Picture a flying taxi able to whisk passengers to their chosen destination within minutes, not hours. A noiseless aircraft free from carbon emissions, capable of vertical takeoff and landing in the heart of a major city. What once sounded like a scene out of a science-fiction novel is cruising toward reality, forever altering the way we move.

Major aerospace manufacturers like Airbus, Boeing, Honeywell and Rolls-Royce, in addition to numerous start-ups, are actively developing a new breed of small battery-powered aircraft that can ferry several passengers at a time. The concept has also attracted serious interest from air carriers such as American Airlines, Virgin Atlantic and United Airlines.

Urban air mobility (UAM), an aviation industry term for on-demand passenger or cargo-carrying air transportation services, relies on vertical take-off and landing (VTOL) aircraft flown with or without a pilot. These vehicles promise to reduce urban congestion and provide new ways for people to travel around cities and rural areas.

Traditionally, aircraft use fuel-burning engines to mechanically turn rotors, propellers or fans. Many new designs, however, use a distributed electric propulsion architecture, in which multiple electric motors can be tilted or turned on and off for vertical takeoff and horizontal flight.

The emphasis on battery-powered, electrically driven craft (eVTOL) also has positive implications for carbon-neutral mobility. And, while some UAM players are sticking to a hybrid model, there’s been a heightened focus on all-electric designs.

As with the e-mobility revolution shaking up the automotive industry, the UAM sector is in the midst of figuring out how to optimize battery performance, while also dealing with weight issues and the more stringent performance reliability standards for aircraft.

 

Design and Assembly Challenges

The design and assembly of VTOLs requires a high-tech harmony of batteries, lightweight airframe components and advanced propulsion systems, all fitted together with the highest levels of precision.

“I think batteries are a real key to this,” says David Wyatt, a technology analyst at market research firm IDTechEx. “It does present a challenge to lithium-ion batteries in that they need to improve in terms of energy density. And, they need to be able to put out a lot of power—especially a high discharge rate. Third, they also need to have a very good cycle life.”

According to Wyatt, the pathways by which eVTOLs in an air taxi application become profitable is very high utilization, which means, like any airline, they need to be in the air a great deal.

“They’re going to cover substantial mileage every day, and they’re going to have to go undergo many charge cycles, potentially daily,” explains Wyatt. “If the battery life cycle is poor, they’re going to have to replace an awful lot of batteries to make the system viable. So effectively, it’s very hard to balance power, energy density and cycle life.”

Improving those capabilities will allow for the different mission profiles that UAM players will be able to fly. But, batteries are one of the key challenges facing engineers.

“You can’t simply take an automotive battery and put it into an [eVTOL] aircraft,” notes Wyatt. “The demands are simply too high. It does take some bespoke work, and it’s certainly a core area.”

VTOL aircraft provide the aviation industry with a game-changing chance to reduce emissions and introduce alternatives that address sustainability issues.

After a three-year R&D effort, a French start-up company called Ascendance Flight Technologies recently unveiled the Atea. The five-seat VTOL boasts a range of nearly 250 miles.

Equipped with modular hybrid engines developed by Ascendance, the Atea features eight rotors integrated into two fixed wings and two horizontal propellers. Two separate propulsion systems are used for vertical and horizontal flight.

A battery is used for takeoff and landing, while a turbine is used for the cruise portion of the flight. The turbine generates electricity to feed an electric distribution system for propulsion.

“The hybrid propulsion system uses a mix of kerosene and batteries for optimized fuel consumption, which results in carbon emissions reduction of up to 80 percent,” claims Jean-Christophe Lambert, CEO of Ascendance. “However, this is only the first step. Future hydrogen-powered aircraft or those using sustainable aviation fuel will be able to eliminate all emissions in flight.”

Lambert sees two paths of product evolution, including a switch to get even greener, through either hydrogen or sustainable fuels to reach net zero emissions, as well as larger aircraft equipped with more passenger seats.

