The purpose of a wind tunnel is to simulate flight, but in the conventional tunnel the models of fullscale aircraft are attached to supports so that aerodynamic forces can be measured. The models are not free. A plane's maneuverability cannot be gauged completely with these restrictions on motion.
Since full-scale aircraft are expensive to build, test fly, and modify-to say nothing of the possible loss of test pilots-Langley engineers decided to build a free-flight wind tunnel. The basic idea was to let a model glide in a wind tunnel that is actually tilted at the aircraft's glide angle. Thus the unpowered model, nose tilted down at its glide angle, remains stationary and horizontal in the rising airstream of the tilted tunnel, much like a hawk or buzzard hovers in air currents. Maneuverability and flight performance are tested as a "pilot" outside the wind tunnel manipulates the model's control surfaces via electrical signals sent through thin wires trailing behind the model.
Two of these tillable free-flight wind tunnels were constructed at Langley: the first, 5 feet in diameter in 1937 and the second, 12 feet across, in 1939. These tunnels were useful with radically new aircraft where no reservoir of flight experience was available, namely, tailless aircraft, planes with delta and skewed wings, and vertical takeoff and landing/short takeoff and landing (VTOL/STOL) vehicles. The 12-foot freeflight tunnel was used into the early 1950s, when it was supplanted by powered models flown in the Langley full-scale tunnel, which had ample flying room in its 30 x 60-foot test section.