THE SUBSTANTIATION OF THE STRUCTURAL INTEGRITY OF THE YAH-64 CONTROL ACTUATORS WITH MATERIAL VARIATIONS

The flight controls of the YAH-64 Advanced Attack Helicopter are powered by four servoactuators. These actuators were designed to be ballistically tolerant through the use of ESR 4340 steel heat treated to a high ultimate tensile strength. Early in Phase 2 of the AAH Program it became evident that the unavailability of high quality E.S.R. material for the servoactuators would jeopardize the flight test program schedule unless a temporary material substitution were made. VAR 4340 was selected as the substitute material. The Army's Black Hawk helicopter also employed high ultimate strength ESR 4340 steel in the control system servoactuators. In 1978 a hydrogen embrittlement problem was discovered in the pistons of the Black Hawk servoactuators, and signalled a potential problem on the YAH-64. A 200 hour static pull test was instituted as an acceptance test procedure, which later proved to be unsuccessful. The ultimate strength of the piston material was subsequently reduced, thus, alleviating the hydrogen embrittlement problem. The problems involved in the structural substantiation of the actuators are discussed in this paper.