“Our conviction is hybrid is necessary and the best way to quickly decarbonize aviation,” says Lambert. “As we are addressing a market for longer range flights, which a battery cannot reach, we’ll stay hybrid.”

Ascendance is also developing a more autonomous aircraft as part of a longer-term strategy. “We are developing the R&D, but we don’t plan to commercialize it until after 2030,” adds Lambert.

 

Providing Electric Power Trains

Rolls-Royce plc has been a leader in aircraft propulsion systems for decades. In recent years, the company has invested millions of dollars to become the aerospace industry’s leading supplier of electric power trains and propulsion systems, including motors, control systems, energy storage and power distribution.

“We want to supply the full system for applications within the UAM environment,” says Gabriele Teofili, business area lead for UAM at Rolls-Royce.

Rolls-Royce recently set a world speed record for electric-powered aircraft (see sidebar below). And, it has played a key role providing technology to companies ranging from Airbus to Vertical Aerospace Group Ltd., a start-up based in England.

“An electric motor gives you the ability to switch from traditional combustion [technology] to a green, zero-emissions transport solution,” explains Teofili. “There are many options for energy storage and distribution, and that’s why you’ll see this as the main choice for the UAM market—zero emissions and simplicity.”

However, because of battery weight constraints, this new breed of aircraft currently have a limited range.

“[If] you’re transporting four people with a maximum weight of 3,000 kilograms, you will see ranges of 100 to 150 nautical miles,” Teofili points out. “You can’t offer more range until the battery range goes up, or you have less payload on board, or different configurations.”

Rolls Royce engineers are currently in the process of developing UAM battery technology as part of a four-year effort that aims to extend aircraft range using the same battery weight.

“We don’t want to wait and stay static with batteries, so that’s why we have an in-house developed battery engineering capability, with the possibility to switch to a hybrid type,” says Teofili. “As aircraft sizes grow from four to nine seats, a turbo generator will be an option, but there will be an emission with a gas turbine. Fuel cells also offer the potential to extend range.”

Some observers believe it’s still early to have a clear picture of how the market will shake out. Today, because there is only development activity and no go-to-market, UAM manufacturers are heavily dependent on external investors.

“I don’t believe the companies involved in UAM design have enough power to influence battery development,” says Pedro Pacheco, an analyst at Gartner Inc. “For that matter, they will basically take the latest and the greatest the market has to offer. Or, in other words, they will rely on the major push happening today in the automotive sector to adapt more evolved battery technologies.”

According to Pacheco, the goal for everyone is achieving high energy density combined with low weight.

“In an ideal world, propulsion systems and battery development would be tied together,” Pacheco points out. “But, in practice, the financial limitations of companies involved in the design of these vehicles mean they need to tap into battery technology that is already on the market or about to be.”

When it comes to established aircraft players like Airbus and Boeing competing with startups like Archer Aviation Inc., Joby Aviation or Lilium GmbH, Pacheco says it’s the typical story of market leaders vs. disruptors.

“Airbus and Boeing do have projects in this space and are [making] interesting progress, but eVTOLs are not their No. 1 priority, especially under the current period of turmoil for the aerospace industry,” explains Pacheco. “Often, such a situation leads to the outcome that market leaders are seldom the main disruptors. However, eVTOLs still have a long time to market ahead of them, which means opportunity for new market dynamics to settle.”

 

Reducing Weight and Flying by Wire

Another huge challenge facing aerospace engineers is reducing the overall weight of eVTOLs. They’re exploring new materials that can be used to create fuselages and wings. For instance, Vertical Aerospace worked with Solvay to develop a composite structure for its VA-1X air taxi demonstrator.

“By providing the performance required to operate safely and maximize range, while facilitating the processes needed for mass-production, our advanced materials will be key enablers to the mass-adoption of eVTOLs,” claims Carmelo Lo Faro, president of Solvay Composite Materials. “With their ability to significantly reduce both weight and manufacturing costs compared to metal, thermoset and thermoplastic composites have an important role to play in UAM technology.”

