Challenges and opportunities for low noise electric aircraft

Abstract

A new class of electric aircraft is being developed to transport people and goods as a part of the urban and regional transportation infrastructure. To gain public acceptance of these operations, these aircraft need to be much quieter than conventional airplanes and helicopters. This article seeks to review and summarize the aeroacoustic research relevant to this new category of aircraft. First, a brief review of the history of electric aircraft is provided, with an emphasis on how these aircraft differ from conventional aircraft. Next, the physics of rotor noise generation are reviewed, and the noise sources most likely to be of concern for electric aircraft are highlighted. These are divided into deterministic and nondeterministic sources of noise. Deterministic noise is expected to be dominated by the unsteady loading noise caused by the aerodynamic interactions between components. Nondeterministic noise will be generated by the interaction of the rotor or propeller blades with turbulence from ingested wakes, the atmosphere, and self-generated in the boundary layer. The literature for these noise sources is reviewed with a focus on applicability to electric aircraft. Challenges faced by the aeroacoustician in understanding the noise generation of electric aircraft are then identified, as well as the new opportunities for the prediction and reduction of electric aircraft noise that may be enabled by advances in computational aeroacoustics, flight simulation, and autonomy.

Low-Frequency Intensity Modulation of High-Frequency Rotor Noise

“…As such, a proper quantification of BPFM and an identification of its source mechanisms is needed. It will inspire new measurement and post-processing procedures to support noise regulations and the assessment of the noise impact on communities [37,38]. For instance, this can be achieved by including BPFM aspects in realistic auralizations of rotorcraft noise [20,39] and future noise prediction tools [21,[40][41][42].…”

On the balance between the tonal and broadband noise of isolated propellers

“…Recently, Greenwood et al 24 presented predictions of the balance of tones and broadband noise using their analytical prediction code for an eVTOL propeller in the “hover” operating condition at a fixed thrust of 2500 N. The rotor had a diameter of 3 m with a solidity of 0.2. The tip Mach number varied from 0.3 (near stall) to 0.8, and tonal noise increased by more than 50 dB over this range of tip speeds while the broadband noise varied by less than 8 dB.…”

Effect of Roughness on the Aeroacoustic Performance of Rotor Noise at Low Reynolds Number

“…Noise production is a large matter of concern for unmanned aerial vehicles (UAV) applications [1]. When looking specifically at quadcopter micro air vehicles, the Reynolds numbers (based on chord length) tend to be lower than on helicopter rotors, typically between 10 4 and 10 5 .…”