Engage the (almost) silent drive

First-generation electric delivery drones and passenger air taxis could be too noisy for many communities, so researchers are attempting to engineer electric thrusters of unprecedented quietness. Paul Marks delves into their chances of success.

On the shores of Lake Holiday in Crossville, Tennessee, a former watersports resort has found an unlikely new role — as an electric aviation innovation lab. On any given day at the startup Whisper Aero, one might find a bunch of aerospace engineers hunched over 3D laser scanners, rapid prototyping software and computational fluid dynamics simulations, I’m told by an executive. It’s all part of an endeavor to solve one of the most pressing problems facing the advanced air mobility revolution: Flying almost silently. 

Specifically, the Whisper team has developed an ultra-quiet type of electric thruster that it says can be scaled for use on electric aircraft of different sizes, whether they are small delivery drones, urban air taxis or all-electric light aircraft flying city-to-city routes. This, they believe, will be key to earning public acceptance for the emerging market NASA dubs advanced air mobility, AAM, typically consisting of eVTOLs, electric vertical takeoff and landing aircraft. 

Will the device be a souped-up propeller that’s digitally shaped, sized and spun at a speed to disturb as little air as possible — perhaps in a special, hushed kind of ducted fan — or will it be something entirely new and exotic? Here is all Whisper CEO Mark Moore would tell me: “It’s a unique propulsion technology, one that lets us drive the noise down to such an ultra-low level that it will blend into the background noise of where drones or eVTOLs are flying.”

Whatever form these mysterious ultra-quiet thrusters take, Whisper’s aim is to sell them to the makers of delivery drones and eVTOL passenger air taxis, as well as to install the thrusters on an electric conventional takeoff and landing light aircraft that Whisper is designing. These planes that would fly fast, quiet city-to-city hops are called Whisper Jets, though the company hasn’t said quite what it means by “jet.”

Whisper’s aims are not unique. A handful of eVTOL makers have their own proprietary technologies for quelling noise, including Archer Aviation and Joby Aviation in the U.S., Lilium and Volocopter in Germany and Vertical Aerospace in Britain. In parallel, research institutes elsewhere in the U.S. and in Europe are also attempting to craft a new generation of quiet thruster designs.

It’s a dynamic that’s setting up a high-stakes, low-noise propulsion competition in the much-vaunted AAM market, which in the U.S. alone is predicted by market researcher Deloitte of London to be worth $115 billion per annum by 2035 — creating, it says, 280,000 jobs.

To hear one observer tell it, this quest for quietness is not a pipe dream; Whisper is succeeding in its aims. 

“I was one of the first to see a live demo of Whisper’s tech,” says Nikhil Goel, a New York technology investor and one of the company’s earliest financial backers. “And I have never heard anything quite like it. From where I was standing, you could barely hear a faint whirring; it was reminiscent of a quiet ceiling fan.”

He adds: “Having seen the plans for the Whisper Jet, I am confident it will change the way cities interact with airplanes forever.”

Goel and half a dozen other venture capitalists, including “Iron Man” actor Robert Downey Jr.’s venture fund FootPrint Coalition Ventures, have invested a total of $7.5 million to fund initial development of Whisper’s technology. The company has also benefited from U.S. Air Force Research Lab funding for some of its early quiet thruster research. 

Progress has been quick: “We’ve already developed our design tool suite and built our first propulsor and tested it. And we’re very excited by the results,” says Moore, who is well known in AAM circles because he pioneered the very concept of distributed electric propulsion during his 30-year career at NASA. And after he left the agency, he developed a number of AAM business models at Uber’s short-lived but influential aerial ride-sharing division, Uber Elevate, acquired in late 2020 by Joby Aviation of California.

Now, Moore believes his team’s experience and Whisper’s innovative design are a winning combination for creating “the next generation of distributed electric propulsion technology, with a focus on achieving ultra-low noise, quieter per pound of thrust than any other company has ever achieved, while still having very high efficiency, low weight, and high reliability.”  

