Tomorrow's transport

Batteries not included: The electric air taxis aren’t quite ready yet

People have been dreaming of flying taxis for over a century. So why aren’t there any? Lewis Page finds out.

Flying cars. They're a thing the human race has been anticipating for a long time, so much so that the idea is often seen as something of a joke. And yet, around the world, people are still trying to build them: there are lots of projects active today, many of them with actual flight-tested hardware, and there have been others before them back to the invention of the aeroplane.

If practical flying cars can ever be built, they would be of huge interest even to those who believe that private car ownership will soon be a thing of the past. That’s because flying cars would naturally offer the possibility of airborne cabs or taxis, often envisaged as something you might find in the smart cities of the future.

A flying taxi, indeed, is perhaps a more realistic idea to begin with than a flying personal car; a taxi operator is able to finance an expensive vehicle and get it to pay its way more easily than an individual owner. That's why, when the first motor vehicles appeared on city streets over a hundred years ago, they were typically cabs rather than private conveyances. Until after World War II, the only way a normal person would ever ride in a motor car was in a taxi (or maybe for military service). Personal cars were rare for many decades after cars took over from horses in taxi service, and it would seem that flying cars may work out the same way.

Certainly big money is now backing the idea of flying taxis or cabs, rather than privately-owned flying cars. Billions have been ploughed into the new wave of air-taxi startups. In March, German air-taxi firm Lilium announced a special purpose acquisition company (SPAC) flotation plan that values it at $3.3bn. Others besides Lilium are heading for stock market listings via SPACs. One of them, Joby Aviation, has been valued at nearly $7bn. Another, Archer, is worth almost $4bn. Morgan Stanley has estimated that the market for aerial taxi rides could be worth $674bn by 2040.

Lilium Jet. Credit: Lilium

But a flood of investment doesn’t necessarily mean the dreams will come true. In truth there are several serious technical obstacles ahead of the flying-cab dream.

First, a flying cab should be able to pick people up and drop them off in the heart of a city. This means that it can’t make use of a runway: it has to be able to take off and land vertically.

The main type of aircraft in widespread use today which can take off vertically carrying a useful load is the helicopter. Helicopters achieve the necessary thrust by having long rotor blades which sweep a large disc area – the helicopter has “low disc loading”, in engineering parlance. This means that it doesn’t need excessive amounts of power to get airborne. A related concept is the tiltrotor, which points its propellers up in helicopter mode to make vertical landings or takeoffs and swivels them to point forward when flying in aeroplane mode, allowing it to fly further and faster than a normal helicopter can.

Despite their low disc loading, both helicopters and tiltrotors of today need to use powerful gas turbine engines if they are to lift off carrying any significant load.

There were flying airport buses, once. It didn’t end well

Way back in the 1940s, people expected that helicopters might become the flying cars of today. They did catch on to some degree, but the fact is that they aren’t suitable for picking people up or dropping them off in city centres, the way a cab does. Helicopters were used for a sort of bus service between New York’s John F Kennedy airport (JFK) and the roof of the Pan Am building, a Manhattan skyscraper, back in the 1960s and 70s: the noise problem wasn't too serious as the pad was 60 stories above street level. Unfortunately, a helicopter preparing to take on passengers suffered a serious accident in 1977, resulting in several deaths. The service was shut down.

Taxi app company Uber has tried to use helicopters as cabs more recently with its so-called Uber Copter offering, which in some locations for some customers included a helicopter flight as part of a journey. For instance, someone travelling from lower Manhattan to JFK could choose to be driven to the existing waterside heliport around the corner from Wall Street, fly from there to the airport, and then get in another Uber car to their terminal or hotel.

The market for helicopter trips does exist, but it’s not a $674bn one.

