An Assessment of Multi-copter Noise in Edgewise Flight

This study examines the acoustic behavior in forward speeds of 0, 15, 30, and 60 knots of manned-size, multi-rotor, eVTOL aircraft in quadcopter, hexacopter, and octocopter configurations. The rotors are assumed to have constant RPM and are controlled through collective pitch, with orthogonal phasing between rotors. All configurations share the same disk loading and hover tip Mach, with the rotor radius decreasing and the RPM increasing as the number of rotors increase. The simulations use the Rensselaer Multicopter Analysis Code (RMAC) for the aerodynamic loads coupled with the PSUWOPWOP code for noise predictions at an observer hemisphere. From the results, it is shown that at higher forward flight speeds, where loading noise becomes more dominant, high elevation angles (below the vehicle) show peak noise for all configurations. Asymmetry in the number of outside advancing blades, such as on the plus quadcopter, causes higher noise levels for the left (advancing) side. In-plane directivity patterns that show large decreases due to signal cancellations in inter-boom noise are shown to progressively diminish at larger forward speeds, with lower reductions for the hexacopter and octocopter configurations. The total acoustic radiated power is also compared with the single rotor for all multicopter configurations. The comparison shows that the acoustic power versus the single rotor power is decreasing as we increase the number of rotors or the forward speed.