There are many potential benefits of electric aircraft, which have been identified by several companies around the world, including all major aircraft manufacturers. However, if urban air services, such as on-demand air taxis, are to become a reality within city limits, engineers must address the problem of propeller-induced noise pollution.

Researchers from the Aeroacoustics research team at the University of Bristol have now experimentally measured the effects of the ground on propeller noise for the first time. The research team found marked variations in the ‘ground effect’ sound characteristics of propellers while operating above ground, as opposed to when operating normally.

When measuring at angles above the ground, they saw an overall increase in noise, with hydrodynamic and acoustic interaction effects playing an important role in the overall trends.

It is envisaged that this research, which was tested at the National Aeroacoustic Wind Tunnel facility, would help develop techniques to reduce aircraft noise during takeoff and landing by modifying the design of planned aircraft structures or landing pads.

Lead author, Liam Hanson from the Bristol Department of Mechanical Engineering, explains: “In light of the need for greener aviation, there has been a push in the aviation industry to develop electrified aircraft.

“There are many potential benefits of electric aircraft that have been identified by several companies around the world, including all major aircraft manufacturers.”

Advanced Air Mobility (AAM) related electric aircraft are an important subcategory that is now under development. Three general categories can be used to classify this aircraft.

The first is an Electric Vertical Takeoff and Landing (VTOL) aircraft targeting Urban Air Mobility (UAM) applications such as air taxis, patient transfers, city roof-to-roof travel and airport transfers.

Electric conventional take-off and landing (VTOL) aircraft, which are being developed for regional air mobility (RAM), are the second category. RAM focuses on both transporting passengers from remote areas and delivering cargo.

The third group, which focuses on videography, small package delivery and the transfer of medical supplies, can be seen as the most commonly recognized electric aircraft, small unmanned aerial vehicles (SUAS) or drones.

Liam said: “Until now, there was no literature on the problem of isolated propeller noise in ground effect.

“Our research attempted for the first time to answer what happens to the sound of the propeller as it operates in Ground Effect and which key acoustic and aerodynamic interactions are most important to understand.

“For the first time, we have extensively measured the noise from small-scale propellers during takeoff and landing when interacting with the ground. We can expect noisier eVTOL aircraft during takeoff and landing if the complex interactions with the ground are not taken into account.”

They are currently testing other strategies to reduce overall system noise based on their improved understanding of propeller noise in Ground Effect.

Magazine reference:

  1. Liam Hanson et al. Experimental investigation of propeller noise in ground effect. Magazine for sound and vibration. DOI: 10.1016/j.jsv.2023.117751