As one journalist noted when Uber Copter launched, the two car rides and chopper flight combined can often take longer than simply driving to the airport, and the $200 ride wasn’t any cheaper than existing helicopter services from the Manhattan heliports to JFK, or other popular Wall Streeter destinations such as the Hamptons and Nantucket. The market for helicopter trips does exist, but it’s not a $674bn one. Today’s helicopters are not flying taxis.

OK, so what would be a flying taxi?

One answer offered by the new wave of air-taxi firms is basically an electric helicopter, often called a “multicopter”. Such designs, rather than a single large set of rotors, usually have many small sets of rotors attached to a frame above the fuselage, each powered by its own electric motor (though China’s eHang attaches the rotor frame to the bottom of the cab).

Perhaps the best known multicopter design comes from Volocopter, a German company which has been working on its aircraft for ten years now. Its latest machine, which it describes as its fourth generation, is called the VoloCity. The previous model, the Volocopter 2X, was described as a “production model”, but didn’t actually go into production.

A multicopter’s many rotors give it almost as large a thrust disc area as a normal helicopter, which means that if it can muster as much power and carry as much energy as a normal helicopter it will offer similar performance.

The power isn’t a problem: electric motors are good at power. Unfortunately even the best batteries can’t contain anything like as much energy for a given weight as a tank of fuel, which means that an air-taxi’s electric motors can’t keep running for long at high output.

Multicopters need to run their motors at high output all the time just to stay in the air, and this means that they don’t offer anything like gas-turbine helicopter performance. The Volocopter 2X had a useful load of just 160kg: it was unable to lift two average 90kg American men off the ground, or even a pair of 82kg Germans, and as such was mostly a single-person machine. Volocopter optimistically described it as “optionally piloted”, but even if the company does someday achieve certification for unpiloted passenger flights the 2X would not be very useful.

At an average speed of 80 to 100km per hour, travelling a flight route of 29km from New York’s JFK airport to Times Square takes only 20 minutes.

The new VoloCity is considerably bigger and more than twice as heavy, much the same size and weight as a normal light helicopter. Volocopter says it can lift two average American men with a small margin left over, and as such will be suitable for piloted taxi operations with one passenger. The VoloCity won’t be able to stay up for long before its batteries go flat, but the company says it can make useful trips.

A recent whitepaper by CEO Florian Reuter says: “At an average speed of 80 to 100km per hour, travelling a flight route of 29km from New York’s JFK airport to Times Square takes only 20 minutes”.

Such a journey would take less than half the time by helicopter, though, and neither helicopters nor VoloCity are actually allowed to land in Times Square.

Voloport. Credit: Volocopter

Advantage: Electric

But this is where Reuter claims an advantage. Volocopter says that its aircraft are much quieter than helicopters, such that one landing 30m away produces a noise level of 78 decibels. A helicopter at such a distance generally produces 100 decibels or more. It’s a big difference, the same as that between ordinary city traffic noise and standing right next to someone using a pneumatic drill. And multicopters should be much safer than normal helicopters, as their many motors and rotors allow for some breakdowns without a disastrous crash.

Reuter believes that governments around the world will permit the establishment of “Voloports” all over densely populated areas like Manhattan. A passenger headed for the airport wouldn’t need to get a ground taxi to one of the three existing Manhattan heliports, but only to the nearest Voloport, which would often be on top of a nearby building. Better yet in Volocopter’s eyes, someone wanting to get a noticeable distance uptown, downtown or crosstown might choose to avoid the crowded subways and city traffic, and instead take a short hop Voloport to Voloport.

And of course, at some point in the future, normal fossil-fuelled helicopters might be banned or heavily taxed, but emissions-free electric Volocopters would not be.

Electric multicopters, however, are seriously limited in endurance, range and speed. Volocopter claims that the VoloCity can make 35km trips. The reality, as with any battery-powered machine, is likely to fall short of the manufacturer’s spec, especially once the batteries have been through a significant number of charge-discharge cycles. JFK-to-Manhattan trips, in the case of Volocopter, are perhaps an aspiration rather than something its aircraft can do today.