Those are bold aims, for sure, and there’s no shortage of competition. Take Harmony Aeronautics, for instance. This spinout from the aerospace research department at Texas A&M University in the U.S. is attempting to engineer quiet-running coaxial propulsors for drones and eVTOLs. Like Whisper, it has done so in part with $150,000 in Small Business Technology Transfer research funding from the U.S. Air Force Research Lab, which on the STTR website described Harmony’s coaxial rotor technology as “revolutionary.” 

And in the U.K., the government-backed Aerospace Technology Institute is funding a $3.8 million research program called Inception, in which the University of Cambridge’s Whittle Lab and the University of Salford’s acoustics department are working with five aerospace firms, including Dowty Propellers, to develop “quiet and safe zero-carbon electric jet engines” — in other words, ducted-fan vectored thrust propulsors, like those spread along the wings of some eVTOL designs, including the Lilium Jet.

However, those involved in this quest for quiet electric propulsors are under no illusions that they will be able to achieve anything approaching silent flight. Electric motors, rotors, propellers and ducted fans have to drive air forcefully to provide thrust, “so there will always be noise associated with that” says Antonio Torija-Martinez, acoustics lead at the University of Salford on the U.K. Inception project. 

Harmony Aeronautics founder Moble Benedict agrees: “There is no shortcut to quieting a rotor that’s moving through the air at speed.”

So why take on such a tough challenge? Fear, basically: What’s driving this new battle against delivery drone and eVTOL noise is a concern that unless the sector’s potential to create irritating community noise can be reduced to socially acceptable levels by the time some operators plan to start passenger flights in 2024 and 2025, AAM services could be stillborn. 

“Without a better understanding of electric aviation noise, its characteristics and why it is different from traditional aviation, the easy answer from the wider public will be to reject it,” says Darrell Swanson, a U.K. consultant specializing in electric aviation issues and strategic adviser to NASA on eVTOL operations. “And without public support for electric aviation, we will not get planning permission for landing infrastructure and the industry will never take off.”

Cutting the noise would result in a virtuous circle, Swanson believes: “In a nutshell, lower noise leads to a higher degree of social acceptance, reducing resistance to the building of AAM infrastructure. More infrastructure leads to more demand, leading to increased competition and lower fares for passengers, increasing the social utility of AAM.” 

However, anyone watching the online postings of the early runners and riders in the eVTOL arena could be forgiven for thinking that the sector’s noise issues are already a solved problem. 

On YouTube, for instance, Joby has posted videos of its tiltrotor prototype variously taking off, hovering and flying over a series of calibrated microphones, showing what seemed a quiet performance. On its website, Joby claims what was quiet in the hover would be “near silent” in the cruise, thanks to the way the six tilting propellers located along the wing and tail operate. This distributed electric propulsion allows eVTOLs like Joby’s to have multiple large-diameter propellers rotating at slow tip speeds, producing low-frequency pressure waves. This would create more of a hum than the well-known “wop-wop” sounds from helicopters, which have an even larger diameter rotor with higher tip speeds. 

The question is: How loud will that hum be in an urban setting, and with a great many eVTOLs plying the airways as our future taxis? 

On LinkedIn, Lilium of Germany showed a flight demo of its eVTOL prototype taking off and flying a number of brief flight maneuvers so viewers can experience the inoffensive “sound profile” of its ducted-fan vectored engines. However, Joby’s flights were in rural locations and Lilium’s at an empty airport, rather than the built-up urban areas that AAM companies are targeting for early passenger service. There, buildings could create concrete canyons where resonating electric rotor noise from multiple eVTOLs might annoy people living near vertiports. But until eVTOLs are safety certified, the operators won’t be able to properly assess their noise in the urban environments they are destined to fly in, says consultant Swanson. 

Some eVTOL designers, buoyed by encouraging lab and ground tests, say they are confident their aircraft will fly quietly — before they have flown. Vertical Aerospace of the U.K., for instance says in an online video that it hopes to achieve “near-silent flight” with its in-development VX4 tiltrotor, aided by a novel rotor design that lets Vertical “tune” noise levels, making it some 30 times quieter than a helicopter. In a January blog post, Archer Aviation of California says its Maker eVTOL prototype flying at 2,000 feet in altitude should be “1,000 times quieter than a helicopter” to a person on the ground. As of press time, Archer had only conducted a low-altitude hover test with the Maker design. 