According to Lo Faro, the companies that build these aircraft need great flexibility in the materials they use so they can experiment with different types of manufacturing processes and design prototypes as easily and rapidly as possible.

“Battery cases require lightweighting, as well,” notes Hill. “Carbon-fiber composite components are ideal, because it can help contain any thermal runaway of the battery.”

Another supplier tackling the UAM weight challenge is Honeywell International Inc., which recently formed a strategic partnership with Denso Corp. to develop electric propulsion units for air taxis and delivery vehicles.

“Weight is everything for these guys,” says Andrew Barker, senior director of sales and marketing for UAM and unmanned aerial systems at Honeywell. “I won’t say it’s more important than cost, but almost, and so that’s certainly something that [we have] looked at very closely in the development and miniaturization of our compact fly-by-wire system.

“That’s a system that just a few years ago would have weighed 50 pounds and now is down in the single pound range,” explains Barker. “It’s the same with our radar technology, another place where miniaturization comes into play.”

Honeywell is providing the avionics and fly-by-wire system for Vertical Aerospace’s VX4 aircraft, which is due to begin mass-production in 2024. It’s also supplying the flight deck and fly-by-wire system for the Lilium Jet, in addition to navigation and automatic landing systems for Eviation’s Alice aircraft.

The compact fly-by-wire system fits in a package the size of a paperback book and offers the redundancy needed to meet the safety and certification requirements for eVTOL aircraft. Its controls are augmented by electronics vs. purely manual controls. The system also reduces turbulence and enables engineers to push the limits of aerodynamics, eliminating the need for heavy hydraulics, control cables or pushrods.

The computer packs the brains of the plane’s flight controls into one system. A flight control computer adds stability by driving electric actuators and dynamically adjusting flight surfaces and motors, resulting in smoothly followed flight paths.

Honeywell’s system features a triplex flight control computer architecture, which provides multiple backup options and eliminates the risk of relying on one system failure. In addition, each computer uses lockstep processing, meaning it has two processing channels that constantly check each other’s work.

“A device in the past may have been only capable of doing one task, but now with multiple core systems, you can partition software, and now one box can serve as three boxes,” explains Barker. “We can reduce our weight not by necessarily shaving ounces and grams off our heatsinks, where you can’t really do a whole lot, but by doing more with less boxes. That’s where we really get our big weight savings.”

 

Cranking Up the Assembly Rate

Production ramp up will play a critical role in the UAM market over the next decade, as aircraft move beyond the R&D phase. To meet projected demand, some companies plan to borrow assembly techniques from the auto industry.

“When we look at the build rate forecast, different OEMs have very aggressive projections, some of up to 3,000 aircraft per year,” says Solvay’s Hill. “But, the market hasn’t industrialized to that rate yet for high-performance composite materials.”

Hill believes the projected build rates and lightweight requirements of UAMs will drive innovative assembly techniques, with advancements of automated processes and novel materials playing a key role, such as fully bonded airframes made out of thermosets and fully welded aerostructures made with thermoplastic composites. Improved dimensional tolerances and one-sided mechanical fastening will also improve quality and reduce production time.

According to Hill, working with a partner that can bring combined expertise from both the aerospace and automotive markets could also be valuable. That’s why there’s recently been strategic partnerships announced between companies ranging from Archer and Stellantis to Joby and Toyota. Automakers such as BMW, General Motors, Honda and Hyundai have also dabbled in UAM technology.

“UAM manufacturers can leverage the electrification developments in the automotive industry to their advantage,” says Hill. “Whether that is power train efficiency, battery systems, structural design efficiency or electric motor performance, having a more holistic approach to material expertise will undoubtedly offer real insights and add value throughout the design and development process.”

