Outer Mold Line Cooled Electric Motors for Electric Aircraft

To be viable alternatives to traditional aircraft, electric aircraft require electric propulsion motors with high specific power, efficiency, and reliability. Motor thermal management is key to developing electric motors that meet these requirements, because increasing motor power density generally increases motor loss density, motor efficiency improves as motor temperature is reduced, and motor winding life is directly related to peak winding temperature. Liquid cooling loops, heat sinks, internal flow, and heat exchangers can all be used to cool electric motors on electric aircraft; however, these components add mass, losses, complexity, and drag to an aircraft. The question in the design of an electric aircraft's drive system therefore becomes how to trade motor performance versus the size, mass, power, and complexity of the thermal management system. This paper seeks to answer the question of what motor performance can be achieved at one extreme end of this motor vs thermal management system trade space where no explicit thermal management system or features are used to cool the motor. This task is completed by designing motors for electric aircraft that are cooled only by the propeller wake on the outer mold line (OML) of the nacelles they sit in and therefore have no thermal management system. A prototype motor designed for NASA's Maxwell X-57 aircraft is used to inform the development of an OML cooled motor design tool. That tool is used to produce a high fidelity OML cooled motor design for X-57 that achieves >96% efficiency at a total mass of ~25.5 kg. The design tool is then used to evaluate the possible performance of OML cooled electric motors for various power levels and thermal environments.