Broadband noise modulation of multirotor aircraft

Rotor broadband noise is typically analyzed over time scales encompassing multiple rotor periods. However, modulation of broadband noise levels with the blade passage frequency has been shown to be significant for human perception of wind turbine and helicopter noise. In contrast, broadband noise modulation has not been extensively studied for aircraft with many rotors, such as unmanned aerial vehicles (UAVs) or advanced air mobility aircraft. In this work, significant broadband noise modulation was measured in flight tests and anechoic chamber experiments of hexacopter UAVs. The amplitude of this modulation depended on the azimuthal phase offsets between rotors, demonstrating the potential for synchrophasing control to reduce broadband noise modulation, analogous to synchrophasing control of tonal noise. If rotors are not synchronized, as in typical flight, the azimuthal phase offsets between rotors vary with time. This variation was found to follow a uniform random distribution, resulting in modulation depth also varying randomly with time. The probability distribution of modulation depth was computed using offset copies of the modulation of a single rotor. These results contribute understanding to how the broadband noise modulation of rotors sum together, and showed that this modulation is likely to be significant in flight.