An Experimental Investigation of Propeller Noise in Forward Flow

The present experimental study characterizes the noise generated by small propellers in static and in-forward-flight conditions. A set of experiments were conducted in the aeroacoustic wind tunnel of the University of Bristol, in which both the aeroacoustic noise and aerodynamic loads were measured simultaneously for the selected propeller blades, having similar propeller diameter (11 inches) and pitch (from 4 to 10 inches) to those commonly used in small unmanned aerial vehicles. The tests were performed with incoming flow velocities from 0 m/s to 20 m/s for a range of propeller rotational speeds from 2500 RPM to 5500 RPM. The results show that the characteristics of noise consist of an overall broadband hump overlaid with distinct narrow peaks, corresponding to the fundamental blade passing frequency and its harmonics, regardless of the flow velocities. Moreover, the synchronous aerodynamic and noise measurements confirm that as the flow velocity increases, the reduction on aerodynamic loads leads to reduction in the overall noise generation by the propeller, indicating a direct correlation between aerodynamic load and propeller noise. This is further corroborated when comparing across propellers with different pitch values. The propeller with the highest pitch of 10 inches consistently produces louder noise than the other propellers. The results shed more lights on the choice of propellers pitch angles and their operational profile with respect to the propeller noise generation.