Experimental study of noise emitted by propeller’s surface imperfections

Recently, noise generation by unmanned aerial vehicles has been identified as an important issue, since they may fly over populated areas, or must often fulfill reconnaissance and surveillance missions. The aerodynamic noise radiating from the interaction between the flow and the blades is more difficult to address and control than the mechanical noise. The reduction of the aerodynamic noise is often performed by design optimizations of the blade geometry, without considering potential surface imperfections that may emerge, for example, from ice or dirt deposition. It is important to evaluate and understand the noise signature emitted by propellers operating in various adverse conditions. In this work, experimental acoustic measurements are conducted to evaluate the noise radiating from several propellers featuring different surface imperfections. To this end, an experimental stand was designed, and various propellers with different blade imperfections were tested acoustically. The sound pressure level shows that while some surface imperfections are not affecting the overall noise, certain types of such imperfections can increase slightly the overall noise level at low rotational speeds, or can decrease slightly the overall noise level at high rotational speeds.