Researchers measured the effects of the ground on propeller noise

Propellers are louder over ground.

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There are a lot of potential benefits from electric aircraft, which have been identified by various companies worldwide, including all of the major aircraft manufacturers. However, if urban air services such as on-demand air taxis are to become a reality within city limits, engineers must tackle the issue of sound pollution generated by propellers.

Researchers in 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 discovered distinct variations in the ‘Ground Effect’ noise characteristics of propellers while operating them over the ground as opposed to when they are operating normally.

When measuring at angles above the ground, they observed an overall rise in noise, with hydrodynamic and acoustic interaction effects playing a significant role in the overall trends.

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

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

“There are a lot of potential benefits from electric aircraft which have been identified by various companies worldwide, including all of the major aircraft manufacturers.”

Advanced Air Mobility (AAM)-related electric aircraft are a significant subcategory now under development. Three general categories can be used to classify this aircraft.

The first is an Electric Vertical Takeoff and Landing (VTOL) aircraft focusing on Urban Air Mobility (UAM) applications such as air taxis, patient transfers, rooftop-to-rooftop journeys within cities, and airport transfers.

Electric conventional takeoff and landing (VTOL) aircraft, which are being developed for regional air mobility (RAM), make up the second category. RAM focuses on transporting passengers from remote areas as well as delivering freight.

The third group, which focuses on videography, small package delivery, and the transfer of medical supplies, can be thought of as including the most widely recognized electric aircraft, small Unmanned Aircraft Systems (sUAS), or drones.

Liam said: “Until now, no literature existed for the problem of isolated propeller noise in ground effect.

“Our research sought to answer for the first time what happens to propeller noise while it operates in Ground Effect and what key acoustic and aerodynamic interactions are most important to understand.

“For the first time, we have comprehensively measured the noise of small-scale propellers during takeoff and landing while interacting with the ground. We can expect louder eVTOL aircraft during takeoff and landing if the complex interactions with the ground are not considered.”

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

Journal Reference:

  1. Liam Hanson et al. Experimental investigation of propeller noise in ground effect. Journal of Sound and Vibration. DOI: 10.1016/j.jsv.2023.117751
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