospheric Wind Tunnel (AWT), began operations in 1930. It provided the capability to study high-lift wings and general problems of stability and control in airframes of the 1930s. The AWT established itself almost as immediately as an exceptionally versatile research tool. Four additional wind tunnels followed with nearly the same size and capability. These revolutionized knowledge about airfoil shapes, airframe aerodynamics, guidance and control systems, and drag reduction. It also aided in pursuing understanding of the pressures on airframes, the compressibility problem, and aerodynamic loads and stresses on the aircraft.
Thereafter, the NACA built another wind tunnel to test propellers. The brainchild of Director of Research George W. Lewis new Propeller Research Tunnel (PRT) was large enough to place aircraft with their propellers operating in the test section. Scaling up was no small problem; the NACA has never built a wind tunnel larger than 5 feet in diameter before. The PRT demonstrated its worth almost at once. In addition to propeller research, it could be used for drag research and NASA engineers found that exposed landing gears contributed up to 40 percent of fuselage drag. Retractable landing gear emerged from this project as the state of the art for aircraft seeking greater speeds. PRT engineers also found that that multi-engine aircraft perform best when engine nacelles were built in-line with the wing. These results influenced every major aircraft of the latter 1930s and may be seen in the shape the DC-3 transport, and both the B-17 and B-24 bombers of World War II.
In addition, the NACA built in the middle 1930s its preeminent wind tunnel before World War II, the so-called “Full-Scale Tunnel” (FST). Built under the direction of Smith J. De France, the FST boasted a 30 x 60 feet test section, with an open throat that facilitated the installation of full-size aircraft. Two massive propellers, driven by two 4,000-horsepower electric motors, pushed air through the test section at speeds between 25 and 118 mph. Once completed in 1931, the FST tunnel building offered an imposing site on the Langley campus with its large air handling system and imposing brick office and research structure. Operating until the 1990s, the FST had a profound influence on the course of American aeronautical research and development. Likewise, a 19-foot pressure tunnel also helped to advance the state of the art in 1939 when completed. Virtually every advanced aircraft of World War II was tested in these two tunnels, so were many commercial vehicles and spacecraft from the NASA era.
Finally, the completion of the NACA’s High Speed Tunnel (HST), with a 22 inch test section, in 1934 enabled engineers to undertake research in the Mach 1 range. Containing a vertical test section, aircraft models were mounted facing downward and a blast of highly pressurized air would provide only a minute of test time to see compressibility flows and aerodynamic flutter on airframes in high speed conditions. This tunnel proved so useful that engineers lobbied for one with a 24-inch test section, put into operation late in 1934. It contained the first Schlieren photography system installed at Langley, allowing engineers to view dynamic airflows near Mach 1. This work eventually made it possible to build 400+ mph fighters for the United States during World War II.
These wind tunnels, from the Variable Density Tunnel to the Full Scale Tunnel and beyond, enabled the NACA to contribute path-breaking research in aeronautics for the United States. They were the instruments that made the agency, which were small and not well-funded, the best in the world at aeronautical R&D by the time of World War II. As historian Deborah G. Douglas concluded in a 1999 essay: “By the late 1920s, the NACA’s Langley Aeronautical Laboratory had begun to earn an international reputation, largely due to the construction of a trio of pioneering wind tunnels (the Variable Density Tunnel that became operational in 1922, the Propeller Research Tunnel in 1927, and the Full Scale Tunnel in 1931).”