An Experimental Evaluation of an Electronic Rotor Phase Synchronization System for Multirotor Aircraft Noise Control

Multirotor electric Vertical Take-Off and Landing (eVTOL) vehicles and many small unmanned aircraft systems (sUAS) utilize distributed electric propulsion. One concept for reducing the noise of these vehicles involves synchronizing the phase relationships between multiple rotors on the aircraft to control the radiated noise in one or more directions. This paper describes the development of a practical electronic phase synchronization method that can be implemented on multirotor aircraft, experimentally evaluates its effectiveness in controlling the noise of a small hexacopter mounted in an anechoic chamber, and investigates how the configuration of the vehicle influences the ability to control noise. Harmonic noise reductions on the order of 10 dB are demonstrated over a 20^◦ azimuthal arc, with limited increases in noise in other directions. Noise amplification by 6 dB is also demonstrated over the same region. Moreover, global noise reductions on the order of 6 dB across nearly all observer angles are achieved using a different combination of rotor phase offsets. The effect of phase controller error on noise control performance is investigated statistically, indicating that phase errors must be limited to about 5^◦ in order to achieve 10 dB of noise reduction. Simulations of the noise control scheme are conducted for vehicles with a range of rotor configurations, achieving similar levels of attenuation or amplification across all configurations, but with greater numbers of rotors providing more directional control over the radiated noise.