A Comparison of Multicopter Noise Characteristics with Increasing Number of Rotors

This study examines the acoustic behavior in hover of manned-size, multi-rotor, eVTOL aircraft in the classical
quadcopter, hexacopter and octocopter configurations. The rotors are assumed to have collective pitch control and operate
at a specified RPM, with orthogonal and tip-to-tip rotor phasing considered. All configurations have the same disk loading
and tip Mach number, with the rotor radius decreasing and RPM increasing, going from the quadcopter to the octocopter.
The simulations use the Rensselaer Multicopter Analysis Code (RMAC) for the aerodynamic loads on the blades, coupled
to PSU-WOPWOP for predictions of propagated noise. From the simulation results, orthogonal phasing between rotors is
shown to produce significant noise reductions along inter-boom bisectors (between 9-14 dB relative to an equivalent single
rotor, at 6lb/ft2 disk loading and 0.51 tip Mach number). Further reducing the tip Mach number not only reduces the
propagated noise but produces even deeper regions of quiet along the inter-boom bisectors (18-25 dB quieter at 3lb/ft2
with 0.36 tip Mach number). An examination of the overall sound pressure frequency spectra indicates that smaller faster
spinning rotors (going from the quadcopter to octocopter) produce more tonal peaks at higher frequencies which results in
penalties in A-weighted noise.