Experimental investigation of contra-rotating multi-rotor UAV propeller noise

Multi-rotor Unmanned Aerial Vehicles (UAVs) are an increasingly popular technology with a range of uses. These small aircraft typically use 4, 6 or 8 single propellers as propulsors, although contra-rotating propellers are sometimes preferred as they reduce the aircraft planform area and add redundancy in case of failure of one or more propellers. To our knowledge, the noise produced by contra-rotating multi-rotor UAV propellers has not been studied previously. To address this, we recently conducted an experimental investigation of such a propeller mounted statically in an anechoic chamber to simulate the propeller operating in hover mode. The effect of propeller diameter, spacing, rotational speed and blade number were investigated. Thrust, power and noise were measured for a very large number of operating points. The results obtained in these experiments were analysed and a theoretical model was used to explain the phenomena and trends which were observed. This Forum article summarises the results of this study which is described more fully in McKay et al. (2019). We are continuing this work to try and understand the physical mechanisms by which these propellers produce noise so that we may use this knowledge to design propellers which produce a minimum level of noise.