Electric aerial propulsion systems are commonly used for many small-scale unmanned aerial vehicles (UAVs), providing a light and powerful method of generating thrust. In the emerging area of aerial-aquatic vehicles, most existing prototypes rely on such systems to propel themselves in both air and water. As the density of water is three orders of magnitude larger than that of air, a spinning aerodynamic body in the medium will experience significantly higher torque at the same speed. This results in aerial propulsion systems to be heavily mismatched underwater, as the required torque is higher than the drive torque that a typical aerial motor can provide. Here, an in-depth investigation of such off-design operation is conducted. Based on numerical simulation, we identify the feasible operating range of such systems and present an evaluation framework that identifies a motor-propeller combination from a component database that maximises underwater performance while ensuring aerial thrust requirements are met.
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