In September 1969, Cheyenne prototype #10 (s/n 66-8835) underwent wind tunnel testing at NASA Ames Research Center, to research the half-P hop and drag issues. The engineers did not realize that the fixed mounts used to secure the aircraft in the wind tunnel would not allow the helicopter to move relative to the rotor, as it did in flight. As a result, there was no natural damping of the rotor pitching motion. The remote controllers' lack of sensory feedback from helicopter compounded the situation. During high speed testing to replicate the half-P hop vibration, the rotor oscillations quickly accelerated out of control and struck the tail boom, resulting in the destructive breakup of the helicopter.