NACA ultimately built a series of two freespinning vertical tunnels at Langley. A 15-foot diameter spin tunnel was placed in operation in 1935. Langley's present 20-foot Free Spinning Tunnel began its work in 1941. In all these tunnels, air is drawn upward through the test section by a fan at the top. After passing through the fan, the air circulates through turning vanes that direct it down into an annular return passage and up through the test section again.
Spin tunnels are extremely simple.
No provisions need be made for mounting a model or measuring aerodynamic forces. A rising column of air is all that is needed. The researcher launches the aircraft model into the rising air by hand from a platform. A flick of the hand imparts a spin and, as the model spins downward, the operator increases the tunnel wind speed until the model's fall is just balanced by the uprushing air, like a circling hawk buoyed by rising thermals. Cross section of the Langley 20-foot spin tunnel.