# Is there any theoretical problem powering the fan with an electric motor [duplicate]

A question that I have been wondering about lately is whether it would be possible to power the fan in a modern hi-bypass turbofan with an electric motor and achieve performance comparable to the kerosene-fueled engine. Or is the 10% of air that flows through the combustion chamber crucial for the operation of the system?

(I wish to ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity)

Edit: I have realized that the "electric" part of the question is really not the core of the question. What I am really interested in is whether it would be feasible to power a plane using only the fan and not the exhaust jet with a performance similar to a current jet (which would make the engine similar to a turboprop with many blades and a cover around it)

• What would power the electric motor? If you wish to ignore the weight of batteries you would need an engine driven generator... do you think this would be somehow more efficient than driving the fan directly? Commented Dec 28, 2020 at 16:08
• I don't understand how you can "ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity" ... if the plan won't fly because of the weight of the engines to provide the power, or because the number of batteries carried would cause the weight to be too much, then it's not a plane, it's a brick that can't take off. Commented Dec 29, 2020 at 13:47
• Why do you think that "the 10% of air that flows through the combustion chamber" could/would not still flow through? Commented Dec 29, 2020 at 15:26
• @erikkallen maybe a better question (or a follow up for this one) would be which thrust apparatus would be most suitable for electric airplanes: ducted fan, propeller, unducted fan etc. Electric motors have some characteristics that differ from turbines and piston engines, and taking these characteristics into account might tip the scales to a surprising direction. The application is of course one variable: what kind of missions would the plane need to fly etc. Commented Dec 29, 2020 at 19:25
• See my comment on the latest answer with the embedded video. It is not clear at all what you are actuall asking, and your edit only makes it worse. Voting to close until you can clarify what you really mean. Commented Dec 29, 2020 at 23:41

Taking into account your wish to ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity, the answer is yes.

You would, however, need to compensate the missing jet engine thrust by increasing the fan thrust.

• Should someone feel irritated by this answer, you're wellcome! Throwing reality out the window does not spawn very good deliberations 🙃 Commented Dec 28, 2020 at 17:04
• Nobody should be irritated by truth. Sometimes these random and theoretical musings just need a blunt debunking. Commented Dec 28, 2020 at 18:17
• Any irritation should be directed at the question -- your answer is spot-on. "If we ignore all the reasons that this doesn't work & isn't a very good idea, can you in theory do this & make it work?" Sure, ignore enough reality & all kinds of silly things become "possible." I think it's entirely fair game to include reminders of how much has to be ignored, with the answer to such a question. +1
– Ralph J
Commented Dec 28, 2020 at 18:47
• @Criggie of course the big difference between the battery-powered electric turbo-pumps on a rocket engine, and electric turbo-pumps for a turbine engine is that one flies for five minutes, and the other flies for five hours. That makes all the difference. Commented Dec 29, 2020 at 0:23
• @Criggie "or reputation or advertising" as in "Look how green we are!" Commented Dec 29, 2020 at 0:39

"using only the fan and not the exhaust jet" does not make sense.

This is because the exhaust jet powers the fan. That's what the turbo in "hi-bypass turbofan" means: the exhaust jet spins turbine blades attached to a center shaft, which thus spins turbine blades in the front of the engine. Those front turbine blades then compress air into the combustion chamber.

Something must power the fan. If you don't "steal" some thrust gas to do it, then some other energy source must, for hours and hours and hours. Therefore, somewhere in the limited volume and weight requirements of the plane, you must also account for an energy source and motor.

## Bottom Line:

Stealing some thrust gas is the most compact and economical method of spinning the fan.

• His ideais to power the fan - electric. That is the whole question. Commented May 24 at 10:33
• @TomTom the electricity has to come from somewhere. Commented May 24 at 10:37
• Multiuple companies working on small size energy generation - i.e. avalance avalanchefusion.com with a really small 100kW fusion core planned to be done in some more yxears. THAT is a small and low weight long enducance energy source for you. Commented May 24 at 11:00
• @TomTom "planned to be done in some more yxears." Hahahaha... vaporware. Commented May 24 at 11:43
• like AI was in 2017. They have good investors and are well reviewed. Also, they are one of about a dozen companies readying fusion reactors - the first going supposedly online in 2028 - and their special thing is the focus on small size. You - would consider reusable rockets vaporware - just before it was demonstrated to work. I definitely put them not into vaporware but into active research because of - credibility. That you can - well - check - instead of acting like a brat. Commented May 24 at 14:37

would [it] be possible to power the fan in a modern hi-bypass turbofan with an electric motor?

