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What if we had wind turbines on planes. These would not power a battery but would instead power a smaller engine. Perhaps once a pilot goes to the cruising level he could activate the wind turbines to come out from the bottom of the wing, or some other more sensible spot. These would spin very fast and in turn, the motion would be converted to electricity. Although this would not be enough to keep the plane flying it would help the plane conserve fuel, similar to hybrid cars.

This would already be in use if it worked, or it would be on the news, so why does it not work? And if it works why not use it?

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  • $\begingroup$ Does this answer your question? $\endgroup$ – Peter Kämpf Feb 12 '20 at 17:56
  • $\begingroup$ Century old idea.... $\endgroup$ – user3528438 Feb 12 '20 at 17:58
  • $\begingroup$ @user3528438 Much older than that. I'm pretty sure da Vinci either designed a PMM, or debunked one. I think one of the Greek philosophers (Archimedes?) had one sketched out. $\endgroup$ – Zeiss Ikon Feb 12 '20 at 18:02
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    $\begingroup$ Just think about what you are asking: You want to put a propeller in the airstream to turn a generator, which will power a motor, which will turn a propeller. Even without an engineering degree you should understand that there are energy losses due to inefficiency in the system and the absolute best you could hope for is to break even. So if you can only break even, why bother? Just fly a glider! It's almost like a photo I saw of someone who plugged a power strip into itself and didn't know why it wouldn't work. $\endgroup$ – Michael Hall Feb 12 '20 at 18:06
  • $\begingroup$ Ram Air Turbines (RAT) are on many aircraft: en.wikipedia.org/wiki/Ram_air_turbine They add a lot of drag, so are not there for any propulsion purpose, as are suggesting. $\endgroup$ – Adam Feb 12 '20 at 18:29
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This is a perpetual motion scheme, and no that would not work as it does violate the second law of thermodynamics.

Said wind turbine scheme like that could generate electricity, but would add additional drag on to the airframe which would have to be compensated for by additional thrust (energy is conserved). In a perfect world the total drag force on those wind turbines could be totally converted into thrust by an electric engine driven by them. However, as they say, there’s no free cup of coffee in this universe. There will be thermodynamic and entropic losses associated with such a design. You could never break even and, even if you could, the additional weight of the systems would have to be accommodated my additional structure, weighing more, creating more drag and requiring even more thrust to overcome.

When I was a child, I had an idea for an airplane which was electrically powered. An electric motor not only drove a propeller but also an electric generator which, in turn powered the motor. My father, an engineer himself, quickly pointed out to me the fallacy of such designs. Until we figure out a means to spontaneously create energy - and that’s not necessarily out of the question if you subscribe to the multiverse and string theory physics! - they remain the stuff of childhood fantasies.

EDIT: Beyond the tongue in cheek vid above, here’s a pretty good one on perpetual motion.

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The simple reason this isn't in use is it won't and can't work.

Under no circumstances can you obtain even as much energy from your array of Ram Air Turbines as they will sap from the aircraft's motion -- it'll be like trying to save fuel by deploying a drag brake.

Why? Because of what gets lumped together as "losses." A windmill turbine is less than 100% efficient in turning wind movement into shaft power, the generator behind it is less than 100% efficient at converting shaft rotation into electrical energy, your electric flight motor (to use this energy) is less than 100% efficient at turning electrical energy back into shaft rotation, and its propeller is less than 100% efficient at turning shaft rotation into forward thrust in the form of airstream kinetic energy.

The result is that you get less thrust from the drive motor than the drag produced by yoru array of wind turbines, and the whole arrangement winds up being worse than just shutting off the engine and gliding.

If you want to get the most distance for the least energy cost, look into self-launching sailplanes. They use fuel or electricity (stored in relatively small batteries) to get from the ground to soaring altitude, then not only shut down, but stow the motor and propeller, and use energy harvested from the atmosphere to fly at cross-country speeds comparable to a lower end light airplane (competition sailplanes can fly point-to-point at a net speed of up to 100 knots in suitable weather)

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Current science does not support the concept of perpetual motion.

You can not extract energy out of nowhere.

Sailing ships, including wind turbine powered ones, can sail against the wind, but they do so by extracting energy from the interface between wind and water - transferring wind's energy to water and using some of it for propulsion.

There is no such interface for an airplane, unless it's sitting on the ground. In theory one could get energy from an interface between thermal currents, but that is better done without a turbine, by a glider.

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