Where's my flying car? Coming in for a landing...

Opener predicts the Blackfly will be for sale in 2019

Readers all know I love robocars and write about the tremendous effect they will have on our lives and cities. But a new technology, running about a decade behind but now real, is coming which could have even more dramatic effects, the e-VTOL or "flying car."

I've written a few pieces here before but I felt it was time for a broad article about what's going on, and why I think that this technology is in a similar position to robocars in the mid-2000s: Some core engineering breakthroughs have taken place, it's going to be big and people think it's crazy. It's the archetype of the failed promise.

So look up and read Where's my flying car? Coming in for a landing soon on Forbes.com


I'd say that the solution to the battery limitations is at hand, and it will be hydrogen fuel cells scaled for the average power draw over the whole flight. Replacing the battery mass with a fuel cell, hydrogen tank, and a supercapacitor to handle the high power draw at takeoff and landing should give us far better power/weight and endurance.

The cost of hydrogen is trending down, and I'd say we're only a couple of years away from the point where the cost per Kw/H of hydrogen will fall below that of petroleum fuels.


Today most hydrogen, as I understand it, is sourced from natural gas so it's not as clean a fuel as electricity from solar/nuke/wind, but that could change in time. One issue is that the vehicle has to be able to fly 5 miles and land after any failure. That would include rupture of the H2 tank or failure of the fuel cell, so you probably also need batteries. The energy density of H2 is good, but what's the weight of the entire system, with two H2 tanks, two fuel cells and say 100kwh to 200kwh of energy after conversion?

The other issue is the lack of infrastructure. All designs today either use electricity, which can be found anywhere or gasoline hybrid which is even easier to get fuel for. If I deploy a hydrogen aircraft today, where can people refuel it?

You might want to look into Nikola's plans for their hydrogen long-haul trucks. Their hardware is great, but the thing that made me sit up and take notice when they unveiled their plan, was their deal with Ryder to put a fueling station at every Ryder location in the USA and Canada.


That might not be that useful for flying vehicles. Anyway, do you have the costs and weights for hydrogen fuel cell power for aircraft? People are definitely aware of it, and hungry for alternatives to batteries and generators, yet I see little talk of it, which is a bad sign.

Most airports are a lot closer to a Ryder location than they are to a welding gasses supplier.

When there's a market for hydrogen for personal VTOL vehicles, I'm sure that we'll see landing zones for refueling made available. Some of them will be in Ryder parking lots, and others will be on rooftops, etc.

As for the weight of fuel cells, I don't know the current figures, but NASA of course tried to save all the weight they could from anything they sent into space. I think that most of the commercial fuel cell providers today aren't really thinking about weight much since they're mostly selling stationary installations.

Boeing flew an HFC demo some years ago, so we know that the power/weight ratio doesn't make flight impossible altogether.


And more recently, there are HFCs available for unmanned aircraft:



Right now batteries are on the edge of being useful. So anything superior will be of high interest, but it is about the math, so if you can get the numbers that helps us figure this out. There are some other issues besides the math of the energy density, of course, like refueling as discussed, and fear of a big tank o' hydrogen in the sky. It might not need two tanks and two fuel cells, because you can also use a design with enough battery for 5 minutes of flight. That may limit travel over water or other places without emergency landing spots.

A little reading reveals:

  • Fuel cells are quite expensive -- $400 per kw. Generating the 30 to 90kw for takeoff is expensive.
  • As such, the design is to have battery for take-off and landing, fuel cell for cruise and recharge
  • Light H2 tanks only hold 300 bar, energy density per volume poor compared to liquid fuel. Tanks are also round, so can't go in the wings, you need a bigger fuselage.
  • Right now people mostly talk about it as a future approach, that the math isn't quite working yet, but they hope it will in the future.

I'd like to know how much of that $400 is materias, and how much of it will yield to economies of scale. We make far fewer fuel cells than catalytic converters today.


I don't study fuel cells enough to know. This was from a presentation on current efforts to put hydrogen fuel cells in aircraft. With the hybrid VTOL/Fixed designs, the problem is you need immense power to take off, and much less to cruise. So you end up having to make a giant fuel cell to use for under a minute, and that's not efficient. Instead wire up your batteries so you can get the burst from them. It's a good design anyway because you need redundancy. Actually, you need multiple battery packs, each with enough power to land because you can't land on just the fuel cell at VTOL spots though you could land at airstrips on just the fuel cell, if you have wheels or are willing to belly land. Though you also have to be ready for battery loss on landing. However, you can do the very low energy landing if you can approach fixed wing, which is you fly in fixed wing and then stall just over your landing spot, except you don't stall because your rotors keep you up, and then you hover down to it. One battery pack could do that.

