Cheyenne prototype #6 (s/n 66-8831) 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.