Larry Page is pouring millions into flying cars

Re:Larry Page is pouring millions into flying cars by nickparker on Friday December 30, 2016 @07:55PM (#13288778)

Surprisingly, they don't actually need to be all that inefficient.

The rolling resistance of a typical car is 0.01 to 0.015 [1], so the effective lift/drag ratio of a car's suspension is about 70 to 100, for just the wheels themselves. Missing from this number are the parasitic losses of all other unsprung mass, aerodynamic drag, and the costs incurred by traveling along roads which are neither level nor straight routes between your start and your end.

Small aircraft have L/D of 10-20 [2], and electric craft will probably fare slightly better if anything because of packaging advantages vs big ICEs.

I might do a more in depth analysis later and write up a Medium post, but my point is: Even just looking at the rolling resistance of the car and the whole L/D of aircraft, the difference is only an order of magnitude. Rolling resistance is one of the smallest contributions to drag in a car, so all up electric cars are probably only 3-6x more efficient than electric aircraft, and combustion cars are probably have a far smaller advantage.

[1]: https://en.wikipedia.org/wiki/Rolling_resistance#Rolling_res...

[2]: https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Examples

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Re:Larry Page is pouring millions into flying cars by raquo on Friday December 30, 2016 @09:40PM (#13289133)

I don't think L/D of more than 10 is realistic for a compact VTOL aircraft even in cruise mode, and then there's the vertical take off and landing component that will cause further inefficiency especially on short trips. But at the same time I don't think it matters that much.

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Re:Larry Page is pouring millions into flying cars by aptwebapps on Friday December 30, 2016 @11:42PM (#13289529)

Depends on the type of VTOL. Quad copter? Not so efficient. Tilt-wing? I imagine it's much better. (Amateur opinion disclaimer)

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Re:Larry Page is pouring millions into flying cars by zevets on Saturday December 31, 2016 @01:05PM (#13292213)

iirc a v-22 is below 5:1 and somewhere around 3:1

The new DEP designs will do better, but exceeding 10:1 seems unlikely

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Re:Larry Page is pouring millions into flying cars by skybrian on Saturday December 31, 2016 @01:58PM (#13292514)

You need compare the whole system: internal combustion engine plus the gas tanks to feed it, versus an electric motor plus batteries.

The energy density of batteries is far worse than gasoline. Also, after you burn gasoline, you don't have to carry it anymore, which isn't true for batteries.

Also consider that for a plane, running out of gas can be fatal, so you can't actually use all the range. There needs to be a safety margin.

And then, consider that a practical flying car can't actually take off or land like a plane, and probably can't have the wingspan of a plane, so you might want to compare to a helicopter instead.

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