Time-Varying Noise of Electric Multirotor Aircraft
Open Access
- Author:
- Gan, Ze Feng
- Graduate Program:
- Aerospace Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 11, 2025
- Committee Members:
- Amy Pritchett, Program Head/Chair
Kenneth Brentner, Chair & Dissertation Advisor
Joseph Horn, Major Field Member
Eric Greenwood, Major Field Member
Michelle Vigeant-Haas, Outside Unit & Field Member
Sicheng Kevin Li, Special Member - Keywords:
- aeroacoustics
rotorcraft
acoustic signal processing
beats
eVTOL aircraft
UAVs
drone noise
urban air mobility - Abstract:
- This dissertation quantified time-varying noise levels of multirotor electric aircraft such as unmanned aerial vehicles (UAVs) and manned electric vertical take-off and landing (eVTOL) aircraft. Signal processing of flight tests and anechoic chamber measurements of hexacopter UAVs found the amplitudes of multirotor broadband noise modulation to be significant. This modulation amplitude of noise levels was strongly affected by small-amplitude stochastic fluctuations in rotor rotation speeds commanded by the flight controller, which are not typically considered in rotorcraft noise predictions. Measured rotor rotation speeds were found to follow Gaussian distributions about the set point commanded by the flight controller, resulting in measured blade azimuthal phase offsets between rotors following a uniform random distribution. These state variable variations caused noise variations not only in time, but between repeated measurements under the same conditions. Accordingly, a noise prediction methodology was developed to characterize these noise variations for multirotor aircraft, given the probability distribution of input states and the time-varying noise of a single rotor. Closely-spaced rotation speeds of different rotors caused acoustic interactions known as beats. Beating effects were found to be significant for multirotor aircraft in repeated measurements. Although beats occurred in all trials, since rotor rotation speeds varied between repeated trials, the resulting noise modulation frequencies and amplitudes also varied between repeated trials. Amplitude-modulated broadband noise signals with closely-spaced modulation frequencies caused significantly audible beating effects, even though only tonal beats are considered in the literature. Tonal beats of more than two rotors generated significant values for the sound quality metric of fluctuation strength. These multirotor tonal beats were characterized using the highest beat frequency when considering differences in blade passage frequency between all rotor pair combinations. The resulting variation of modulation amplitude between consecutive beat periods was characterized over the fundamental period of the modulation, which consisted of the least common multiple of all beat periods. Synchronizing the phase offsets between rotors significantly reduced the amplitude of broadband noise modulation, as well as beating effects. If such synchrophasing control is not implemented, the work in this dissertation demonstrated that rotorcraft aeroacoustic analysis should consider not only time variation of noise levels, but also effects of the flight controller and the resulting variations in the noise modulation between repeated flights.