 The project suffered a setback on 12 March 1969 , when the rotor on prototype # 3 ( s / n 66 @-@ <unk> ) struck the fuselage and caused the aircraft to crash , killing the pilot , David A. <unk> . The accident occurred on a test flight where the pilot was to manipulate the controls to excite <unk> oscillations ( or half @-@ P hop ) in the rotor ; <unk> is a vibration that happens once per two main rotor revolutions , where P is the rotor 's rotational speed . The accident investigation noted that safety mechanisms on the controls had apparently been disabled for the flight . The investigation concluded that the pilot @-@ induced oscillations had set up a resonant vibration that exceeded the rotor system 's ability to compensate . After the investigation , the rotor and control systems were modified to prevent the same problem from occurring again . 