Ascendance’s Lambert points out that the market is asking for more aircraft than have traditionally been produced, which requires an evolution of production lines. “You can take inspiration from auto production, and you have to think about your production footprint,” he explains. “If your [market] is worldwide, you’re likely to need a global production strategy.”

The companies that succeed in the UAM sector will come up with a cost-effective manufacturing process that enables them to produce many more aircraft than low-volume aerospace manufacturers have traditionally produced.

“The goal is to get to a level of production where there’s an economy of scale savings, so the aircraft become cheaper,” adds IDTechEx’s Wyatt. “But, I don’t think anyone’s that far down the line development wise, where they have those kinds of structures in place yet for mass-production, or even automated systems for that.”

 

Clearing a Regulatory Path

As the UAM ecosystem moves forward solving the challenges of weight, propulsion, automation and guidance systems, regulation—and in fact, the development of the regulatory guidelines themselves—are another critical component that needs to be addressed.

First, there is a safety accreditation similar to every traditional aircraft. However, the trickier bit lies with next-generation autonomous flight systems that will also have to receive safety approval. On top of that comes unmanned traffic management systems, which are needed to coordinate all aircraft in the sky, to make sure they don’t collide or enter restricted no-fly zones.

Many of these regulations have not even been created yet, which poses a major conundrum for eVTOL makers. The aviation sector has always been one of the toughest concerning safety regulations, and that certainly won’t change with UAMs.

“However, this is not compatible with a typical start-up entrepreneurial approach that often is about learning by trying,” says Gartner’s Pacheco. “As such, there isn’t a way to get eVTOLs to market fast.”

Regulators and manufacturers need to address this roadblock to speed things up. On one hand, regulators need to create regulatory sandboxes that allow companies to try and test their vehicles in real life, as this will be a valuable part of development. On the other hand, eVTOL makers must come together, formulate and apply strict safety standards to ensure the competition is fair and that no one is taking shortcuts.

“It has to be a collaboration between public and private players,” warns Honeywell’s Barker. “The Federal Aviation Administration (FAA) and European Union Aviation Safety Agency (EASA) obviously are the main two certification bodies; they’re setting that tone.

“It’s important to make sure we are helping with that communication in any way possible, because this is new technology,” says Barker. “There’s new airspaces, these are new vehicle types and there’s going to be much more communication required between the vehicles. There are still a lot of problems to solve before we [will] see these aircraft in widespread use.”

 

Making the Business Concept Fly

Hovering over all other issues is what potential applications are most realistic from a technical and economic perspective.

“Moving to electric propulsion helps the cost equation, but not disruptively,” notes Gartner’s Pacheco. “Autonomy removes the pilot’s salary out of the equation but also makes time to market considerably longer.”

Pacheco believes early eVTOL applications will revolve around use cases that prioritize greater convenience and speed, like urgent transport of goods or emergency services. Another use-case scenario will be air taxi service for affluent passengers around large cities or between airports and city centers.

“The fact it’s electric—and eventually autonomous—will open eVTOLs to a slightly larger market than helicopters, thanks to the lower price, even if it won’t be massively larger,” says Pacheco. “Addressing public concerns is important, but there isn’t much eVTOL makers can do prior to these vehicles being operational.”

The initial debut of smart phones is a good example of human beings at first being averse to change and slow in understanding the need for a disruptive technology.

“Acceptance will come, but first the technology needs to come to the market in a convenient and cost-competitive offer,” predicts Pacheco. “For now, the best eVTOL makers can do is to avoid accidents at all cost, as each one of these events builds a major negative perception in the public.”

Eventually, autonomous operation will become an important element of the UAM business model, as the removal of a human pilot frees up a seat to a paying customer.

“The goal is certainly for these aircraft to be able to fly themselves, but I don’t see that happening in the next decade,” says IDTechEx’s Wyatt. “I think the regulatory environment will necessitate having a pilot on board who can take over the aircraft should it require that.”

