Lost in the Noise

With rules yet to be written, aircraft yet to be tested, and public perceptions yet to be determined, aircraft noise poses complex questions for the makers of advanced air mobility.

Textron eAviation Nexus current design

NASA’s Urban Air Mobility (UAM) Noise Working Group (UNWG) and Acoustics Technical Working Group (ATWG) meetings at Langley Research Center, Virginia, in March addressed tools and techniques of aircraft noise measurement. It also gave electric vertical takeoff and landing (eVTOL) pioneers a forum to tout quiet progress, even though noise testing of urban air taxis has just begun. Textron eAviation engineering lead JD Terry described the Nexus tilt-prop air taxi in his electric portfolio and noted aircraft typically used airports built outside towns. “What’s unique about this is we’re trying to start in cities, in more populous areas... Keeping them as quiet as possible is going to be one of the [primary] requirements to gain acceptance.”

United Airlines and Archer Aviation have already trumpeted plans to link Newark International Airport with lower Manhattan and O’Hare International Airport with the Chicago Loop in 2025 with Archer’s Midnight air taxis. The all-electric Midnight uses six tilting propellers and six lift-only propellers to take-off and land vertically, and accelerate for fixed-wing lift to reach cruise speeds of 139 kt (267 km/h). According to Archer Aviation acoustics manager Ben Goldman, aircraft noise at 1,500–2,000 ft (450–600 m) cruising altitudes should not be audible over background city noise. Ambitious plans call for the first “conforming aircraft” to fly in early 2024 and gain airworthiness certification later in the same year. With successive technology insertions, Goldman acknowledged, “This first vehicle will be our worst; all the others get better from there.”

Early Testing

NASA has just begun to noise-test such unconventional air taxis. As part of the agency’s National Campaign, Joby Aviation flew its six-tiltpropeller S4 prototype over NASA microphones in California. Overflights at 60 and 100 kt (110 and 185 km/h) measured low-frequency noise induced by tilting propellers, and the dominant higher frequency noise from interactions with wing and tail surfaces. Flyover audio documented the noticeable difference between the electric air taxi versus common helicopters and fixed-wing airplanes. Joby computational fluid dynamics and aeroacoustics lead Jeremy Bain noted there are, as of yet, no set noise requirements for advanced air mobility (AAM). He said, “I don’t think there’s going to be a one-size-fits-all national standard... It’s important to communicate with communities.”

The Archer Aviation Midnight air taxi aims to connect airports with city centers in 2025. (Archer image)

NASA’s computational aeroacoustic simulations agreed with the Joby overflight data at low frequencies, but the mix of noise sources and airflows made predictions less certain at higher frequencies. A single propeller from Joby’s S4 prototype entered the wind tunnel at the Ames Research Center’s National Full-Scale Aerodynamics Complex in February to better isolate noise sources and help researchers understand the acoustic impact of variable-rpm propellers, airframe interactions, and shielding and scattering effects. Tests of the Moog SureFly eVTOL multicopter at NASA Glenn Research Center this summer will measure noise at altitudes greater than 15 ft (4.5 m), first in hover and then in forward flight.

NASA Langley began low-speed aeroacoustic wind tunnel tests of three- and five-bladed tilting propellors last year in the 14-by-22-ft (4.3-by-6.7-m) wind tunnel and looks to improve tools used to predict broadband noise of proprotors in axial flight. A new proprotor test stand measures multi-axis loads at different pitch and yaw angles and over a range of propeller collective settings. Follow-on tests in 2025 will run tiltprops at different incidence angles with a new phased microphone array.

The in-person panel of speakers at the NASA UAM Noise Working Group meeting. L-R: Rizzi, Goldman, Bain, Ismail and Henderson. (NASA photo by Mark Knopp)

Public noise perception likewise demands new tools that can measure how annoying air taxi noise might be in different settings around the United States. Damon Joyce of the National Parks Service Grand Canyon Quiet Technology Incentive noted that the level of observer annoyance associated with the noise of helicopters and airplanes is perceived to be different, but the difference is not statistically proven. NASA researchers found drone noise was more annoying than ground vehicle noise of the same level. To test annoyance levels in different regions, NASA’s UNWG last October developed a remote psychoacoustic test that used headphones to replicate air taxi noise inperson. The objective is to assemble AAM vehicle sounds and gauge human response in localities around the United States.

