Previous in series: Why would I not want a flying car?

How close to a true VTOL does a flying car have to be to retain the advantages we would like?  If you have to keep it at an airport, you have to drive there and back in a separate vehicle, obviating many of the advantages.  If it can fold up its wings and drive around town, like the Terrafugia, it gets most of them back. (The alternative is the modular convert-a-plane, where you leave the wings at the airport.)

Yet there remain a number of reasons why you might want to be able to takeoff and land anywhere. The landing part is obvious: safety. If the vehicle can come in for a soft landing more-or-less straight down, it doesn’t need to get close to things like houses and people while moving at highway speeds. Luckily, an apparatus which can do that quietly and efficiently was invented back in the Renaissance by Leonardo da Vinci: the parachute. Some small aircraft actually have whole-vehicle emergency shutes today; it seems reasonable that various combinations of chutes, wings, and high-lift devices (flaps, etc) could be invented that would be deployed for takeoff and landing and folded for high-speed flight. A STOVL car might, for example, be able to takeoff from a straight stretch of road, or from your driveway if it were long enough (mine is), and land anywhere, which would make it enormously more convenient than having to operate out of an airport.

Convenience aside, there is another reason to want a VTOL vehicle. The more power you have in a plane, the safer. Storms, wind shear, clear air turbulence, even airliner wakes can toss small craft about and crash them.  Power is safety — and of course power is speed, as well.  So in the long run, you would expect flying cars to tend toward VTOL.

And it doesn’t have to happen all at once.  If you have a thrust equal half your weight, for example, you can get to 40 mph in a 100-foot takeoff roll, and then have plenty of oomph to shove low-stall-speed high-lift devices through the air, and still climb at about a 30-degree angle.  There would be room for lots of takeoff runways in the average suburb, any mall parking lot, and so forth.

There is, of course, a catch. In modern technology, VTOL means noisy.  Helicopters are considered annoying 20 decibels quieter than street noise, because of the whup-whup nature of their characteristic sound. Small plane props are quite loud as currently used, and if you put enough of them, and enough power into them, to take off vertically, they’d be hellacious. Ducted jets like the Harrier, and rockets, are earsplitting.

On the other hand, the actual wind noise (as opposed to the rotor noise) produced by a helicopter taking off is a not particularly noisome whoosh. So it is physically possible to throw down a column of air that will lift a car-sized vehicle in a non-objectionable way, if you can find some way to generate the column quietly.

One possible answer is to use lots of small thrusters instead of a few large ones. Lots of small motors also have a reliability advantage in that they only lose a small fraction of their total power when one fails. In current technology, this isn’t seen because many small motors are much more expensive (to make and maintain) and not as efficient as one big one.  However, if we imagine some steps along the Feynman Path so that we have a high-precision microtechnology, we might be able to get into the increasing power-to-weight regime of electrostatic motors to decreasing scale. Then all we need is good fuel cells.

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