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This is for an electric airplane (so no fuels for combustion):

  1. Are propellers the only option?
  2. Is there a turbine-jet possible with only air intake, electrically compress, and a turbine and nozzle and get equal or more thrust than a propeller like a water-jet but then with air?
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    $\begingroup$ You might like to state what sort of altitude and performance you're thinking of. A slow plane intended for use up to say 3-5000 ft will probably be more efficient with a propeller, very high altitude like above 30000 ft might be more efficient with a higher exhaust/nozzle speed, like your option "2". $\endgroup$ – Andy May 28 '15 at 10:33
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    $\begingroup$ Propellers are not the only option. I'm thinking of a ducted fan. $\endgroup$ – D_Bester Apr 14 '16 at 14:57
  • $\begingroup$ If you need super flight, you would need to use a jet, e.g. use intake to slow the flow to subsonic, compress it further, and jet it out as a supersonic flow. You couldn't do it propeller or even ducted propeller because ducted not not they don't run well with fast flow (even high sub sonic) and at fast speed. $\endgroup$ – user3528438 Sep 1 '17 at 23:40
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  1. Propellers in some form are really the only option. It is currently the only efficient way to generate thrust with air as a medium. Even jet engines are in some sense just a very elaborate system of propellers.

  2. The reason for the turbine is to create an environment to efficiently burn fuel (suck, squeeze, bang, blow) to generate as much thrust as possible. Hence, in an electric aircraft this is irrelevant. You might be interested in the Airbus E-Fan, which uses ducted propellers increase efficiency. However, it is still 'just' a propeller with an electric motor in the middle.

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    $\begingroup$ It's not about efficiently burning fuel, it's about efficiently converting heat to work. For more info, check the Wikipedia article on the Brayton cycle. $\endgroup$ – Sanchises May 28 '15 at 13:09
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You may choose whatever propulsion you prefer, but need to accept the consequences. An electrical turbine is easy to build, but will need a lot of power for heating the core flow. Instead of burners you will need to install heater elements, which will necessitate a longer flow path and more volume between compressor and turbine, but by and large the electric turbine engine will look much like a fuel-burning one.

You can also remove all the turbo machinery and only drive the fan with a big electric motor. But that would have lower efficiency than turning a propeller instead, so only when your flight speed will cause supersonic propeller tips might an electrically powered fan be a better alternative.

Therefore, at speeds below Mach 0.7 propellers will give you more thrust per kW of power, and their efficiency is best at low speeds. The much lower energy density of electric storage will demand flight at low speed, or your flight time will be very short. Therefore, propellers are the best, but by no means the only solution.

A potentially even more efficient method might be ionic thrusters where a high voltage is used to ionize and accelerate air in order to produce thrust. Designs using ionic thrusters would use the whole airframe for thrust creation and would need to be run at tens of thousands of Volts, but their potential efficiency surpasses even that of low-speed propellers - in theory. The linked article, however, is an embarrassment to MIT. A 1 kW jet engine that produces only 2 N thrust has to fly very fast (500 m/s, to be precise) while the 110 N cited for the ionic thruster at the same power mean that it could not had moved faster than 9 m/s.

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    $\begingroup$ @VladimirCravero: He didn't say it would be a good idea. $\endgroup$ – Fred Larson May 28 '15 at 17:05
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    $\begingroup$ @FredLarson Yep but that's like saying that gas is inefficient to propel boats because if you throw it off the stern the ship barely moves $\endgroup$ – Vladimir Cravero May 28 '15 at 17:47
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    $\begingroup$ @VladimirCravero: A turbine extracts energy from fluid flow. Other kinds of devices like impellers and fans are used to convert energy into fluid flow. Using electric heat to drive a turbine jet would be silly, but fundamentally the purpose of a turbine jet is to convert expansion of gases into thrust, and there aren't a whole lot of ways to make most kinds of gas expand electrically other than by heating. $\endgroup$ – supercat May 28 '15 at 21:24
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    $\begingroup$ Ionic Thrusters! Now there's a marketable phrase! $\endgroup$ – Koyovis Sep 17 '17 at 3:31
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    $\begingroup$ @Stanislasdrg: Yes, if you have enough battery capacity to accelerate this fast. Also, you should do that at high temperature and high altitude, so density and Mach are low. I would prefer slow-moving, large propellers. See here for more. $\endgroup$ – Peter Kämpf Sep 26 '18 at 18:43
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You do not have hot combustion gases in an electrically powered engine. The only gas available to you is the air around you, and some form of propeller is the most logical way to accelerate that air.

