Fatigue life of bearings supporting edgewise rotary wing loads in E-VTOLs

Electrically powered, thrustborne aircraft are a rapidly growing class in the advanced air mobility sector. In forward flight, these craft present an edgewise rotor to the air, which generates more lift on the advancing side of the disc, creating a moment about the shaft end. For low blade-counts, this effect dynamically varies at blade-pass-frequency. Unlike conventional helicopters, E-VTOLs have insufficient weight budget to package mitigating solutions in the rotor hub, resulting in substantial adverse loading being transferred directly to the propulsion machine drive end bearing. This can lead to a significant reduction in fatigue life. In the example given, a steady moment load reduces the expected fatigue endurance of these bearings by a half at air speeds as low as 10 m s − 1 relative to the pure-axial loading experienced in hover. A logarithmic decrement in endurance with advance ratio is observed. The extremely dynamic loading of a two-bladed rotor further reduces the expected endurance, to up to an order of magnitude worse than a steady shaft end moment. This paper presents a workflow for the concept designer which allows expected fatigue life of E-VTOL propulsion machine bearings to be computed from the aircraft performance envelope. An analytical treatment is developed for load distribution in the bearings subject to combined eccentric loading and is used to determine stresses in the bearing raceways. The resulting computationally efficient analysis is combined with a rotary wing flapping force model and bearing fatigue life assessment to illustrate the magnitude of the bearing loading challenge for E-VTOLs. • E-VTOL rotor loads have a deleterious effect on bearing life. • Low blade-count rotors produce dynamically varying shaft-end moments. • A toolchain for evaluating bearing life based on rotor configuration is presented.