UK Researchers Found More eVTOL Propeller Noise Than Expected

A team of researchers from the University of Bristol in the UK studied propeller noise near the ground, finding a clear increase noise just above landing pads, rooftops and other features. Their work suggests electric Urban Air Mobility may not be as “quiet” as touted.

Bristol’s Aeroacoustics research team published its findings in the current issue of the Journal of Sound and Vibration. The team found that propellers in Ground Effect (where the airflow around an airfoil changes when near the surface) show a substantial increase in broadband noise levels. Most meaningfully for eVTOLs, they found “significant acoustic reflection” for a propeller in Ground Effect.

For the urban cityscapes which UAM proponents have long claimed will be its natural home, that signals potential noise above the levels which eVTOL makers have claimed will be orders of magnitude below traditional helicopter noise. The Bristol study notably tested sound in the UK’s National Aeroacoustic Wind Tunnel facility.

The eVTOL noise demonstrations done for the media thus far have almost all been flown in open, wind affected environments. A Joby Aviation video from 2021 in which company founder JoeBen Bevirt waxes prosaically about his dream of “building an airplane that I could fly from the meadow amidst the Redwoods where I was born,” is a good example.

As Bevirt does a stand-up on an open airport ramp with few trees visible, it’s difficult to determine exactly how far he is from the Joby eVTOL or whether he’s wearing a body microphone. The presence of sound dampening fog is notable as well. The air is cold, as evidenced by Bevirt’s breath, another atmospheric sound deadener. It is decidedly not the Redwood meadow he alludes to nor anything like a cityscape.

The Bristol researchers identified additional (high pitched) tonal peaks for a propeller operating in Ground Effect. Where water is present, as in many city harbors, there will be hydrodynamic interaction that affects sound.

Lead author Liam Hanson observed that for UAM operations within urban areas to become practical, engineers will have to tackle the issue of sound pollution generated by their propellers. The same holds true for high volume small commercial drone (delivery, mapping, monitoring, security UAVs) operations.

The authors raised an obvious aspect of the character and pitch of the sound created by such vehicles in noting that the propellers used by eVTOLs and UAVs are smaller than the helicopter rotors, usually rotating at higher speeds. Their noise characteristics differ from what is known in the existing sonic knowledge base, indicating that further research is required.

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

The testing done in the National Aeroacoustic Wind Tunnel facility, the Bristol team says, can inform strategies to reduce the noise of aircraft while taking off or landing – by either changing the design of the landing pads and vertiports from which they operate or, tellingly, by changing the design of proposed aircraft architectures.

While these are reasonable suggestions, implementing them will take time and money, resources that would-be eVTOL makers are beginning to run short on.

The research should surely be of interest to movers and shakers within the commercial UAM space but also to the military. In April, four USAF pilots remotely flew a Joby S4 eVTOL at the company’s Marina, California, manufacturing facility.

The flights were early preparation for the delivery of Joby aircraft to Edwards Air Force Base, California. The Air Force, through its AFWERX innovation arm, is evaluating eVTOL aircraft as part of its Agility Prime program which seeks to accelerate the commercial market for advanced air mobility vehicles.

An AFWERX release noting the first remote flight by USAF pilots also highlighted that the innovation agency recently entered into a third extension of its contract with Joby which began in 2020. The release specifically noted that, “The [contract] extension enables options for Joby to deliver up to nine of its low acoustic signature, zero-operating emissions aircraft to the Air Force and other government partners.”

How low the Joby aircraft’s acoustic signature really is in various operating environments is something the Air Force, NASA and others will begin learning when the first two Joby aircraft are delivered to Edwards in early 2024.

A query to AFWERX and AFRL regarding their knowledge of and reaction to the Bristol Aeroacoustic study was not answered by press time. Should the Air Force offer thoughts on the subject, this piece will be updated.

In the meantime, the University of Bristol researchers are now conducting additional tests on methods to potentially reduce the noise of entire eVTOL systems (aircraft/landing pad/rooftop/vertiports). Their findings will be of continued interest and are yet another reminder that technology claims and real-world results routinely differ.

If UAM and high density small UAS services are to become ubiquitous – contrary to the non-issue it has been portrayed as – noise will have to be dealt with.