This range problem is why most air-taxi startups favour something other than a simple multicopter. There are all sorts of variations out there, but all of them involve landing and taking off vertically under propeller thrust and transitioning to aeroplane-style flight once moving horizontally. Aeroplane flight using lift from wings requires much less power than vertical thrust, and allows any aircraft to achieve much greater speed and range. This is why aeroplanes arrived 40 years before helicopters did. It’s also why the US military spent so much in years, lives and money to develop the V-22 Osprey, the only tiltrotor flying in any numbers today. It can go much further and faster than a helicopter can, while still taking off and landing without benefit of runway.

JFK-to-Manhattan trips, in the case of Volocopter, are perhaps an aspiration rather than something its aircraft can do today.

Tiltrotors of one sort or another are a popular idea in the air-taxi world, and this could make sense as it is much simpler to build an electric tiltrotor than a turboshaft one like the Osprey. It’s also comparatively easy, with electric drive, to use lots of propellers: as with multicopters this brings safety and reliability benefits as the machine can keep flying even if it should lose a motor or a prop (or even two).

One prominent electric tiltrotor design is that of Joby Aviation, set to list on the NYSE through the SPAC Reinvent Technology Partners at a projected valuation of $6.6bn. Joby was originally seed funded 10 years ago by JoeBen Bevirt, a reclusive serial entrepreneur who had previously made his money from the Gorillapod bendy camera tripod among other things.

Joby’s current aircraft is a five-seater design with six tilting props, four on the wings and two on its V-shaped tail. It has flown, though mostly unmanned thus far, and its maker claims 250km range and 300+ kph speed. It is also very quiet: the company has recently released a much-viewed video of Bevirt doing a piece to camera as his aircraft lifts off not far behind him, during which he can still be clearly heard.

If we build it, they will come

Joby describes its current machine as a production prototype, says it has an agreed pathway to certification with the Federal Aviation Authority, and expects to be certified for piloted passenger carrying in 2023. Bullishly, Bevirt contends that the only limit on Joby’s revenues will be the number of aircraft it can build, and the company is to break ground for its first major factory this year. Some of its pre-SPAC venture funding came from Toyota, so there is at-scale manufacturing expertise on tap.

There are other kinds of copter-plane combo transitioning craft out there. Some don’t tilt their propellers, instead having separate, dedicated vertical and horizontal thrusters. This is referred to in the air-taxi sector as “lift and cruise”. Drag from the lift fans when cruising is a problem here, as is the weight of two sets of equipment, but the approach does have the virtue of simplicity. Contenders in this area often look like relatively conventional aeroplanes with lift fans attached. Examples include the Cora from Wisk Aero, spun off from Google co-founder Larry Page’s Kitty Hawk venture, and the PAV from well-known crazy aircraft firm Aurora Flight Sciences, nowadays owned by Boeing.

Yet another design approach is the use of ducted fans, where the propellers are inside round or tubular cases. These ducted fans can be separated from the aircraft or built into it.

One company taking the built-in path is Lilium, which encloses small fans inside tubular ducts mounted in rows along the wings and canards of its aircraft. The company calls these “jets” and its aircraft is known as the Lilium Jet. The electric jets can swivel to point down for takeoff and landing, and then point back in the cruise. Significantly, the planned Lilium Jet has maximum takeoff weight of more than three tonnes, triple that of the Volocopter and half again that of the current Joby 2.0. This means that it can carry more – six passengers plus pilot – and lift more battery.

The hefty aircraft and the jets’ small discs mean high disc loading and heavy energy drain in the hover, and Lilium admits this is a weak point. But the ducts act to cut down noise despite the high loading and Lilium claims similar noise levels at the pad to its competitors. Once in the air and flying, the Lilium Jet’s large battery and streamlined, closely integrated fans should mean better range and speed than its competitors. The company says “250+” km and cruising speed of 280kph, about the same as Joby, though it does pointedly specify that its range is “with reserve”, and sometimes it says 300km.

