Test and Modeling of an Aquatic sUAS Propulsion System

There is a growing interest in the design and performance of aquatic small uncrewed aerial systems (sUAS) to accomplish tasks in both aerial and aquatic domains. To support the development of these systems, an aerial propeller-motor combination was tested in both air and water and the test data was then compared to two computational design models. One model was based on the software of XFOIL and QPROP, and the other relied on classical momentum theory. While an aerial propulsion system can function underwater, it operates at a much lower range of RPM in the denser fluid. Application of partial dynamic similarity with respect to torque and/or thrust coefficients revealed that RPMs in water should be approximately 3.5% of those in air – an effect verified experimentally. Test and model results were compared, and the underwater operating conditions were mimicked in air using a hysteresis dynamometer to isolate the underwater operating point of the motor from the thermal/fluid effects from operating the motor underwater, and the potential un-modelled physics in the propeller analysis from the difference in fluids. The dynamometer tests revealed that the motor was unable to reach the operating conditions of high torque loads at low RPMs in air. This led to separate fits in air and water addressing the thermal motor model parameters that improved the matching of the model results. The low-order design model and QPROP models sufficiently predict system performance in both fluids, but further investigation is necessary to more fully understand an identified “power wall” phenomenon in underwater operation.