Wyatt also doesn’t foresee the FAA and EASA allowing for completely autonomous flights, especially over urban environments, without considerable testing. But, that hasn’t stopped companies like Boeing subsidiary Wisk Aero LLC from testing unmanned eVTOLs in remote locations, such as New Zealand.

“I suspect autonomous UAM is the goal of most of the OEMs in the space at the moment, but it’s a longer-term strategy,” warns Wyatt.

“There’s still a lot of work to be done on control of low-level air space,” Wyatt points out. “Once you start getting to a volume of multiple aircraft all trying to use a limited air space, even the design of the ground infrastructure needs a lot of capital to make it a viable low-cost form of transport. There’s certainly a lot of work still to be done.”

Whether the aircraft operate with fully electric or hybrid propulsion designs, it is not yet clear to some observers how the UAM concept will function as a transportation service.

“The traditional view of these aircraft is of them buzzing between skyscrapers, but that’s not where they’re going to be most useful,” claims Wyatt. “I don’t see them used for short hops. The benefit for eVTOLs is 50- to 100-kilometer journeys connecting poorly served areas. Especially locations that are in some way separated by something that makes the ground-based journey take a long time [such as a lake or a mountain].”

From Wyatt’s perspective, the ability to do a direct flight, cover that A to B distance in the best time possible, will provide a significant time savings and provide the fundamental economic basis for UAM development.

A flying taxi will not solve all of the world’s current ecological and transport problems, but this is not really the point: The idea is to allow more transport options for bringing people in and out of cities.

“We are going to improve the way people are transported, not only by the capability, but by numbers, with fewer cars and polluting transportation needs, thanks to the extension capabilities from eVTOLs,” says Rolls-Royce’s Teofili. “I want to be in a place where I can live more comfortably, and not throw away a lifetime spent in traffic. It’s about quality of life, which is very important from my point of view.”

 

Rolls-Royce Sets Electric-Powered Aircraft Record

The “Spirit of Innovation” recently set several world records for electric-powered aircraft, reaching a top speed of 387 mph. Photo courtesy Rolls-Royce

Rolls-Royce recently set three world speed records with its all-electric “Spirit of Innovation” aircraft. The state-of-the-art plane reached a top speed of 387.4 mph during runs conducted at the UK Ministry of Defense’s Boscombe Down experimental aircraft testing site, located in southern England near Stonehenge.

The aircraft reached a top speed of 345.4 mph over a 3-kilometer course, smashing the existing record (set by the Siemens eAircraft powered Extra 330 LE Aerobatic aircraft in 2017) by 132mph. In further runs, the Rolls-Royce plane achieved 330 mph over 15 kilometers (182mph faster than the previous record) and broke the fastest time to climb 3,000 meters by 60 seconds, with a time of 202 seconds.

“The advanced battery and propulsion technology developed for this program has exciting applications for the Advanced Air Mobility market,” says Warren East, CEO of Rolls-Royce. “Never in the history of FAI record attempts has there been such a significant increase in speed over such a short time, highlighting the rapid pace at which electrification of aerospace is advancing.”

The “Spirit of Innovation” was propelled on its record breaking runs by a 400 kilowatt (more than 500 hp) electric power train and the most power-dense propulsion battery pack ever assembled in aerospace. Rolls-Royce worked in partnership with aviation energy storage specialist Electroflight and automotive power train supplier YASA. Aside from being a stunning technical achievement, the project and world record runs provided important data for future electric power and propulsion systems for all-electric urban air mobility and hybrid-electric commuter aircraft. The characteristics that air taxis require from batteries, for instance, are very similar to what was developed for the "Spirit of Innovation."

“Electric flight is set to be as transformative for mobility as the jet engine was 70 years ago,” claims Tim Woolmer, Ph.D., chief technology officer at YASA. “It’s thrilling to see our ultra-high performance, super-low weight electric motors powering the ‘Spirit of Innovation’ to these great speeds, and to know that [this project] takes us one step closer to emissions-free electric flight becoming a commercial reality for all.”