Damage Tolerant Design of the YAH-64 Main Rotor Blade

This paper describes the damage tolerant concepts that were developed and applied to the structural design of the main rotor blade for the Hughes YAH-64 Advanced Attack Helicopter under development for the United States Army. The major design goals are defined as high invulnerability and damage tolerance to the 23mm HEI projectile threat, including the capability for 30 miniites of flight after damage, light weight, high reliability, and a minimum fatigue life ot 4500 hours. The basic design parameters for the main rotor blade, as a function of the helicopter system, are summarized. Structural design concepts used to attain the design goals are presented. These include a hybrid composite structure composed of high strength, high fracture toughness stainless steel and fiberglass: a deep structural chord of approximately 50 percent of the blade chord; a multispar box beam with fax orable fracture and failure mechanisms built in, an ultra light, yet rugged, aft section design: caretully sculptured redundant root doublers and root fittings; and a redundant blade tip design. Failsafe concepts and fractiire modes are discussed, including the use of fiberglass as a crack arresting material for the stainless steel, slow crack growth, geometrical crack stopping mechanisms, and alternate load paths. Verification of survivability concepts on ballistically tested specimens is presented, incliiding fatigue testing of a ballistically damaged main rotor blade section demonstrating a residual flight capability of greater than 5 hours after damage.