“The number one issue for the eVTOL industry to successfully scale up is noise.”
There may be a few who disagree with that statement from Mark Moore, CEO of Whisper Aero, but it’s a point that’s hard to argue.
“To operate regional air mobility or urban air mobility as a financial success, you need 20 operations per hour and hundreds per day,” Moore explained. “The eVTOL industry is going to be looking at using local airports, and they are always surrounded by local communities. Even from the top of parking garages and other elevated vertiport sites within cities, if the take-offs and landings are noisy, people will not put up with it. There are already complaints about drone delivery. Even a 30-pound [13.6 kilogram] drone 100 meters [328 feet] away is a lot of noise.”
While Whisper Aero isn’t specifically developing an eVTOL aircraft, the company is focused on designing quieter vehicles, such as its nine-passenger Whisper Jet eCTOL. They’ve developed the Whisper demonstrator drone in order to do some of their research and development — mainly as a way to test and validate its propulsion system. Its first tests have already been completed with the U.S. Department of Defense.
Moore and his team describe their work as having engineered the best way to convert electricity into thrust with the highest efficiency and lowest possible noise level.
Ian Villa, Whisper’s chief operating officer and chief product officer, noted that while some major eVTOL players like Joby Aviation and Archer Aviation are very much focused on noise — and have been from the start — other companies aren’t, and that could lead to very serious consequences.
“The market will determine what happens, and when I say the market, I mean local community acceptance,” Villa said. “Smarter companies have been spending time on this, and communities are already speaking up. If companies aren’t already planning for very quiet design and operation, they are going to be screwed.”
NASA acoustic noise research
Researchers at NASA’s Revolutionary Vertical Lift Technology (RVLT) project are working on how to reduce noise in a number of areas for eVTOL configurations.
The research will allow NASA to develop design tools that manufacturers can use to reduce noise impacts where eVTOLs will take off, fly and land.
“This follows a long history of NASA research for conventional transports and helicopters to understand and reduce airframe and engine noise,” reported Susan Gorton, NASA RVLT project manager.
Gorton stressed that there are many design and operational factors that influence the noise of eVTOL vehicles and how people respond to them.
“For UAM [urban air mobility] eVTOL configurations, RVLT is working to develop and distribute noise prediction tools, accurately model and predict UAM noise sources, develop techniques and methods for assessing UAM acoustic footprint during operations, obtain high-quality validation data for noise prediction tools, characterize eVTOL noise through both flight and wind tunnel testing, and conduct psycho-acoustic research to characterize human response to the new sounds of UAM aircraft,” Gorton said.
Moog SureFly eVTOL project
As part of RVLT, during summer 2022, researchers from NASA’s Glenn Research Center traveled to Cincinnati to acquire noise data in collaboration with Moog. The Moog SureFly eVTOL hovered over an array of 28 ground-level microphones.
“We’re currently considering what NASA measured on our vehicle, giving us the opportunity to change the design and operation,” said Paul Stoelting, Moog director of aircraft group growth and innovation. “It’s pretty encouraging.”
He said in order to minimize noise during operations, “we can direct the noise based on direction of flight. We are also making adjustments to our dual quad rotor, developing customized propellors. A vehicle’s air foil is the other operational noise factor, but we’ll be flying at a lower speed than some of the other fixed-wing eVTOLs.”
Relating to noise from power generation, Stoelting said the Moog team is going to be doing additional studies on rotors and hubs to increase efficiency.
“From a power generation perspective,” he explained, “greater efficiency can lower noise level.”
Checking in with other stakeholders
Although details aren’t available, Archer “is going to great lengths to make our aircraft minimally-disruptive when it comes to noise,” said an Archer spokesperson. “Our production aircraft, Midnight, is being developed as a high-performance, high-safety and low-noise aircraft that will be significantly quieter than other aircraft like helicopters.”
In terms of what else Archer is doing to ensure success, the company said that it is committed to boosting public awareness of the future of transportation.
“It’s our priority to educate local populations in the cities we plan to operate in about the benefits of eVTOL travel and the role it will play in their daily lives,” the spokesperson said.
Helicopter Association International (HAI) has been doing this for many years with the Fly Neighborly program, in which operators and pilots work jointly with community stakeholders to mitigate aircraft sound. The Fly Neighborly/Environmental Working Group consists of industry leaders who donate their time studying sound issues and developing solutions to resolve them.
“HAI believes in maintaining community compatibility, working collaboratively with operators and residents to find compromises that allow both sides to see positive results,” said James Viola, president and CEO of HAI. “Earlier this year, we conducted webinars focusing on the sound emitted by several popular helicopters. During the presentations, industry experts suggested alternative flight operations that would reduce the impact of sound on residents, including flying at different altitudes, finding alternative routes, and landing in such a way as to reduce sound levels.”
Background noise level is key
Whisper’s Moore and Villa point out that decibel level is not the only metric that should be used to measure noise.
“If you look at how the FAA [Federal Aviation Administration] regulates noise, they look at an entire day of noise events and the associated decibel levels, and that’s incomplete,” Moore said. “The human ear picks up on certain frequencies for our survival and we have to look at all of them. The issue with helicopters has been that they make low frequency noise, and it’s not dampened by the environment.”
The Whisper team has therefore been pushing for a new metric called “time-varying loudness” that factors in not just decibels but also background noise.
“If you talk to anyone in acoustics, this metric is well known,” Moore said. “The FAA needs to apply it to all aircraft — not just eVTOL — but it’s still early days for the FAA to try to implement. We have to make sure the levels of everyday flight in the future doesn’t disturb communities — at all. It’s not just about annoyance, it’s about health. But I’m convinced this will be industry and city-led relating to ground noise community acceptance.”
Moore added that “in terms of aircraft and drone noise certification, these standards will build on existing standards. The FAA has a lot of history using their existing metrics, such as EPNL [effective perceived noise level] for helicopters. Overall, FAA and the French Civil [Aviation] Authority will lead on this. ICAO [International Civil Aviation Organization] already has a paper published on the topic as they also push for longer-term improved standard adoption.”