The problem for people trying to make sense of all these comparisons, says the University of Salford professor Torija-Martinez, is that in this nascent AAM market, companies rarely compare like with like. 

“I would be very careful with [comparing] noise figures because there is a problem with nomenclature,” he says. “What do they mean by 30 times quieter? In terms of acoustic energy? In terms of decibels? Or what?” 

On top of that, with companies jockeying for early market dominance, commercial secrecy is very much the watchword. None, says Benedict of Harmony Aeronautics, are going into much detail about how their thruster quietening technology works. 

But it’s not only the AAM airframe makers who are keeping schtum; would-be quiet propulsor maker Whisper Aero, too, is saying absolutely nothing about how its quiet electric thrusters work. 

“Our first drone product won’t be operational until 2023, so we don’t see any benefit in letting others know how the unique technology we’re developing works until we need to,” Moore told me. “It’s natural that when others see what we’re doing, they will want to do similarly.”

This secrecy even applies to the five patent applications Whisper has already filed on its thruster-hushing technology. “None of our patents will be made public for several years, with many more patents actively being filed as we scout out this new frontier of electric propulsion,” says Moore. 

While a patent application is normally published 18 months after filing, a U.S. patent applicant can file a “request for nonpublication,” which if granted will keep the invention secret until the inventor deems it commercially safe to publish the patent specification.

So how might it be possible for an electrically driven rotor or vectored-thrust ducted fan to be hushed? What design features could be modified to make a difference? Thankfully for the curious, Harmony Aeronautics and the Inception team were willing to share at least some details with me.  

Harmony Aeronautics was spun out in 2018 when Benedict and his colleague David Coleman, a PhD student who was then developing a robotic hummingbird-like UAV, decided to enter Boeing’s GoFly X-Prize challenge. The goal: To build a single-person vertical takeoff and landing aircraft, measuring no more than 8.5 feet (2.6 meters) in any dimension, which can carry a 200-pound (91-kilogram person) a distance of 20 miles (32 kilometers).

And there was a further proviso. The sound pressure level of the aircraft’s noise had to be less than 87 dBA, a special version of decibels weighted for the human ear’s frequency response, at a range of 50 feet (15.2 meters) from the aircraft. “That means the noise from the aircraft would only be like that from a truck or a lawnmower at that distance,” says Benedict.

There were two issues with noise they had to deal with: Its magnitude, as measured by its sound pressure level in dBA, and its frequency range — which is a strong measure of how psychologically annoying a noise tone is to the human ear. “Our objective was to make it not only reduce the magnitude of the noise, but also to reduce the frequency so humans would perceive the noise as quieter.”

The Harmony team settled on an electric rotorcraft design powered by a pair of contrarotating coaxial four-blade rotors and set about achieving that demanding specification. Knowing that on any rotorcraft the noise sound pressure level is proportional to the fourth (or higher) power of rotor tip speed, Coleman and Benedict eschewed the small rotors popular on many drone and eVTOL designs, which would have had to spin very fast, creating quite a din.

“Instead, we went with a big rotor with more blade area so we could move the blades at a much lower speed and produce the same thrust,” says Benedict.

They also innovated with the blade shape to reduce the noise caused when contrarotating blades pass over each other (that is, as a clockwise-moving one crosses a counterclockwise-moving one).

“During that blade crossover, a sudden, impulsive noise is produced because of interactions between the blades,” says Benedict.

With regular, straight-edged rotor blades, the entire upper and lower blade will overlap at the same instant, producing a strong impulse noise. Their answer was to bend the blade into a dog-leg shape, sweeping half of it backward so the entire blade span never overlaps simultaneously. “This means that at any instant, only two points on the blade are actually overlapping, so we’re basically smearing out that interaction over a longer period of time — simply by the virtue of the shape,” says Benedict.

They have patented this noise-quashing idea, he adds.

Next, they addressed the annoying frequency issue, knowing, says Coleman, the range to avoid is between 1 kHz and 5 kHz. On a piano keyboard, that’s approximately the top two octaves. Imagine music only being played at that end of the keyboard, and it’s easy to see how annoying it would be.

