An Assessment of Oil Free Bearing and Electric Machine Technology for Optimized Rotorcraft Propulsion Systems

Abstract

A technology state of the art assessment is conducted on oil-free bearings and electric machines that are required to enable the optimized rotorcraft propulsion system. An optimized rotorcraft propulsion system incorporating a foil air bearing supported Oil-Free engine coupled to a high power density gearbox using high viscosity gear oil is initially explored. Foil air bearings have adequate load capacity and temperature capability for the high-speed gas generator shaft of a rotorcraft engine. Managing the axial loads of the power turbine shaft (low speed spool) will likely require thrust load support from the gearbox through a suitable coupling or other design. Employing specially formulated, high viscosity gear oil for the transmission can yield significant improvements (~2X) in allowable gear loading. The complete, optimized propulsion system also requires a means to start the system as well as to provide electrical power for operation as well as vehicle use. Electric machine technology, which can enable an Integral Starter Generator for this system, is explored. Oil-free bearing and electric machine technology are found to be in a high state of technology readiness in the engine power class up to 20,000 shaft horsepower and have been demonstrated at a lower state of readiness for engine power class up to 50,000 shaft horsepower. Theoretical and experimental technology development is required to assess the scalability of this technology for the full spectrum of aeroprouplsion engine size class.