Industry experts agree that electric drive motors and slower propeller tip speeds will enable eVTOL aircraft to have a significantly lower external noise signature compared to helicopters, but that does not necessarily translate into instant public acceptance.
Juliet Page, a scientist with the U.S. Department of Transportation’s Volpe National Transportation Systems Center, said it is important for eVTOL manufacturers to avail themselves to the variety of integrated, simulated, and dual-use aircraft noise models that have been developed over the years by the FAA, NASA, and the U.S. Department of Defense, as well as the latest advanced acoustics models, in fashioning their aircraft to be as quiet as practicable. Page said data is required to develop consensus standards and best practices for the industry.
“Noise is a key area that needs to be addressed in the eVTOL community in order to scale up operations and make [the industry] economically viable,” she said. This involves “operational noise, the physics and mechanics of the vehicle and how you fly it, and community engagement.”
Page stressed the importance of the latter: “There are noise challenges in various areas of the country when it comes to helicopters and it is often difficult to put the genie back in the bottle.” She said industry groups, including HAI's “Fly Neighborly Committee,” can assist eVTOL OEMs and other industry stakeholders with community engagement.
She urged eVTOL OEMs to “go out to the community in advance and understand their concerns, then design future flight operations and (UAM) skyport locations with community stakeholders. This will help avoid trouble downstream and will produce goodwill from the community, if they are engaged from the get-go.”
Page pointed out that, while the federal government regulates aircraft and airspace design, localities are still preeminent when it comes to the critical issue of where to build skyports and other infrastructure essential to supporting a network of vehicles with limited range. Trying to design vehicles and operations in an environment where every locality has differing noise standards is simply impractical.
“Heliport and vertiport use are prescribed by the local jurisdictions. In all of these locations, individual restrictions come into play.” She said local jurisdictions need to “adopt uniform standards rather than a patchwork of different rules from one city to the next.”
At an eVTOL acoustics forum on Wednesday at Heli-Expo 2020, she implored those in the industry to get involved and do the research to develop these standards. "There are tools in existence now [to help], but more validation needs to be done,” Page said, adding that this means going out in the field with microphones and measuring the sound levels of vehicles.
Ben Goldman, the acoustics lead for eVTOL maker Joby Aviation, stressed that the perception of noise and its sound is just as critical in winning public acceptance as actual decibel levels. “Quiet helicopters can still be annoying,” he said, but added that the independent, multi-rotor system designs presented an opportunity to “tailor” noise so it is less objectionable to the human ear via “psychoacoustics.”
“Independent rotor systems give designers an enormous amount of flexibility to tailor noise,” he said. Goldman said that disc loading, blade speed, location, shape and thickness are all factors in the equation as is blade vortex interaction. But offloading blade noise via vehicles transitioning onto a fixed-wing for cruise flight, not only provides better vehicle economics, it also significantly reduces noise, he noted.
“The quiet revolution is here,” he said. “I hope you didn’t hear it coming.”