Bursting the EVTOL Bubble

Spend any time on aviation, business, traditional and social media, and you will hear how electric-powered vertical take-off and landing (eVTOL) aircraft will slash commute times from hours to minutes within three to five years.

The trouble with the above paragraph is that it could have been written at almost any time since 2018, including the three-to-five years bit.

The eVTOL (also called urban air mobility or UAM) business has stayed airborne without delivering returns in the zero interest rate policy era, as investors have been tempted into everything from the stock market to crypto to Bored Ape NFTs in the quest for big returns. However, as financial markets tighten, investors will start to ask questions. Here are ten that really need to be asked:

Louis Blériot crossed the English Channel in 1909 with just 25hp at his disposal. Why have none of the eVTOL manufacturer's completed a similar feat? (WikiCommons)

Joby has flown a 134nm distance on electric power, but this was with no payload and on a closed circuit, with only one energy-draining take-off and landing.

The range problem has two roots: eVTOLs may be green but they are not particularly efficient. A small helicopter with a 36ft rotor diameter has six times the disk area of an eVTOL with six 6ft diameter rotors, and the power required for vertical flight is inversely proportional to disk area. Carrying so much engine power and battery capacity adds weight throughout the flight and, for eVTOLs with separate lift and propulsion systems, the stopped lifting rotors add drag.

Combine an aircraft that is already not very efficient with battery power – which today has a 30:1 disadvantage in energy density (Watt-hours/kg) over hydrocarbon fuel – and you have a problem. Batteries that can provide the energy density to deliver the 25 mile round-trip mission, with four vertical segments, are simply not yet available.

Until someone demonstrates that reference mission, eVTOLs are still at the equivalent point of waiting for Louis Blériot to cross the Channel.

Should we hold out hope for ever-developing battery technology?

The lithium-ion (L-ion) battery has had an impact on travel comparable to steam and the internal combustion engine. Battery development continues, but there is no sure sign of another such jump in energy density now that plug-in electric cars have attained a range that most consumers can live with. The industry is focused on reducing cost, improving durability and fixing safety issues.

However, eVTOLs need more than just energy density. Automotive propulsion systems have a light duty cycle, using only a small fraction of their power at constant speed. Conversely, the eVTOL cycle is not only more intense but has frequent peaks. In battery terminology, this is defined as C-rate. A C-rate of 1 is the rate that will discharge a battery in one hour; a 10C rate will do so in six minutes and a C/2 rate will take two hours.

An early 2024 paper from Oak Ridge National Laboratory looked at the high C-rates incurred by eVTOL batteries during take-off and landing. The researchers noted that “very limited experimental data sets exist in open literature investigating L-ion batteries under these extreme power conditions.” The researchers determined that an eVTOL battery would require a 45sec 15C energy pulse to provide take-off power. The state-of-the-art batteries under test, in simulated eVTOL operations, showed major loss of performance after 85 cycles.

Will current designs evolve from prototypes to production vehicles?

The eVTOLs flown so far are effectively ‘X-planes’, comparable to other VTOL aircraft and helicopters in their size and complexity. Such high-disk-loading, transitioning aircraft with electrical power and flight-critical integrated flight/propulsion control are completely new to commercial flight.

Aerodynamicist, Dr Richard Brown of Sophrodyne Aerospace, a Scottish-based consultancy, has conducted computational fluid dynamics (CFD) simulations of highly loaded multi-rotor configurations. His work shows that the interaction of stream-tubes through multiple rotors is complex, each rotor affecting its neighbours differently as forward speed changes. The highly loaded systems will be susceptible to vortex ring state in rapid descents, and interaction with the ground will be complex, with an uneven outwash pattern, including high-velocity jets.

Developers of eVTOL vehicles intend to manage this complexity through tightly integrated flight and propulsion control systems (IFPCS) which must not only be reliable but highly responsive.

If a motor fails, the IFPCS must determine which one has failed, and compensate by reducing power in the opposite quadrant of the vehicle, while increasing power on the remaining rotors to maintain level flight. If there is turbulence, the IFPCS needs to apply differential power, without misinterpreting the phenomenon as caused by a failure – which could lead to a cycle of over-correction.

Today, even a conventional aircraft, like the Textron Denali, appears to be taking ten years from launch to certification – so quoting less than that for an eVTOL seems ambitious.

What happens if the battery runs out before you reach that vertiport?

Mike Hirschberg, Director of Strategy for the Vertical Flight Society in Fairfax, Virginia, has called proposed UAM flight reserve rules – 30 minutes of reserve power beyond their planned flight times during the day and 45 minutes at night – an “industry killer.” Instead, he calls for performance-based rules because eVTOLs can “land anywhere,” like a street or a field.

Of course, an eVTOL landing on a roadway with an exhausted battery is going to block traffic until it gets a charge, while its passengers face hours of delay. Urban open areas have obstacles in and around them. A mass event (like an unexpected squall passing over an airport) could mean that dozens of eVTOLs are looking for emergency landing spots in the same area. Reducing reserves will mean that any operational glitch will face pilots with many choices and no time to pick the least bad one.

Are eVTOLs really going to be 'flying taxis'?

Uber leaves owner/drivers to deal with capital, operating, maintenance and insurance costs, and offloads navigation to third-party platforms. UAM operations cannot work like that and will have to support three different businesses.

UAM models start with companies that operate and support vehicles – public air carriers – and employ pilots. A second group of businesses are ‘providers of services to UAM’ (PSUs). A PSU would be responsible for low-altitude airspace designated for UAM, air traffic management (ATM), weather and hazard surveillance, and flight planning, including the allocation of vertiport slots. A third set of companies would operate vertiports and there are indications that some investors are already securing rights to likely spots.

