Effect of tip Mach number on propeller noise in edgewise flighta)

Advanced air mobility represents efficient and sustainable transportation through the use of electric vertical takeoff and landing (eVTOL) aircraft, which will see their propellers operating in edgewise flow for a significant portion of their mission profile, giving rise to challenging aerodynamic and aeroacoustic features, different from the conventional helicopter rotors. The present experimental study examines the effect of the blade tip Mach number on the acoustic signature emitted by a propeller operating in edgewise flight conditions. Both aeroacoustic and aerodynamic measurements were performed for a 12-in., two bladed propeller over a wide range of tip Mach numbers (MT) from 0.12 to 0.56, and inflow velocities from 0 to 24 m/s. The results confirm the highly sensitive nature of the propeller noise to the tip Mach number. The overall sound pressure level (OASPL) in the plane-parallel to the propeller's plane of rotation, scales with MT3 to MT4 at low tip Mach numbers (MT<0.3) and with MT6 at MT>0.3, for the no inflow velocity case. Similar trends are observed for the plane perpendicular to the plane of rotation, but with noise scaling up to MT8. At the highest inflow velocity case, the OASPL scales with MT3 to MT7, depending on the observer location and MT, highlighting the increased complexity and sensitivity of noise directivity at higher inflow velocities. Tonal noise is the dominant source of noise at lower inflow velocities, with an increased contribution of broadband noise at higher inflow, possibly leading to the larger deviations of the tip Mach number scaling observed. These findings offer valuable insights into the acoustic behaviour of propellers during edgewise flight conditions, which can be leveraged to design more efficient, low-noise propellers in an eVTOL application.