 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 . 
