Time-varying broadband noise 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. Time-varying broadband noise 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 and anechoic chamber tests of hexacopter UAVs. Envelope analysis showed that the modulation depth depends on the azimuthal phasing between rotors, demonstrating the potential for synchrophasing control to reduce broadband noise modulation. If rotors are not synchronized, as in typical flight, the phasing between rotors varies with time. This phase variation followed 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 noise modulation of rotors sum together, demonstrating that broadband noise modulation is likely to be significant in flight.