Fatigue of rotorcraft gears: overview and prospects of improvement

Abstract

The helicopter transmission design is a critical aspect due to the complexity of the components and the potential severity of a failure. A Rotorcraft Transmission Safety Working Group (RTSWG) has been set up in 2017 with the major Helicopter Manufacturers and EASA, addressing some recommendations to improve design and increase safety. During the years, Leonardo Helicopters has promoted some research activities in collaboration with Politecnico di Milano focused on the determination of the tooth strength under bending load. Following an extended test campaign, Wohler curves have been produced for the main materials used for transmission gears. The Safe Life analysis is now based on specific Wohler curves implemented on dedicated software which elaborates the flight data in a full fatigue spectrum, as for the rotor components. Flaw Tolerance requirements are complied with Flaw Tolerance Safe Life/No Growth analyses, performed in accordance with the Threat Assessment. Damage tolerance tests have been carried out on LH planet gears with simulated spalling, demonstrating adequate performance and robustness of design. Future research activities will be focused on the characterization of the contact fatigue on a back-to-back gear test rig for the definition of the endurance limits for pitting failure mode.