The propellers are louder on the ground, the researchers found

The effect of ground on propeller noise has been measured experimentally for the first time by researchers in the Aeroacoustics research team at the University of Bristol.

In findings, published in Journal of Sound and Vibration, the team found a marked difference in the noise characteristics of the propellers above the ground, known as the ‘Ground Effect’, compared to when they were operating normally. They noted an increase in overall noise when measuring at an angle above the ground, with hydrodynamic and acoustic interaction effects being key factors for the overall noise trend.

It is hoped that this research, which was tested at the National Aeroacoustic Wind Tunnel facility, can inform strategies for reducing aircraft noise during takeoff or landing, either by changing the design of the landing strip or by changing the architectural design of the proposed aircraft.

Lead author Liam Hanson explained: “Given the need for more environmentally friendly aviation, there has been a push within the aviation industry to develop electric aircraft.

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

However, if urban air services such as on-demand air taxis become a reality within city limits, engineers will have to address the problem of noise pollution, generated by the propellers.

An important part of the recently developed electric aircraft is for Advanced Air Mobility (AAM) purposes. This aircraft can be broadly considered to fit into three distinct categories.

The first is an Electric Vertical Take-Off and Landing (eVTOL) aircraft that focuses on Urban Air Mobility (UAM) applications such as air taxis, patient transfers, rooftop-to-rooftop trips within cities and airport transfers.

The second category is the Electric Conventional Take-Off and Landing (eCTOL) aircraft which is being developed for Regional Air Mobility (RAM). RAM focuses on cargo shipping, short haul flights, and the transfer of passengers from rural areas.

The best known electric aircraft, Small Unmanned Aircraft Systems (sUAS) or drones, can be considered as a third category which focuses on videography, delivery of small packages and transfer of medical supplies.

Each of these categories of electric aircraft often uses a propeller or rotor to generate thrust for takeoff and landing. Most importantly, eVTOL aircraft operate in urban areas with large populations and consequently the noise generated by aircraft is critical to understanding and mitigating where UAM allows.

The propellers used by planes are smaller than those of helicopters that have been used for years, are usually much smaller in diameter and rotate at higher speeds. As a result, the noise characteristics are very different from existing knowledge, so further research is needed.

When eVTOL and sUAS aircraft take off or land from rooftops or landing strips, the propellers may experience Ground Effect, an aerodynamic phenomenon that changes the performance of the propellers.

Changes in propeller aerodynamics in Ground Effect change the acoustic performance of the propellers and cause complex interactions.

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

“Our research seeks to answer for the first time what happens to propeller noise when operating in Ground Effect and what are the main acoustic and aerodynamic interactions that are most important to understand.

“For the first time we have comprehensively measured small-scale propeller noise during takeoff and landing during ground interaction. Obviously we can expect louder eVTOL aircraft during takeoff and landing if complex interactions with the ground are not considered.”

Based on their new understanding of propeller noise in Ground Effect, they are now conducting additional testing on various methods to potentially reduce system-wide noise.

This research was sponsored by Embraer SA and the SilentProp Horizon 2020 project (agreement number 882842).

Paper:

‘Experimental investigation of propeller noise on ground effect’ by Liam Hanson et al in Sound and Vibration Journal.