Through 1932 NACA's wind tunnels were all subsonic. Indeed, one might ask why NACA should even consider building higher-speed wind tunnels when supersonic flight was contemplated only by a few visionaries. Actually, NACA began designing its first high-speed tunnel in 1927, a time when commercial aircraft still cruised at 100 to 150 mph. The raison d'etre for a high-speed tunnel was that while most airplanes were a long way from the sonic barrier, their propeller tips were not. Actually, some racing planes and military craft had already reached Mach 0.5 (about 350 mph). Already a few aerodynamicists could see where the future lay; they began studying this new speed regime.
When Joseph S. Ames became Chairman of NACA in 1927, he gave priority to high-speed wind tunnels and the development of transonic and supersonic research capabilities. Shortly thereafter, John Stack of Langley initiated design studies of a small, highspeed tunnel. One of his first obstacles was the lack of electrical power to run such a tunnel at Langley. The power required to operate a wind tunnel varies as the third power of the wind velocity. Existing wind tunnels rarely topped 120 mph, only one-sixth the speed of sound. The number 6 raised to the third...
...power is 216, a stupendous increase in power requirements for Mach 1 operation. In searching for a power source, Langley engineers noted that a large reservoir of energy was stored in the 5200 cubic feet of air compressed to 20 atmospheres in the variable density tunnel-energy that was thrown away each time the tank was blown down to change models. Dr. Lewis asked, "Why not use it?" That is, why not pipe the VDT exhaust through a much smaller tunnel and use its jet as a high-speed windstream. Thus the Langley 11-inch High-Speed Tunnel (11" HST) was born.
The 11-inch HST was set on end, with the test section oriented vertically. Small models were aimed downward into the mainstream air entering at the bottom of the tunnel. A special design feature of the 11-inch HST was the use of an annular injector down stream from the test section. The blast of highvelocity air from the VDT exhaust entrained the slower moving mainstream air and accelerated it to high speeds. Both were exhausted upward from a cone-shaped nozzle. The injection scheme permitted runs of only about 1 minute before the VDT's pressure plummeted to useless values. These short runs, however, were sufficient to demonstrate the sharp rise in drag, the loss of lift, and the changes in pitching moments that occur near Mach 1.
So successful was the 11-inch tunnel that a bigger one with a 24-inch test section was quickly designed. It was put into operation in October 1934 and also used the exhaust air from the VDT. The first Langley schlieren system was installed in this tunnel. Engineers were now able to view the dynamic phenomena occurring as air near Mach 1 flowed over airfoils and fuselages. By simultaneously viewing flow phenomena and recording the pressure distributions over various wings and propellers, aerodynamicists could pinpoint isolated areas where shock waves formed or airflows separated and therefore limited the useful speed of the entire airfoil. By correcting airfoil design at these local trouble spots, the performance potential of the entire airfoil could be raised.
With the help of the 24-inch High-Speed Tunnel, NACA in 1939 was able to develop and provide aerodynamic data on a family of new high-speed airfoils for the American aviation industry. These airfoils quickly found their way into the high-speed aircraft propellers that powered the 500-mph American fighters that dominated the skies in the latter part of World War II.