FARNBOROUGH, England — Hundreds of millions of venture-capital dollars have poured into flying taxi projects in the past few years. At the Farnborough Air Show, concept vehicles were everywhere, though not one was flying.

The spark for this urban air taxi frenzy is the possibility of electric flight.

Batteries will never power a Boeing-sized airliner; they’re too heavy with not enough power. But they do have the potential to electrify small aircraft — like flying cars with four or so passengers.

These new concept vehicles typically lift vertically like a helicopter by means of multiple small rotors on top, hover for a moment, then transition into horizontal fixed-wing flight like an airplane.

A video on the website of Vertical Aerospace, an aspiring British electric vertical take-off and landing aircraft (eVTOL) company, gives an idea of the hype.

CEO Stephen Fitzpatrick speaks about Vertical’s prototype VX4 air taxi, which the company says should fly for the first time later this summer and be certified to fly passengers in 2025:

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“The X4 is going to be 100 times quieter than a helicopter. It’s going to be zero carbon. It’s going to be a fraction of the cost,” Fitzpatrick said. “Most important of all, it’s going to be 100 times safer.”

It has never flown. But how’s that for promise?

The full-size VX4 mock-up at Farnborough is enormous — 43 feet long with a wingspan of 49 feet — much bigger than others on display.

Designed to carry four passengers and a pilot, it’s a tiltrotor concept.

That means the four rotating fans on the leading edge of the wings, after lifting the vehicle into the air, swivel down to become propellers in forward horizontal flight.

Only two or three eVTOL companies have successfully flown the transition from vertical lift to horizontal fixed-wing flight.

And tiltrotors specifically are a challenging technology.

The U.S. Marine Corps’ Osprey tiltrotor, designed by Bell Helicopter and Boeing, suffered a series of crashes with multiple fatalities that generated serious doubt about its safety.

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Leonardo’s AW609, the world’s first commercial tiltrotor aircraft, is now close to being certified — fully 20 years after its launch.

The idea that a technology as new and radical as the VX4 could be certified for commercial passenger flights by 2025 is attractive to venture capital investors.

To aviation people, familiar with the rigors of certification, it’s entirely implausible.

“It is optimistic,” admitted Nate Isbell, Vertical’s head of marketing for North America, as he gave a tour of VX4 inside a Farnborough exhibition hall.

Asked about how the transition from vertical hover to horizontal flight will be safely performed, he said simply, “the Honeywell flight control software will take care of that.”

If the VX4 ever were to become the mass public transport that Vertical wants it to be, it might literally blacken the skies.

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The giant black-painted aircraft looks as menacing as a pterodactyl or a taxi that might be summoned by Darth Vader.

At least the marketing guys at Wisk, the air taxi company majority owned by Boeing, had the sense to paint their relatively little air taxi bright yellow.

Boeing selects its air taxi

The Wisk two-seater taxi, 21 feet long with a 36-foot wingspan, has 12 vertical lift fans and a pusher propeller at the back for forward flight, with fewer moving parts and simpler technology than a tiltrotor.

Except, that is, for one important detail: Unlike the VX4 and all the other current eVTOL concepts, Wisk has no pilot. This taxi is self-flying.

Looking into the cockpit of the Wisk, a passenger might be a little disconcerted to see what looks a bit like a helicopter layout, but with no visible controls.

Inside the glass bubble of the prototype shown at Farnborough, there are just two passenger seats. Step in and it will take off by itself.

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That might be harder to get right than a tiltrotor.

Seasoned aerospace analysts, such as Adam Pilarski of aircraft consultancy Avitas, are universally skeptical of the business case for these air taxis and the timelines projected for their entry into service.

“This is sexy for some and people invest a lot of money in it, especially financial people,” Pilarski said. “I don’t think that it changes aviation hugely.”

Sash Tusa, an analyst with Agency Partners in London, said the Silicon Valley tech mindset at these startups — to try and fail and try and fail until eventually the technology works — is “wholly antithetical to air transportation.”

And though all the startups concede that they need to appeal to regular commuters to make money, consider the pitch at the website of Archer, another Silicon Valley aspirant:

“Archer users can take a morning hike in Marin before hitting the office, and then unwind with an evening wine tasting in Napa Valley — all with time to spare.”

That’s an image of “regular commuters” from an insular high-tech world.

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A note about air taxis from Agency Partners during the air show suggested to investors that they might succeed “in cities which have a small wealthy population who are fearful of a much larger poor population (Sao Paolo for example, where helicopters are already widely used).”

Perhaps coincidentally, Gary Gysin, CEO of Wisk, in an interview at the air show said he has 20 congested global cities in mind as the target market — and then named Sao Paolo, Mumbai, New York, Los Angeles and London.

Will air taxis make money?

However, Gysin insisted that the economics of the business won’t pan out unless the air taxis carry high volumes of people.

“We want to get to $3 per passenger mile so a college student can afford it,” he said. “If it’s only premium service, it will be a small business.”

