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I was just curious because the air is very compressed in the engine and some things do run on compressed air. And what if the engine was powered all by electricity?

PW geared turbofan cutaway view

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    $\begingroup$ Where does the compressed air come from? Or the electricity? You will need some energy source on board that is not too heavy. Kerosene is best for the time being for its energy/weight ratio, though this does not exclude some people (such as Airbus) from working on small prototypes of electrically-powered planes. Not yet up to big aircraft, though. $\endgroup$ – ALAN WARD Aug 7 '15 at 8:05
  • $\begingroup$ You can get thrust out of a balloon… but it'd need to be a pretty big one to lift a 737. Then it'd be more efficient to use it as a blimp then as propellant. $\endgroup$ – bjb568 Aug 7 '15 at 11:34
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    $\begingroup$ @bjb568 You can get lift out of balloon. It doesn't generate thrust. It either gets blown around by the wind or needs some other form of engine to propel it horizontally. $\endgroup$ – reirab Aug 7 '15 at 18:52
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    $\begingroup$ @reirab: Haven't you even blown up a balloon, and then let go of the nozzle so it flies around the room? What causes that, if not thrust? $\endgroup$ – jamesqf Aug 7 '15 at 19:12
  • $\begingroup$ @jamesqf Ah, I wasn't thinking of that kind of balloon. That kind of balloon doesn't work as a blimp (i.e. doesn't generate lift) unless you fill it with Helium or Hydrogen. I was thinking a hot-air balloon. That kind of balloon is just a small compressed air tank, which doesn't scale well because of the problems mentioned in David Richerby's comment on R.J. Jones' answer. $\endgroup$ – reirab Aug 7 '15 at 19:16
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A typical modern turbofan engine produces about 85% of its thrust from bypass flow, i.e. "compressed air", that never goes through the actual jet core.

However, as others have mentioned, in order to get the compressors spinning and sucking air in to begin with, you still need an energy source of some kind to turn them and, for lack of a better alternative, current designs still rely on jet core flow to turn both sets of turbines and, by extension, their associated compressors.

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Where would you get the compressed air from? Jet engines "contain" compressed air because the engine itself compresses it using the energy from burning fuel. Asking if you could power the engine by that compressed air is like asking if you could power a car engine by the rotation of the gearbox: it's confusing cause and effect.

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The engine is compressing air to more efficiently burn fuel. If you remove the fuel, there is no reason to compress the air. Other systems that run off of the engine bleed air only do so because it is a convenient source. The 787 uses a bleedless model that runs off of electric power instead of engine bleed air.

Electric airplanes are certainly an option, but a propeller is a better option than a turbine engine. You could however keep the duct around the prop.

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In modern aircraft's jet engines (both turbojets and turbofans), compressed air is used:

  • For starting the engine, spooling it up until the shut-off valve is opened and the fuel starts to enter and burn in the combustion chamber;
  • For running auxiliary systems, like de-icing and pressurizing equipments;

This is not true for some newer engines like the General Electric's GEnx that use an electric starter or little jet engines (including APUs) and sometimes turboshafts (turboprops).

In aviation it's common to refers to the systems running/producing compressed air as 'pneumatic' systems or 'bleed-air'.

This, for instance, is the part of the overhead panel of a Boeing 737 used to manage pneumatic systems (taken from http://www.b737.org.uk/pneumatics.htm):

Boeing 737 pneumatic panel

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No. Air is compressed because we have jetful to run the compressor in the first place. No fuel, no compressed air (or very very few, from to drag induced by windmilling the engine).

Have a look at https://en.wikipedia.org/wiki/Jet_engine for more informations

A plane with compressed air tanks would also not be possible, as it requires A LOT of air to move an aircraft

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    $\begingroup$ It would be possible. Just not practical. :) Lots of things are possible. Physicists worry about those. Far fewer things are practical and those are what engineers are concerned with. $\endgroup$ – reirab Aug 7 '15 at 18:55
  • $\begingroup$ @reirab i would like the mathematical proof :) $\endgroup$ – Antzi Aug 7 '15 at 19:54
  • $\begingroup$ No need for a mathematical proof when we've had working models since at least the 20s. The problem is that it doesn't scale up to anything useful. It works fine for very light R/C planes, though. $\endgroup$ – reirab Aug 8 '15 at 6:05
  • $\begingroup$ @reirab That's the point, theses kind of things might not scale up. But if you consider the R/C as genuine planes I guess you are right :) $\endgroup$ – Antzi Aug 8 '15 at 12:51

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