The Design Improvement of a Rotary-Wing Aircraft Frame Considering Fatigue Life and Fracture Characteristics

The present study evaluates and investigates the fatigue design and fatigue life as the crack propagation occurs around the fastener of the mainframe of the central fuselage (CTR) of a rotary-wing aircraft designed based on the conventional safe-life method. For this work, the static and dynamic strain data were acquired according to flight conditions and fastening conditions between the mainframe and rail fitting (bracket). Furthermore, the fatigue life of the mainframe of rotary-wing aircraft was overestimated because of the conventional stress equation, for generating the stress spectrum according to the maneuvering conditions. To improve the existing stress equation, the modified stress equation was proposed by taking into account all force components transferred from the operating rotor system. Based on this equation, by employing the modified Goodman equation and Miner’s rule, the fatigue life of the mainframe of the CTR fuselage is calculated by the S–N diagram. Thereafter, the predicted fatigue life by considering simultaneously the damage of low-cycle fatigue and high-cycle fatigue according to the various flight conditions were compared with the actual life of crack occurrence. As a result, it showed that the predicted fatigue life (655 FHrs) was in a good agreement with the actual life (656.7 FHrs) of crack occurrence using the modified stress equation, respectively.