I feel like the physics of enough energy to get airborne is very hard to overcome. All practical modes of air transportation are crazy loud (a lot of energy). Maybe if clever glide effects can be harnessed (like an ultralight or glider) and shrewd use of updrafts found with extremely good AI it can sort of be done. Still, doesn't seem like it's coming in a decade for mass civilian use. I also worry that the flying car meme will detract from progress on the absolutely solvable problem of making surface cars drive themselves. I'll try to keep an open mind and hope that my entire neighborhood doesn't trade their current vehicles for ones louder than that obnoxious neighbor's Harley.

Well, you are right about distraction from robocars. They will compete with robocars for talent and transportation.

Going up is expensive. VTOL vehicles are not efficient for short trips. Depending on the vehicle, however, once they get up, they can fly fixed with with much less energy to their destination. Indeed they can regain all that energy used in climbing if they do it right. Depending on where you land, you fly fixed wing down to not far above it and convert as quickly as you can for rotor descent. Effectively you stall the fixed wing flight but you don't stall because the rotors keep you up.

The other energy gain is the route can be direct. The route in the air might be half the miles of the route on the ground, depending on where you are.

I have to disagree with you that VTOL is inefficient for short trips. Just for an example, when I was living up in the hills in Santa Cruz, I was only about ten miles as the crow flies from Infinite loop. That's a four-minute flight for most of these vehicles currently under development. When a four-minute hop in the air can replace a 30-minute drive, that's a pretty big win, without even considering the fact that I'd be taking far less weight along for the trip in the air than on the ground.

Also, 4 minutes in transit means a lot less opportunity to collide with anything than I risk in 30 minutes on the surface, approaching other vehicles nearly head-on, with only paint to enforce the separation.


I mean a short trip on the ground. Take-off in a VTOL uses a lot of the energy. The numbers will vary, but enough energy for the ground car to travel a handful of miles -- on the ground. It's a clear win from the hills.

The noise issue if not completely solveable might require takeoff/landing away from residential areas. While there are some obvious suitable places such as parking lots, rooftops, even if these are more convenient than park and ride facilities, people still resist multi stage journeys.
Perhaps there is a way around this.
The old flying cars were not useful because they were a hybrid. They were never good as cars or aircraft.
What if the air taxi was docked to a self driving car chassis for driving the last mile.
The aircraft lifts off from the chassis leaving it to carry another aircraft to fetch another passenger.
The economics of this as a last mile solution would seem to be similar to using robot taxis as last mile connection for bus/train.

Yes, that's similar to one of the airbus designs. See the Pop up Next.

I am not so sure that transfer from flying to ground vehicle is all that difficult, so much of a burden as to justify something as complex as this. There are other reasons for the vehicle to be able to move on the ground, though.

Why is it hard to switch vehicles if it's always seamless -- one vehicle waiting for you when you arrive. Get up, walk 20 feet, sit down. Not a big deal.

I agree if transfer can be made seamless, the hassle is minimum, especially for commuting where luggage is usually a single small backpack or briefcase.
It will of course need to be proven to users that they really don't have to wait twice each trip for any length of time.

The nice thing is that robots don't mind waiting, and the cost to have them wait is not high (it may be more opportunity cost than anything.) If your transfer vehicle waits for you, right where you get out, it's pretty easy. It's not even that bad to haul a larger suitcase. I mean nobody wants to do that, but the question is, do you want a big complex mechanism to make it happen.

Turns out, the reason for the flying vehicle to be able to travel on the ground is to get it off the landing pad, which is a busy resource. It can roll. It can hover-glide. Or it can fly off somewhere else. But it can't stay on the pad if pads are scarce.

I wonder if strategically placed towers on higher ground could offer similar advantages as leaving from the tops of buildings? The vehicles could land at the base of the towers and take on passengers before an elevator arm raises the vehicle up several hundred feet before the vehicle takes off. The arms could be continuously lifting several vehicles at once on different sides before retracting and heading back down.

The down sides would be the large costs and limiting the areas that can be launched from. Against this is the separation of the landing site from the takeoff positions allowing faster turnaround, and reducing the noisiest and most energy draining part of the flight.

That requires a lot of specialized infrastructure. What is the value of the crane lifting the vehicles? Saving them a little power on takeoff? Seems a lot just for that. Could use other tricks for that, like beamed power. Or perhaps a cable that disconnects? Thought it needs to transmit a lot of power.

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