Aeroacoustics of a Scaled eVTOL Rotor Undergoing Harmonic Rotational Velocity Changes

Abstract

Current eVTOL certification regulations rely on measurements of the vehicle under controlled, and mostly steady conditions, whereas actual flight operations often involve transient maneuvers and continuously varying rotor operating states. To assess differences in real unsteady operations, this work focuses on eVTOL rotors experiencing unsteady rotational speeds, which are common in electric motors controlled by automatic controllers. The aeroacoustic behavior of a scaled eVTOL rotor undergoing harmonic rotational velocity changes is experimentally investigated in the anechoic wind tunnel of the Delft University of Technology. This was achieved by imposing a harmonic variation in rotational speed of different frequencies ( fℎ) and amplitudes. It is observed that the differences between steady state and unsteady conditions are governed by the reduced frequency parameter, kℎ, which depends on the harmonic frequency and the dimensionless amplitude, λ. The analysis reveals that harmonic variations in rotational speed result in a significant decrease in the rotor’s mean thrust relative to the equivalent static rotational speed, reaching up to 18%. Tonal noise is found to spread around the BPF harmonics, forming a hump containing multiple peaks with a frequency range proportional to the harmonic amplitude of the rotational speed. At higher harmonics, the broadened humps around adjacent BPF tones overlap, resulting in stronger spectral tonal content. In terms of sound pressure levels, a maximum decrease of 2 dB in the tonal noise occurs when the harmonic frequency varies from 0.5 Hz to 2 Hz. At the highest tested frequency, 5 Hz, the tonal noise increases again, and the thrust no longer follows the rotational speed variation as closely as at lower frequencies. This suggests the onset of divergent unsteady behavior, causing deviations from the quasi-steady trends observed at lower frequencies.