In the early days of wind tunnel design, every effort was made to make airflow in the tunnel test section as uniform as possible. Only under such idealized conditions could the aerodynamic forces acting on a plane in level flight be measured accurately. Unfortunately, aircraft do not remain long in level, straight-ahead flight; they roll, turn, and pitch. When an airplane rolls, the wing tips move much faster than the wing near the fuselage. The aerodynamic pressure built up by virtue of the wing's rotation produces a force opposite to that desired by the pilot for the roll maneuver. Aircraft designers wanted to know just how large this roll-created resisting or "damping" force was. They also wanted to know more about the forces encountered when an airplane flew in a curved flight path.
To answer such questions, NACA built a special Stability Tunnel at Langley. Placed in operation in 1941, it was a simple, continuous-flow wind tunnel. It had two interchangeable test sections, each about 6...
Langley stability tunnel, rim-driven rotating paddles set the air
swirling to simulate the rolling action of the model.
 feet in size. Into the first test section was built a set of rotating paddles that started the air swirling as if it were a giant (but gentle) eggbeater. The second tunnel section actually had a curve built into it to simulate curved flight.
For many years the stability tunnel provided data for predicting the maneuvering performance of aircraft and missiles. Its eventual demise was hastened by the perfection of oscillating model techniques, which could be employed in conventional tunnels, as well as better free-flight, powered models that could be flown in the cavernous full-scale tunnel. In 1958 the stability tunnel was dismantled and reassembled on the campus of Virginia Polytechnic Institute and State University at Blacksburg, Virginia, where it now functions as an educational and research and development tool.