 Cheyenne prototype # 6 ( s / n 66 @-@ <unk> ) began conducting weapons testing at Yuma Proving Ground , Arizona , demonstrating the ability for the gunner and pilot to accurately fire on separate targets on each side on the helicopter . Towards the end of 1970 , the Army funded work on TOW missile guidance and night sighting systems . Prototypes # 6 and # 9 were also tested and evaluated at Yuma Proving Ground from 30 January to 23 December 1971 , to determine if stability and control systems were sufficient . Deficiencies were identified in lateral directional stability , uncommanded motion during maneuvering , high vibration , and poor directional control during sidewards flying . 