The only way to get ticket prices down, he said, is to eliminate the pilot and fly autonomously. He said that’s why Wisk is pursuing that goal even though it knows some piloted air taxi will get to market sooner.

“The rest of the industry will have to go to self-flying to make money,” Gysin added. “It’ll be a money-losing proposition with pilots in aircraft.”

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The truth is that even participants in the now crowded field of air taxi startups see the risk in the project.

Gysin estimates only four or five among Wisk’s many startup rivals are likely to survive.

The eVTOL sector is “an incredibly good place to lose money,” Tusa said.

Even Boeing Senior Vice President Chris Raymond, the company’s chief sustainability officer, in an aside during a pre-Farnborough briefing, acknowledged the uncertainty of the business case for this new urban air mobility sector.

“We don’t really know where that market’s gonna go. We don’t know what the prices are going to be,” Raymond said. “There’s a lot of uncertainty about the future.”

Interesting work for engineers

And yet, however dubious the business case, even the skeptics admit that the technology is real and that the engineering underway is really impressive.

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For these flying cars to work, they must not only make that difficult transition without faltering from vertical to horizontal flight, they must also travel through low-flying airspace while avoiding buildings and other air taxis. Then they must reverse the transition to hover and land.

It may be difficult to win public and political acceptance for swarms of flying taxis.

Here in London, for example, because of safety and noise concerns, helicopters are restricted entirely to flying along the Thames River.

In U.S. cities there are similar issues, with campaigns to restrict helicopters flying in New York.

In San Francisco, there are 32 helipads, all unused because of public opposition.

For Wisk, the added complication is that its taxis must navigate such airspace autonomously, guided by sensors and controlled by computers.

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The vehicles will be monitored from the ground by humans whose job will be akin to that of air traffic controllers more than pilots.

Yet Boeing Vice President Brian Yutko, Chief Engineer, Sustainability & Future Mobility, believes Wisk, backed by Boeing, has the technology expertise for its self-flying taxi to succeed.

Wisk’s stated goal is, within five years of beginning service, to have more than 2,000 of the taxis flying 1.7 million flights, moving 40 million passengers in mostly short, quick hops around cities.

“Are these big, audacious goals? Yes,” Yutko said. “But … actually the timing is right.”

He said all the various advanced technologies needed to make the Wisk aircraft fly and safely navigate the airspace — the electric power systems, the software, the artificial intelligence, the autonomous systems — “it’s all in such a state that it’s just ready to start to intersect with one another where you actually can imagine taking such a transformational step.”

Boeing is all-in on the project.

Yutko, a star engineer at Boeing, is on the board at Wisk, which is a joint venture between Boeing and Google founder Larry Page’s Kitty Hawk Corp.

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Of the 500 employees at Wisk, about 100 are Boeing engineers based at the company’s Silicon Valley offices.

Wisk has built a series of nine full-scale aircraft prototypes and, unlike Vertical Aerospace, has flown about 1,600 flight tests.

The fifth generation two-seater model on display at Farnborough completed more than 400 flights without passengers, including multiple transitions from hovering to forward flight and stopping in the air to turn around 180 degrees.

In September, Wisk will unveil its four-seater sixth-generation vehicle that will replace that one for certification and mass production.

Wisk is proceeding with care. It’s not pushing a specific timeline. “These things will fly when safe. Not before,” Yutko said.

“We think that the Day 1 milestone is to meet the system safety threshold that will allow us to operate in the first place,” he said. “Those operations may be limited initially, as we start to roll out the service. It won’t be an everyday thing for you to take to work on Day 1.”

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Yutko said Boeing sees Wisk as an opportunity to explore the boundaries of research into autonomous flight.

Simply pushing to the target of certifying an eVTOL aircraft to the same safety standard as commercial airliners will drive progress, he said.

And Boeing hopes too, Yutko said, that this may become “a pretty big business” eventually.

That will take patience. Gysin said the total investment to get Wisk through to begin operating passenger service will be about $2 billion.

In a sector awash with money, the chief beneficiaries right now are the teams of engineers working on this. They’re pushing the boundaries of aviation technology more than has been done in decades.

Some of these projects may succeed, at least in achieving the technology goals, if not in making money.

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It’s likely therefore that flying cars are coming, though maybe not to your door or mine.

Farnborough 2022: Essential stories from the big air show

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• Delta, Boeing confirm big 737 MAX order after 11-year ‘drought’

• Commercial Airplanes CEO outlines Boeing’s engineering landscape and Puget Sound’s place in it

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• Seattle-based aerospace company makes Moses Lake a proving ground for new electric airplane projects

• ‘Morale booster’: Boeing wraps up successful Farnborough Air Show

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• Flying cars are all over the Farnborough Air Show: Will they take off?

• More coverage of the Farnborough Air Show 2022

Correction: An earlier version of this story incorrectly said none of the small handful of air taxi prototypes that have successfully demonstrated the transition from vertical to horizontal flight are tiltrotors. Joby is one of those and its lift fans do rotate to become propellers.