Experimental Investigation of a Small Drone Propeller Aerodynamics in Forward Flight

The aerodynamic performance of small drones is difficult to predict. Their small-scale propellers operate in a challenging fluid dynamics regime characterized by low Reynolds numbers, where complex transitional phenomena usually occur around the blades' surface. In the present work, an experimental campaign is carried out on a small isolated propeller in forward flight conditions. The main scope is to investigate how the performance and the induced flowfield are affected by the presence of a crossflow, which can be representative of the forward flight condition for a typical drone's propeller. As the crossflow velocity is incremented, considerable increases in thrust and power are measured. The resulting experimental loads are compared with semi-empirical laws derived from the momentum and blade element momentum theories. These laws have found extensive use in the preliminary design of helicopters and are also found applicable to the hereby investigated propeller with appropriate tunings. Furthermore, an analysis of the propeller's wake is conducted via planar particle image velocimetry. Significant differences in terms of flow topology and flowfield statistics are deducted with respect to the hovering condition. Ultimately, the vortices shed from the propeller's blades are identified and characterized, and a focus is posed on the vortices' trajectory and circulation. With respect to the hovering condition, the strongest vortical structures are advected in more confined trajectories, while the weakest structures are instead broken down under the effect of the crossflow.