Noise, vertiports, electrical grids may prompt questions.

Developers of advanced air mobility aircraft are progressing toward mid-decade goals of gaining airworthiness certification for their products, with flight tests being accelerated and production facilities laid out.

“The momentum is really building,” Joby Aviation founder and CEO JoeBen Bevirt told Bloomberg News at the future-of-mobility UP.Summit in Arkansas last month. Joby is working toward launching service in 2024 with its six-tiltrotor, four-passenger, single-pilot S4 electric vertical takeoff and landing (eVTOL) aircraft.

Advanced air mobility (AAM) participants are developing campaigns to persuade residents and businesses to accept — and even embrace — eVTOL flights in and over their neighborhoods. Successful AAM operations will rely on frequent flights by unfamiliar, low-flying aircraft on all-new routes between freshly built, heliport-like structures scattered around urban and suburban areas.

JoBen Bevirt Joby
JoeBen Bevirt

“We’ve seen a two-phased approach to urban air vehicles,” Matheu Parr, customer business director for Rolls-Royce Electrical, said. “Phase I has been the technical/regulatory stage of airframers, propulsion system companies, and regulators coming together” on how to bring electric-powered, short-range commercial air operations to fruition safely. Rolls-Royce is developing a portfolio of products to support those operations, and broader, longer-range ones to follow, as part of its “absolute focus” on achieving Net Zero aviation by 2050, he said. That portfolio includes developing complete electric propulsion systems for eVTOL OEMs Vertical Aerospace and Eve Air Mobility, in which Rolls-Royce has invested.

Mattheu Parr Rolls-Royce Electrical
Matheu Parr
Rolls-Royce Electrical

Phase 2 started late last year and is focused on infrastructure, detailing eVTOL use cases “that clearly demonstrate societal benefits … and working with the public to understand their concerns and ensure that we get a good reception to entry into service for these aircraft,” Parr said. That phase has the strong focus of everyone from airlines, airports and airframers to propulsion suppliers. “We need to demonstrate how these aircraft really move to democratizing mobility” and transporting people “in a very different way and at an affordable cost level,” Parr said.

Mattheu Parr Rolls-Royce Electrical
Jeffrey Engler
Wright Electric

What is AAM?

AAM is a widely used term without a common definition (often used interchangeably, for instance, with urban air mobility). For our purposes, it aims to use electric or hybrid-electric propulsion systems and distributed power systems for aircraft that will make affordable flight accessible to a broader group of passengers (often with on-demand-like services) and also reduce aviation’s carbon footprint.

“On a per-minute basis, there is nothing other than space travel that is worse for the environment than air travel,” said Jeffrey Engler, founder and CEO of Wright Electric, an upstate New York company committed to producing lightweight electric power systems for aviation and eliminating carbon emissions from all flights under 695 nautical miles (1,288 kilometers) by 2040.

AAM includes:

• Urban air mobility (UAM), which involves vertical-lift flights of about 85 nautical miles (160 kilometers) or less within “mega-cities” of more than 10 million residents.

• Longer vertical-lift AAM flights, beyond roughly 85 nautical miles, to and from mega-city suburbs.

• Regional air mobility (RAM), using fixed-wing electric/hybrid aircraft with ranges up to about 700 nautical miles (1,295 kilometers) for intercity services.

The leading eVTOL professional technical association, the Vertical Flight Society, groups that sector’s developing aircraft into five categories:

• Vectored-thrust eVTOLs use any of their thrusters for lift and cruise. An example is Joby’s S4.

• Lift-plus-cruise ones employ completely independent thrusters for cruise flight and others for lift (with no thrust vectoring). Example: Airbus’ four-passenger CityAirbus.

• Wingless (multicopter) eVTOLs use thrusters only for lift. Example: the EHang 216 from Guangzhou, China’s EHang.

• Hover Bikes/Personal Flying Devices are single-person eVTOLs that so far are wingless, multicopter configurations on which the operator stands or rides a saddle. Example: Stockholm-based Jetson AB’s Jetson ONE.

• Electric rotorcraft are electric helicopters or autogyros that use rotors for lift and thrust. Example: Dallas-based Jaunt Air Mobility’s slowed-rotor, compound, four-passenger Journey.

That society tracks eVTOL developments in an online directory that lists about 680. Only about two dozen had actually advanced to flying prototypes by early this year. The vast majority had been flown remotely, with no passenger.

A Growth Market

Estimates of the AAM worldwide market vary widely, depending in part on whether they include military applications and aircraft other than eVTOLs, according to Adam Cohen, senior research manager at the University of California at Berkeley’s Transportation Sustainability Research Center. Projections of the global market range from $74 billion to $641 billion by 2035. In the U.S., projections for the passenger market range between $2.8 billion and $4 billion by 2030.

The consulting firm Deloitte notes that last year marked a milestone for the AAM market. In 2021, eVTOL companies garnered $5.8 billion in investments. That compared to $4.5 billion reported between 2010 and 2020. The broad special-purpose acquisition corporation (SPAC) craze, fueled by accounts flush with Covid-related relief funds and by very low interest rates that left investors hunting higher returns, helped eVTOL firms get funding through public stock sales. Joby Aviation was the first company to go public, Deloitte said, followed by Archer Aviation, Lilium, and Vertical Aerospace. This year, rising interest rates and increased regulatory scrutiny have slowed SPAC activity.

