Battery-powered flying taxis projects are doomed — hydrogen is the only fuel that makes sense for vertical take-off aircraft designed for urban settings.
At least so says Rafi Yoeli, founder and chief executive of Israeli aviation company Urban Aeronautics, who recently signed a partnership deal with HyPoint to use the US startup's company’s fuel cells in its CityHawk vertical takeoff and landing (VOTL) craft.
You’d think developing a flying car would be tricky enough on its own without adding a relatively new fuel like hydrogen to the mix. But Yoeli says battery-powered aircraft will simply not have enough power to fly.
“If there was a breakthrough in batteries we would switch to those. But there is no such thing on the horizon,” says Yoeli, who has spent 17 years developing the CityHawk vehicle. “As a high-power, 100% environmentally friendly fuel, hydrogen is key to the future of VTOL aircraft.”
“All the eVTOLs powered by batteries have large rotors and wings and limited payloads. They are worse in all respects than helicopters,” says Yoeli. These limitations, he believes, will stop electric flying taxis from becoming truly useful in cities.
For one thing, big aircraft will be limited in where they can land in cities — they will need special landing pads and vertiports. Yoeli, on the other hand, intends for CityHawk to be no bigger than a car, and able to land anywhere in the city.
This would make CityHawk valuable, for example, as an air ambulance. At the moment air ambulance helicopters often struggle to get close enough to the person they are sent to rescue.
“There is a strong demand for air ambulances that can land anywhere, that could land on the sidewalk next to the person who has just had a heart attack, rather than landing in a field several hundred metres away and the paramedics having to run the rest of the way.”
Size vs power
Building a vertical takeoff craft is all about tradeoffs between power and the size of your fan or propeller blades.
You can go for low power. You can even lift an aircraft off using just human pedal power — the Aerovelo team did this in 2013, But the rotors have to be huge — the Aerovelo model was the size of a tennis court and could only be flown for a few seconds inside a covered sports stadium.
This is not practical for a busy urban setting.
Lifting something smaller — with, say, the same footprint as the average car — would need a more powerful motor and more powerful fuel. Which is where hydrogen comes in.
Yoeli lays out the numbers starkly:
Jet fuel has an energy density of roughly 12,000 watt-hours per kilogram. Hydrogen has some 33,000 — nearly three times as much as jet fuel. Lithium-ion batteries have, at best, an energy density of 300 watt-hours per kilo.
While electrical propulsion systems can be made more efficient to help compensate for this, there is still a big shortfall to make up. This is one of the reasons many electric flying taxis have a relatively short flying range. And it also means many of them have big rotors. Volocopter’s array of rotors, for example, is about 9 metres in diameter.
Urban Aeronautics’ CityHawk looks more like a car, with two big fans embedded (one in front and one at the rear).
It is a design that borrows from the Piasecki Airgeep, an experimental flying car developed for the US military in the 1960s. The car flew and handled relatively well (if noisily) but the project was ultimately abandoned as being unsuitable for combat. Yoeli has refined this design with the more modern engines available today from partners such as Safran.
Renewed hopes for hydrogen
Urban Aeronautics is not the only aircraft company to be looking at hydrogen. UK startup ZeroAvia recently completed the first-ever flight of a commercial-scale electric and hydrogen-powered aircraft in the UK. For big aircraft, hydrogen is seen as the only real alternative to jet fuel — and ZeroAvia hopes to be flying 20-seater planes within three years.
After being dismissed for some time as being too expensive and impractical, hydrogen is enjoying something of a renaissance at the moment. Germany recently published a national hydrogen strategy with plans to invest €7bn in ramping up production capacity. The EU, meanwhile, is expected to publish a hydrogen strategy next month promising up to €15bn investment in increasing the production of green hydrogen.
Hydrogen aircraft could, therefore, soon have a more plentiful fuel supply to reply on.
Yoeli plans to have CityHawks flying on city streets in around five years — certification from aviation authorities will take at least that long. The company, which has so far raised angel financing but is now looking to raise a further round in order to help turn its prototype into an aircraft that can be produced at scale.
In one way, Yoeli is grateful to the electric flying taxi competitors that have grabbed so many of the headlines recently. The big funding rounds for companies like Joby, Lilium and Volocopter have added to the credibility of the sector.
“10 years ago investors thought we were crazy. We’d walk into a room with our flying car pitch and they would throw us out,” chuckles Yoeli. “Now we are taken more seriously.”