We’ll be able to focus on connecting entire regions with high-speed transport, rather than trying to persuade you that we’re quicker than a crosstown journey on an underground train or bike.

Lilium is also different to other air-taxi firms in that it says straight out it’s not interested in flights of less than 20km. You won’t be able to catch a Lilium jet for journeys between pads in the same city. Remo Gerber, Lilium COO, flags up the same problem that Uber Copter illustrated.

In a recent blog post he writes: “Taking a short hop from Lower Manhattan to Grand Central Station in New York, or from the English Garden in Munich to the main station, is not only impractical (you’d need hundreds of landing pads in one city) but it won’t actually save you any time as you’ll likely need to travel to and from the vertiport as well as check-in for your journey…

“We’ll be able to focus on connecting entire regions with high-speed transport, rather than trying to persuade you that we’re quicker than a crosstown journey on an underground train or bike.

“In fact, contrary to the idea of ‘urban air taxis’ we think there will only ever be a handful of routes in our network that are shorter than 20km.”

Rather, Gerber says, Lilium jets would focus on places like his native Switzerland. Switzerland has excellent transport infrastructure but journey times between cities by road or train are still long, often over two hours. The Lilium Jet, he thinks, would be very popular for trips such as Zurich-Geneva, which it could do in 45 minutes as opposed to nearly three hours by car or train.

This would not be airport-to-airport, of course: there would be city-centre Lilium “vertiports”, not unlike Volocopter’s Voloports, just not nearly as many in any given city.

What’s wrong with Oakland, then?

Another likely region for Lilium, according to Gerber, would be the San Francisco Bay Area. Rather than hopping about within San Francisco itself, Lilium customers might take short flights among the various communities around the Bay, perhaps commuting from Hayward to Palo Alto for $25 each way (for whatever reason, the speculative Lilium map doesn’t envisage a vertiport in Oakland).

There is one huge issue hanging over all these companies, and that is the matter of battery capacity.

So there’s a spectrum, perhaps, among the electric vertical-lift startups. At one end there are short-hop companies like Volocopter and at the other companies like Lilium seeking to link different cities – but still from downtown pads rather than out-of-town airports. And in the middle there’s Joby, claiming to be the best of all worlds.

There is one huge issue hanging over all these companies, and that is the matter of battery capacity. It’s more or less openly acknowledged by most of them that their projected endurance, range and speed figures do not refer to the prototypes they have flying today. Rather, these figures are based on their current aircraft – but equipped with better batteries which they think they might be able to get in the near future.

Yes, an electric multicopter or tiltrotor or ducted-fan craft can lift off, fly, and land again. But it’s safe to say that with current battery tech, very few of the current aircraft have stayed up for long or gone very far.

We can say this because the mathematics of disc loading, weight and battery power are relatively well understood, and the sums have been done by other people than the electric air-taxi firms. NASA engineers, for instance, have recently said that battery packs capable of storing 400 Watt-hours of energy per kilogram of weight (400 Wh/kg) are a requirement for useful vertical-takeoff electric aircraft.

It’s important to note here that there’s a big difference between the specific energy a single lithium-ion cell can hold and the amount a whole lot of cells assembled in a pack can hold: cells can’t just be packed together in a mass, they need to be insulated from each other and usually provided with cooling systems and other additions. It’s not uncommon, when battery cells are either charging or discharging quickly, for them to overheat: if precautions aren’t taken, cells can catch fire and then set off other cells, in a phenomenon called “thermal runaway”.

All this means that assembled, ready-for-service battery packs have much worse specific energy figures than their individual cells, so that almost miraculous 600 Wh/kg cells would probably be needed to achieve NASA’s required 400 Wh/kg packs.