Fly and Listen

NASA Langley’s Senior Researcher for Aeroacoustics, Dr. Stephen A. Rizzi, leads the UNWG with Brenda Henderson at the NASA Glenn Research Center. The UNWG was established in 2018 to create and support a community of acoustics experts from industry, academia and government agencies to identify, discuss and address noise issues associated with UAM vehicles and their operations. Since its inception, the semiannual UNWG meetings have regularly attracted over 300 participants from over 125 organizations across more than a dozen countries. Its 2020 consensus white paper, entitled, “Urban Air Mobility Noise: Current Practice, Gaps, and Recommendations,” has served as a roadmap for coordination of activities tackling a set of high-level goals intended to address barriers associated with UAM noise that may impede vehicle entry into service.

The workshop panel and questions from specialists showed industry is likewise coming to grips with the acoustic complexity of electric air taxis. Though Bell helicopters, McCauley propellers and Textron Aviation business aircraft all gave Textron eAviation noise databases applicable to the Nexus air taxi, Terry observed, “What we’re predicting [versus] what we’re actually seeing, that’s what we need to get on the test stands and understand. We believe that our in-house databases are going to work and they’re going to work well, but again, they haven’t been validated.” Terry added, “Our guess right now is that takeoff for us is going to be the loudest.” He conceded, “We need to get the aircraft up in the air and figure it out.”

Joby’s Bain told the Langley conference that his company’s air taxi has registered hover noise of just 45 dBA with observers 1,640 ft (500 m) away, and takeoff noise only 65 dBA from 330 ft (100 m) away. “It tries to blend in with the environment, really a quantum leap from helicopters.” Joby tested a range of propeller blades to achieve lowest tip speed and balance low noise and best performance. The company plans to publish some data for select agencies to validate its acoustic tools.

A screenshot of Joby’s “sound parade” video, comparing the noise level and tonal quality of its S4 with existing aircraft. (Joby)

According to Lilium aeroacoustics engineer Till Heinemann, his company’s ducted fan jet with 30 tilting fans in its canard and main wings will be especially quiet in cruise mode. The company has so far flown two test aircraft about one-third the size of the production vehicle. Heinemann said, “We have been sharing quite a bit of data with EASA,” the European Union Aviation Safety Agency. He added, “We do know the loudest is transition from hovering phase to forward flight.” Heinemann said AAM developers needed new modeling approaches to capture dynamic effects in maneuvers and investigate duct geometry with asymmetric and dynamic flow fields.

Archer’s Goldman said the company had evaluated the noise of isolated motors without propellers installed to better understand contributing noise sources. His company had used the DUST aerodynamic modeling tool from Milan Polytechnic and PSU-WOPWOP from Penn State to predict noise. Goldman offered, “Right now, we’re focused on our takeoff and landing noise, so we want to find a good toolset for takeoff and landing.” Goldman also wished for tools to quantify community noise perception and better in-flight noise monitoring instruments to better understand different sources.

Hyundai’s Supernal unit has studied the varying blade vortex interactions of its SA-1 tiltrotor air taxi, but lead acoustic engineer Yahia Ismail noted work on cabin noise generated by multiple rotors turning close to the aircraft skin will be a significant focus of the company’s ride comfort research. Ismail said the Supernal goal is jetliner cabin noise. He observed that AAM developers need better microphone arrays for their test work. He told the workshop audience, “Once we have started production, we don’t mind sharing data.”

AAM panelists were concerned that current regulatory noise requirements may not be relevant to the way air taxis are allowed to operate. Ismail from Supernal said he expects more stringent requirements to be implemented over time and noted, “There will always be a compromise between the performance and noise... If we bring aircraft into the market [that are] very loud, we don’t want the noise to be the decisive factor.”

Goldman from Archer observed that the industry needs regulations better than the FAA Part 36 Stage 2 limits established in 1988. “I am very concerned that the lack of stringency will allow [operators] to hurt public acceptance. The public will see [air taxis] as something loud flying around, and that will lead to restrictions.”

Terry from Textron eAviation said, “For this industry to go forward, there are going to have to be noise regulations put on it. We can’t have these things flying everywhere all the time making the kind of racket we’ve had in the past.” He concluded, “If we go out there, safety has to be the utmost priority. After that, how loud are they? Let’s start with as quiet as can be and get the general public used to [AAM]. We’ll have to do that first or we’ll kill our own industry.”