

And the VTOL hybrid job to follow, promise engineers

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But I'm not a pilot, and I hardly ever drive past a couple of airstrips during a journey. What about me?

But what of the TF-X? After all, most of us aren't pilots. Most of the journeys most of us make don't pass near a couple of handy airstrips which would let us do part of the trip airborne in a Transition, weather permitting. Where's the flying car for the rest of us? Is it really feasible?

At first glance the answer would seem to be a pretty firm no - it isn't.

A simple comparison with the only tilt-rotor aircraft actually flying in any numbers - the MV-22 Osprey - doesn't look good for the TF-X. An Osprey lifts off weighing 50,000lb (22.7 tonnes) using 9 megawatts of power. The thrust discs of its big props are loaded at around 20lb per square foot.

We can see from the vid that the TF-X's prop discs are around a hundred feet2. With a megawatt of lift power available, it's clear that the TF-X is expected to have heavier disc loading than the Osprey and as such it won't generate as much lift per watt. In rough envelope-scribble terms the TF-X's maximum all-up weight must be a good bit heavier than 2,000lb and a good bit lighter than 5,000lb. Say 4,000lb to be kind - Terrafugia didn't care to give any specifics.

Subtract 800lb for people and 400+lb for the cruise engine at the back and its fuel - presumably it's a relatively conventional lightweight internal-combustion job or something similar (again, Terrafugia are reticent). Assume for the moment that the airframe, wingtip rotor pods, avionics etc weigh nothing. That leaves a maximum of 2,800lb for a battery pack which is able to put out 670 kilowatts (Terrafugia did specify that the engine will be feeding power to the rotors as well during hover operations, so the requirement is 900 horsepower not 1200). That battery pack needs to be able to survive a road crash safely as well - this is a flying car, remember.

One of the best road-safe certified power packs you can buy (provided you have deep pockets) is the one in the Tesla Roadster battery supercar. It can put out 215 kilowatts: three of them could provide almost all the juice we need to get the TF-X off the ground.

Terrafuiga TF-X VTOL flying car takeoff

It can't do this for long, though - and there'd be a lot of stuff blowing about

Unfortunately three Roadster battery packs weigh 2,970lb on their own. That's too heavy already, and we haven't even allowed for the weight of the airframe, rotors, motors etc. And that's not the only problem: those Tesla powerpacks can't sustain that level of output for long. The theoretical maximum is just under 16 minutes, but in real life it would be much less.

Terrafugia wisely specify that the TF-X should be able to attempt vertical landing three times in a row with little if any engine recharging, allowing for aborted approaches or other problems:

Prior to departure, the operator selects a primary target landing zone and backup landing zones. If the TF-X™ calculates insufficient energy on board to conduct last minute aborts at the first two sites and safely navigate to and land at the third within a 30 minute reserve, or if the forecast weather in any of the three landing zones would be outside the allowable limits, or if any of the selected landing zones are in temporarily restricted airspace (TFRs), departure will not be allowed until appropriate landing zones are selected.

Ten minutes or less of hover time to go up and down three times would seem like a big ask. And anyway the batteries are much too heavy. We put this to the Terrafugia people, and they said:

Basing your numbers on the Tesla battery pack is not representative of the state of the art in battery technology today - it is representative only of what is certified for automotive use. Tesla's packs are designed for long range and crash - not high power-to-weight and crash. Our concept relies on higher power density cells than Tesla uses, combined with a different strategy for crash. These cells exist today but are not yet certified for road use.

That's a fair point: there are battery technologies which theoretically might offer a considerably lighter battery pack with the necessary power - though there aren't that many which would leave much of a margin for airframe, motors, rotors etc. Whatever Terrafugia are eyeing, they'll have to make it road-crashworthy as Tesla did. And the fact remains that the Tesla packs, optimised for endurance as they are, are already pretty marginal on the length of hover time they could deliver. Switching emphasis to power rather than energy would in all likelihood cut this time even further.

Terrafugia did more or less admit this last, saying:

Your point that there will be a limited hover time is correct. We estimate the time will be sufficient for two or three aborts before a horizontal landing would be required.

Which is fair enough again: if the computer autopilot couldn't get the TF-X down before the battery approached exhaustion, it could always revert to aeroplane flight and head off for a runway, where it could make a rolling landing under engine power alone.

Still though, when you reflect that enough ordinary light-aircraft engines to provide the required hover horsepower would only weigh 1400lb-odd, you have to wonder just why the enthusiasm for batteries among PAV inventors these days. To be sure, electric thrust would be quieter and less prone to breakdown than normal engines, and the need to put heavy weights at the wing tips is reduced: these are good things for flying cars.

The fact is, however, that those light-aircraft engines have always been there and yet there are very few operational tiltrotors in the world, and those there are belong to exotic branches of the US military with special requirements (namely the Marines and the Special Ops command), not commercial organisations or wealthy individuals. They aren't powered by piston engines either; they need the improved power-to-weight offered by gas turbines. So it would seem that tiltrotors are just not simple to make, and the chance of genuinely making battery-powered ones (which can fold and unfold rotors in midflight, too) is pretty slim.

Looking at the troubled development of the Osprey - and the struggle Terrafugia is having getting the relatively pedestrian Transition to work - you have to say that the TF-X is not, as Terrafugia says, really "what is physically possible with today's technology". Even if it was, the rotors and rear engine would probably still make it too noisy to land and take off in urban areas - and its high disc-loading might make this very unpopular even if it were quiet enough. (Ordinary low disc-loading helicopter downwash is serious enough. The Osprey's has been known to cause serious devastation, and the TF-X's wash, while not as widespread as the Osprey's, would be even more intense: albeit not aggravated as the Osprey's is by downward-pointing jet exhausts.)

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