State of the art assembled battery packs in service as at late 2020 can’t even manage 200 Wh/kg. Lilium, for one, quietly admits that maximum range of 261km – considerably less than the 300km the optimistic Gerber was assuming for his Swiss and Bay Area plans – would require a Lilium Jet with a battery pack capable of 320 Wh/kg, far better than anything now available.

CityAirbus. Credit: Airbus

Yes … it is odd that the existing aerospace sector hasn’t done this already

This probably accounts for the fact that when big, grown-up aerospace companies build electric demonstrators their performance figures are unexciting and they don’t always bother carrying on. Unlike the air-taxi startups, whose investors naturally want their projects to proceed, established aerospace firms’ investors are often asking why their money is being spent on something other than known, profitable technologies.

Sikorsky built a battery-powered helicopter back in 2010, and estimated that it might be able to stay airborne for 12 to 15 minutes with only the pilot aboard, but understandably decided not to bother flying it. Helicopter giant Bell thinks its Nexus craft might be ready for use in the “middle to late 2020s”, and has said that for anything other than short-range flights Nexus will need a gas turbine generator to help the batteries out.

Airbus has already decommissioned its Vahana tiltrotor, which never remained airborne for more than seven minutes at a time, and says its CityAirbus four-seater multicopter can only stay up for 15 minutes. This suggests realistic range – without safety margin – of not much more than 20km, boding ill for the real-world ability of a Volocopter to get from Times Square to JFK. The CityAirbus is likely to be mothballed fairly soon, to await the appearance of batteries which would make it useful.

It might not be such a very long wait: people all over the world are working hard on better batteries, not only for aviation use but many other important applications. Current lithium-ion technology probably can’t be pushed a lot further, but new chemistries are expected to deliver improvements in the near term.

There’s one exception, perhaps, to this general picture of waiting for new batteries. That’s Joby. Bevirt has told Forbes that his battery pack can already hold 235 Wh/kg, significantly more than any other in the world today, though he adds that the pack “is hard to decouple from the rest of the aircraft”. This suggests that various cunning ploys have been used: for instance some of the necessary insulation of the pack might also serve as part of the aircraft’s structure.

Such close integration may present problems. It seems likely that it won’t be feasible to quickly swap battery packs in and out of a Joby aircraft, which is a good thing to be able to do as it means an aircraft needn’t sit on the pad recharging for long periods between flights. Volocopter, for one, says its aircraft can have a fully charged pack snapped in to replace a drained one in just five minutes.

JFK-to-Manhattan trips, in the case of Volocopter, are perhaps an aspiration rather than something its aircraft can do today.

Bevirt may have a plan up his sleeve to help with this issue, however. Joby filed a patent application last October for a cooling unit intended to be hooked up to its aircraft batteries while charging on the ground, which would let them be charged up fast without overheating them. It might also help cool the pack down after the heavy thermal load generated by a vertical landing. Takeoff would heat the cells up even more, but this could be dealt with by air cooling once at speed in the cruise: fast flight makes air cooling work much better. It’s possible that Joby’s aircraft doesn’t carry much thermal management equipment at all.

Nevertheless, even with his stripped down nigh-on-miraculous battery pack, Bevirt’s aircraft would still be an astonishing achievement if his range figures are true. It would be pretty much the lightest and most efficient airframe ever seen in a commercial aircraft, by some margin.

Or it may be that Joby’s range figures are in fact, like those of his competitors, based on the assumption that when series production begins there will be better batteries to be had. Tellingly, last year Joby expected to have aircraft in commercial service as of 2023. This year, it’s 2024.

There is, however, a confirmed and documented case of a Joby aircraft making a flight of more than 150km. This happened last July, and the prototype flew from the company’s secluded heliport at the bottom of a disused quarry near Bonny Doon, California to a different facility, far off within the Fort Hunter Liggett military test range in Monterey County.

Disappointingly for air-taxi fans, however, the electric aircraft made that flight slung beneath an ordinary Bell 205 turbine helicopter.

We may be waiting for our electric air taxi rides a while yet.