Recent progress in AAM efforts include the following (see boxes):

Noise Reduction Efforts

To gain the public acceptance upon which AAM commercial success depends, industry leaders agree that it’s critical to achieve a clear understanding of how eVTOLs generate noise, how that affects and annoys people under and around their flights, and how OEMs and AAM operators can mitigate those effects. Manufacturers have worked individually to reduce the noise of their aircraft designs, but the industry’s work to identify, manipulate, and regulate eVTOL noise is beginning in earnest.

Joby and NASA partnered for noise testing. NASA’s analysis of the results were no more than about 65 dBA within about 328 feet (100 meters) of the flight track. NASA image.
Joby and NASA partnered for noise testing. NASA’s analysis of the results were no more than about 65 dBA within about 328 feet (100 meters) of the flight track. NASA image.

NASA in June published results of two weeks of mid-2021 noise tests with Joby’s pre-production eVTOL prototype at its Electric Flight Base near Big Sur, California. NASA deployed its Mobile Acoustics Facility containing wireless acoustics measurement systems. Researchers arrayed 58 microphones under the eVTOL’s flight path to capture its acoustic footprint. Joby agreed to allow the test results to be published.

“This was the first full-scale advanced air mobility vehicle that we were able to test,” said Kyle Pascioni, a NASA Langley Research Center aeroacoustics research aerospace engineer and the acoustics lead of that agency’s Advanced Air Mobility National Campaign. “We were able to acquire data on representative conditions of essentially all phases of flight.”

Kyle Pascioni NASA
Kyle Pascioni

The tests looked at objective data on the eVTOL’s noise — how much sound pressure it generated in overflight, transition, approach, landing, departure, and hover — and concentrated on the frequencies to which the human ear is most sensitive, a process called A-weighting. “For instance, 3 kHz is the most sensitive frequency of the human ear, so that’s weighted the most,” Pascioni said.

Joby trumpeted its prototype’s acoustic signature of 45.2 A-weighted decibels (dBA) during the flyover at 1,640 feet (500 meters) and 100 knots (185 kph), a level it said would be “barely perceptible against the ambient environment of cities.” NASA researchers focused on sound signatures at lower altitudes. The target was about 350 feet (107 meters) to near touchdown. That simulated near-vertiport operations and enabled researchers to get the best signal-to-noise ratio and resolution. According to a paper on NASA’s analysis of the results, hovers, approaches and departures were all no more than about 65 dBA within about 328 feet (100 meters) of the flight track. Approaches were noisiest.

The noise tests did not assess more subjective perceptions, or psycho-acoustic perspectives, including the annoyance level, if any, of eVTOL flights. That work, which may inform eVTOL noise regulations, remains to be done. “When you start getting into the local communities and even the federal regulators, there’s a lot we need to know about this if we want the AAM system to really scale,” said NASA AAM mission manager Davis Hackenberg. “We don’t want to go into urban environments and scare everybody away from the system.”

Supply of raw materials, particularly for batteries, will be a challenge as electric vehicle sales increase. Benchmark Mineral Intelligence graph.
Supply of raw materials, particularly for batteries, will be a challenge as electric vehicle sales increase. Benchmark Mineral Intelligence graph.

Infrastructure Conundrums

Another key hurdle is developing the networks of eVTOL vertiports that will underpin flight operations and the local “microgrids” to enable them by providing aircraft with electrical power for battery charging stations. “If you’re an all-electric system and you’re dependent on electricity, it’s about $1 million a mile,” said Rex Alexander, president of Five-Alpha, who may be the world’s leading expert on vertiports. “If you’re looking at having a large vertiport, you’re probably going to have to have your own substation. Well, if there is one thing people hate more than heliports, it’s substations.” He added that a substation can take two to four years to build.

More Supply Chain Drama

A longer-term challenge is the supply of raw materials, particularly for batteries. Low prices for critical minerals — lithium, graphite and cobalt — depressed investment in mining over the last decade, said George Miller, a senior price analyst for the mineral-reporting company Benchmark Mineral Intelligence. That, combined with the rapid growth in electric vehicle sales, means demand for those minerals will outstrip their supply over the next decade, drive up prices, and constrain availability.

A “gigafactory” produces batteries for electric vehicles at large scale. On average, each year one consumes 88,000 tons of flake graphite and 49,600 tons of synthetic graphite (both of which are used to manufacture anodes for lithium-ion batteries), 27,500 tons of lithium, 6,000 tons of cobalt (for the batteries’ cathodes), and 21,000 tons of nickel. Miller said today 300 new gigafactories are planned for construction.

“These are growth rates you won’t see for any other commodity market in our lifetime,” Miller said. Before 2010, one new lithium mine could meet excess battery demand for one to five years. “We need to see multiple world-class assets coming to production on a yearly basis from now on out until 2030 to account for this growth in battery demand.”

George Miller Benchmark Mineral Intelligence
George Miller
Benchmark Mineral Intelligence

Other supply chain challenges include the need to process minerals specifically for end users and time required to qualify their supply for safety-critical applications. “This is a multi-year process for Western automakers especially and requires materials from miners, cathode manufacturers, and anode manufacturers to pass through several stages of development” before they can be qualified for an automotive, or eventually an air mobility, value chain, Miller said.

“The good thing for advanced air mobility is because operations at scale are a decade out, there is actually time to account for that demand” for greater supply of critical minerals, he said.