The nozzle you're imagining can be used to turn a large, slow stream of air into a faster but smaller air stream. This can't provide thrust by itself as it isn't a powered engine part.

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    $\begingroup$ The electrically-powered compressor is the "powered engine part" in the second scheme in the question. $\endgroup$ – David Richerby May 28 '15 at 14:45
  • $\begingroup$ @DavidRicherby: Indeed. But the scheme presented there (with a turbine) doesn't really make sense. The compressor is already electrically powered and doesn't need to be powered by the turbine. Nor would the turbine be able to extract energy from the combustion gases. $\endgroup$ – MSalters May 28 '15 at 14:56
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Ducted fans rather than external propellers increase the performance at high speeds, so gets round most of the problems.

You aren't really going to want to carry fuel AND batteries, so some sort of ducted fan is the best solution for a high speed aircraft.

While they do look somewhat like Jet engines, they are basically just prop engines in a tube, so they can better control the air-flow, which at high velocities, can cause issues for exposed props.

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    $\begingroup$ That does raise the interesting question, can you use a ducted fan rated for cruise power, and inject and burn fuel into the compressed airflow (like reheat or afterburner) for takeoff power? Yeah the compression ratio is probably too low... $\endgroup$ – Brian Drummond Aug 22 '17 at 22:17
  • $\begingroup$ No, they are nothing like jet engines. $\endgroup$ – Drenzul Aug 24 '17 at 8:13
  • $\begingroup$ @Drenzul: He didn't say that they were like jet engines - just that, from the outside, they look kind of like jet engines (due to the big round cowling duct). $\endgroup$ – Sean Oct 5 '18 at 21:53
  • $\begingroup$ Suggesting that they can use a similar afterburner system as Jet engines would imply he thought there was more similarity between them than there is, probably because from the outside they do look similar. $\endgroup$ – Drenzul Mar 8 at 14:15
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For your requirement to use a battery for silent takeoff and landing, electrically driven propellors (or ducted fans) would certainly be good enough.

The requirement you mentioned for 350 mph to 550 mph is really only for cruise, which presumably would use a turbofan (or possibly turboprop) - and recharge the battery at the same time.

So I would guess what you want is a fairly high performance ducted fan which can be driven by either electric motor for silence or by a conventional turbine at cruise - a bit like a turboprop. The question is then how do you arrange for an exchange between the two power sources. (I assume two sets of engines wouldn't be possible, though perhaps they could be?)

(Edited to add... your electric motors still need full power to takeoff. So why not run the motor all of the time, and use an auxiliary turbine engine to provide the electrical power at cruise? Not unlike a hybrid car.)

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  • $\begingroup$ Yes, two sets of engine. 1. Battery for silent takeoff. 2. Combustion engine which transfers to electricity which then drives the electronic propellor. $\endgroup$ – Arturo May 29 '15 at 14:43
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    $\begingroup$ In other words - something like a more powerful version of the E-Fan (which MikeFoxtrot linked to) with an auxiliary turbine to generate electricity at cruise. (Provided suitable motors can be found, and a ducted fan design which can provide the necessary performance both at climb and cruise.) $\endgroup$ – Andy May 29 '15 at 15:00
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    $\begingroup$ Keep in mind, much of the noise for prop drive aircraft is the propeller itself. Kind of like how an electric fan makes whole bunches of noise, even though it's electric. $\endgroup$ – Jay Carr Sep 19 '17 at 14:56
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The main difference between air and water is that the second cannot be compressed, while the first can and when you do, you heat it up loosing a cospicuous amount of energy. The more you compress it and the more time you wait before releasing it, the more energy you loose.

A propeller also compresses the air: this is necessary to accelerate it, but it allows to achieve high air velocities with the minimum amount of compression. It is also a very simple and light design, moreover one of the motivation to have an internal water compressor is the possibility to filter it, which is not required with air.

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