“That is the reason we are irritated by the sound of a drone or a chainsaw or a baby crying,” says Benedict. “All these noises, or all these pressure fluctuations, happen in this frequency range.”

To avoid such audible angst, they designed their coaxial rotors to run at speeds creating sound in the hundreds of hertz range, cruising at just 200 Hz. And in tests of Aria, their remotely-piloted 250-kg helicopter, the sound pressure level at 15.2 meters measured a mere 73 dBA, a feat Benedict describes as “phenomenal” given the 87 dBA GoFly requirement.

Unfortunately, a systems failure caused Aria to roll over and crash while taxiing in a slow hover, so it was not entered in the GoFly competition.

“That was one of the saddest days,” says Benedict. However, he says Boeing, undaunted by that crash, is still interested in researching U.S. Defense Department applications for the startup’s innovative coaxial rotor blade, while Harmony explores AAM applications.

Blade architectures are front and center for members of the U.K. Inception project, too, as its members study electric ducted fans — initially single rotor modules, and perhaps dual-rotor contrarotating units in a follow-up project — to see how best to design them for noise abatement. And in a sign that alleviating AAM community noise is starting to matter big time, the Inception researchers, like Harmony Aeronautics, are “assessing sound quality metrics, too” — the so-called psychoacoustics, says Torija-Martinez.

The kind of design parameters that could be modified to improve the noise performance of a ducted-fan electric jet include rotor tip speed, the number of rotor blades and how those rotors interact with features within the duct’s roughly cylindrical architecture. “We are optimizing the tip speed of the blades, the spacing between those blades, and the other structures within the fan, such as the [air] inlet guide vanes and outlet guide vanes,” says Torija-Martinez.

But not everything comes down to the aircraft itself: The acoustic environment a drone or eVTOL operates in, known as its soundscape, could also be used to mask noise. For instance, says Torija-Martinez, operators might be able to fly delivery drones along busy roads so that traffic noise is hiding the sound produced by the UAVs. Similar thinking, he says, could inform those seeking sites for vertiports — using traffic routes to help define flight corridors.

Whisper Aero appears to have just such masking plans in mind. In a statement about its quiet propulsor technology on its U.S. Air Force STTR award webpage, the company says the analysis of its technology “indicates an acoustic signature below all current eVTOL development efforts, with the potential to blend into ambient urban soundscapes.”

It’s clear that Moore and his colleagues at their lakeside innovation campus in deepest Tennessee are certain that beating noise is the key to unlocking AAM.

“I’m convinced the future is electric, that sustainability matters, and that in the modern world, we’re all closer and closer to each other,” he says. “So the winner is going to be whoever’s the quietest and has the greenest set of technologies. And I think that’ll be us.”

Harmony Aeronautics and the seven-strong Inception consortium may beg to differ, however. And in any case, Harmony’s Benedict has some words of caution about the limitations of the aerodynamic noise-reducing tricks they all crave:

“You can’t beat physics,” he warns. 

About Paul Marks

Paul is an award-winning journalist in London focused on technology, cybersecurity, aviation and spaceflight. A regular contributor to the BBC, New Scientist and The Economist, his current interests include electric aviation and innovation in new space.

“From where I was standing, you could barely hear a faint whirring; it was reminiscent of a quiet ceiling fan.”

Nikhil Goel, Whisper Aero investor
An experimental drone with multiple propellers is positioned within a wind tunnel for testing, mounted on a metal frame with various sensors and wires attached.
The larger surface area on the coaxial rotors developed by Harmony Aeronautics is key to the Texas startup achieving a quieter noise profile. Pictured here during January 2019 wind tunnel tests at Texas A&M University, the rotors have flown on Harmony’s 22-pound one-third-scale cargo drone prototype and a larger 550-pound demonstrator. Credit: Harmony Aeronautics
Top view of a white aircraft with a large black propeller on the wing, parked on a concrete surface with shadows cast by a grid-like structure above.
Joby Aviation announced in early January it had received FAA approval to begin test flights with a second prototype of its five-seat electric rotorcraft design, pictured here at Joby’s Marina, California, facility. Credit: Joby Aviation

Engage the (almost) silent drive