So useful was the NACA 7 x 10-foot atmospheric wind tunnel of 1930 vintage that the onset of World...
...War II produced a 4-year backlog of test requests by the Army and Navy. Congressional authority to construct two more 7 x 10-foot tunnels at Langley, in addition to those already operational at Ames, was quickly forthcoming. The tunnels were built side by side and both went into operation in 1945.
One tunnel had a maximum airspeed of 300 mph, while the second could reach Mach 0.9 (about 675 mph). A 14, 000-horsepower electric fan powered the latter, but a 1600-horsepower electric fan sufficed for the former, as one would expect from the law that power requirements increase as the cube of the speed. Originally, neither 7 x 10-foot tunnel incorporated any unique or startling design features, but later ingenious modifications greatly enhanced their value in aerodynamic research.
In 1956, as interest in VTOL and STOL craft intensified, a 17 x 17-foot test section was installed in the settling chamber upstream of the test section in the 300-mph, 7 x 10-foot tunnel. The settling chamber provided appropriate conditions for testing craft making the transition from hovering to cruising flight. Whereas the test area of the 300-mph tunnel was expanded for low-speed work, the test section of its high-speed twin was constricted by a carefully designed "bump.'' Air flowing over the bump was accelerated to the transonic range even though the main airflow remained subsonic. This modification, though crude, led to a qualitative exploration of the transonic range that was just opening up after the conclusion of World War II. Many of the early X-series of aircraft that helped pierce the sound barrier went through tests on the transonic ''bump" in this tunnel.