A Semi-empirical Method to Predict Motor Heat Transfer Coefficient for SUAS Conceptual Design

The brushless DC motors of small unmanned aerial systems typically rely on axial air-cooling. Modeling the motor's thermal dynamics requires knowledge of the convective heat transfer coefficient. However, the coefficient is an extrinsic property which depends on surrounding conditions. As such, motor manufacturers provide no relevant data which stymies conceptual design and modeling. This paper presents an analytical method to predict the motor's heat transfer coefficient in various operating conditions using readily-available parameters. The paper also shares experimental measurements of heat transfer coefficient collected on a custom-instrumented hover stand. The analytical predictions matched experimental trends for a range of operating shaft speeds. The experimental measurements indicate that the heat transfer coefficient increases with shaft speed before reaching a maximum. The combined results provide guidance for eVTOL conceptual designers in (1) predicting a motor system's steady-state temperature, (2) maximizing convective cooling, and (3) understanding tradeoffs for compact and thermally-conductive motors.