However, UAM must be like Uber in one respect: dynamic pricing. Otherwise, the hourly, daily and seasonal urban travel market is unlikely to be served profitably. The three groups of companies will share revenue through fee structures.

Would you spend $100,000 on training to fly an eVTOL? (CAE)

UAM will need pilots, at least initially. It will also need pilot-trained supervisors, even if later vehicles are uncrewed. (They will never be autonomous – there will be a massive human-supervised system directing them.)

For some 60 years, as military pilot forces shrank and airlines grew, low-paid starter jobs and costly training courses have been a workable model, because pilots bank on a big-jet future. The UAM sector will need too many pilots for that model, so how will pilots be tempted to spend $100,000 on training, if they worry that the next stage in the UAM business plan is to make them redundant?

Passengers will (mostly) climb aboard and airliner without fear. Does the same apply to an eVTOL?

This might seem a silly question if you watch promotional CGI in which commuters ride smoothly in quiet glass-walled pods, sailing serenely over traffic jams and crowded trains.

Of course, neither flying in a small aircraft nor the weather is always like that, but not everyone knows that. The fraction of the population who have flown in such conditions is so small that market research means little. It is important to remember that eVTOLs will fly at low altitude, often in cloud with no ground visibility. Rain, let alone hail, will make its presence felt and heard. Lightweight eVTOLs will bounce around in turbulence, whether caused by storms, high buildings or urban thermals. It could be a more exciting ride than the passengers expect.

Heating and air conditioning in greenhouse cabins will be limited to what energy the batteries can spare.

Helicopters have ten times the accident rate and 17 times the per hour fatality rate of large commercial aircraft. How will eVTOLs compare?

In their early years of service, eVTOLs will be newsworthy. In urban environments, webcams of one kind or another will capture most operations in daylight, and eVTOL users will stream their experience. Accidents are inevitable – the question is what rate will the public accept?

Part 135 operations – public-transport aircraft under 30 seats, and helicopters – have ten times the accident rate and 17 times the per hour fatality rate of large commercial aircraft. At that rate 1,000 eVTOLs would suffer only one fatal accident per year – but eVTOL operations will face inherently higher risks compared with the average Part 135 service, just because of shorter trip times, combined with low altitude and weather.

Comparison with helicopters is less encouraging. A 2020 Flight Safety Foundation study of passenger helicopter safety – excluding emergency medical services and other non-transport activity – shows a fatal accident rate six times the Part 135 average. At that rate, a 1,000-strong eVTOL fleet – on the low end of what eVTOL backers would see as typical for one large metropolitan region – would incur 10.8 fatal accidents per year, about one per month.

The Uber Elevate report of 2017 that started the eVTOL boom envisaged that the fatality rate of eVTOLs could be lowered “to one-fourth of the average Part 135 rate, making VTOLs twice as safe as driving.” However, it fails to say how. Proponents cite ‘redundancy’ with multiple motors and propulsors, and a high level of automation – but redundancy never ventures out without her disagreeable wingman, complexity. The more functions, the harder it is for the automated systems to work out what has gone wrong and take the right corrective action.

The future airspace environment could be complex and dynamic. (NASA)

If eVTOL operations expand as the industry expects, and as the business model demands, the mission-management task will be challenging. In near-real time, passenger trip requests will generate a proposed flight plan and a fare quote, and a paid booking will allocate a vehicle, firm up a flight plan (deconflicted from other plans) and reserve vertiport slots. If a metro has 1,000 eVTOLs flying multiple trips per hour, there may be a new trip generated every second.

If there is a disruptive event in this complex sequence, how many other events are disrupted in consequence? Say that an eVTOL glitches at a high-demand downtown vertiport and it cannot fly until a part is replaced. The system economics does not support stocks of parts, let alone maintenance crews, at hundreds of pads and ports.

If it is stuck there for 90 minutes, five or more vehicles that were intending to use that pad in that time are on a ground hold. In turn, each of them is blocking the missions that were flight-planned to use the pad where they are holding. And each of those vehicles is booked for several later trips that will now have to be replanned.
Will the average anomaly in the system disrupt less or more than one other mission? If the answer is ‘more’ you have an exponential problem.

Will eVTOLs really relieve the congestion on our roads?

This is a scale problem. London Heathrow (LHR) Airport’s busiest day of 2023 – the day on which it is most important to reduce congestion – was 22 December, with 250,000 passengers served. Historically, LHR has a stable 2:1 ratio of origin and destination traffic to connecting passengers, so eVTOLs would have to accommodate over 16,600 passengers to replace just 10% of rail or road traffic. Four-passenger eVTOLs at an 80% load factor would have to perform over 5,200 movements (landings and take-offs) to do this. That is one landing and take-off every 24 seconds between 0600 and 2300, on average – but there would be morning and evening peaks. Even discounting peaks and assuming ten-minute turnarounds, the operation would require well over 200 aircraft and 30 eVTOL pads – a more realistic number might be 50 or more. Does anybody want to ask NATS what they think of that?

Even if a UAM network can move tens of thousands of people per day, commuter trips in large cities are counted in millions. One fully loaded subway train carries as many people as more than 200 eVTOLs. How many eVTOLs will it take to make a dent in ground congestion?

That points to a world where eVTOLs, at best, are tools for the better off, linking the urban business centre to remote areas off the major road and rail networks, at no small cost in energy. Where is the social good in that?