Yes, but it would not be the most efficient way to convert electric energy into thrust. The superior way would be to attach a large, slowly spinning propeller to that engine. Since electric airplanes have a poor power-to-weight ratio once their range becomes practical, the available power should be used most efficiently. Jet engines only come into their own at flight speeds above Mach 0.7 when propellers run into increasingly unsurmountable compressibility problems.

and achieve performance comparable to the kerosene-fueled engine?

Not quite because the fan-motor combination is likely heavier than the jet engine. Small electric motors can weigh as little as 0.1 kg/kW but larger ones struggle to reach even half of that. Siemens demonstrated a GA-sized engine with 5 kW/kg, and scaling this up to airliner power levels would most likely result in no more than 2-3 kW/kg. Even with 95% efficiency, 5% of power will be turned into heat, creating cooling problems for large electric motors.

is the 10% of air that flows through the combustion chamber crucial for the operation of the system?

Yes, it is. Its mixing with fuel of high specific energy and combustion provides the mechanical energy to drive the fan. Its much higher exit speed at the nozzle also lets those 10% contribute a disproportionally higher thrust.

• Ignorant to the fact that efficiency for energy is not a factor once you get small fusion reactors generating the fuel - small enough to sit on a desk, 100kW output... avalanchefusion.com/tech coming in a couple of years (from now, not 2020). Commented May 24 at 10:34
• @TomTom Seriously? Fusion has been just around the corner for half a century and will continue to do so for the foreseeable future. Commented May 24 at 19:27
• By governments, yes, and they STILL assume ITER will take 30 years. Look at space travel - NOTHING changed until a private company came up. Helion has binding contracts to deliver electricity from a 400MW fusion facility IIRC 2028. They plan to have their first plus-energy-reactor this summer. I know about a dozen private companies that all plan to have reactors running by 2030 and all are financed by large investments. You know what also was around the corner for a century (okish): AI - and AI is here. Robots. Robots are nearly here (look at the demos). Future? NOW. Commented May 24 at 20:46
• @TomTom Business plans designed to deceive gullible investors are not infallible prophecies. Get real! Commented May 24 at 22:12
• Ah, so now everyone working on something new is looking for gullible investors. Maybe you should talk to a psychiatrist? As I said - plenty of examples making your opinion looking utterly stupid, and here we have a whole industry working on this problem. Believe what you want - reality generally does not care about you. Commented May 25 at 9:13

I believe this has actually been done; please see this video and decide if this meets your theoretical question setting:

This Airbus custom demonstration aircraft flew from England to France using two battery-powered electric ducted fan engines, which could be considered similar to or the same as electrically-powered high-bypass (indeed, 100% bypass) turbofan engines.

At 20:55 the pilot stops the fans prior to getting on the runway and you can see via the front that there are fan blades and a small central core section inside the nacelles. At 21:05 there's a good look from straight aft of the same.

Takeoff is at 24:30 and performance may or may not match your definition of "comparable to a kerosene-fueled engine", but this is reality, not theory, so they were obligated to consider battery and motor limitations.

• This answer just triggered me to downvote the question because it is now completely unclear to me whether the OP is simply asking whether electric powered flight was possible, (which I presumed was common knowledge...) or asking about some hybrid combination of thrust from an electrically driven fan augmented by a gas turbine. The answer is a good one, if the question is about purely electric ducted fans... Commented Dec 29, 2020 at 23:32
• I saw the word "theoretical" in the question, and how existing answers all seemed to address the limitations of generating the power to drive the fans in the first place, and I tried instead to address the unanswered aspect of the question: simply whether, in theory, any kind of all-electric-driven (not-really-)turbofan could function as well as current kerosene examples. The answer is obvious (yes, push air, make plane go) but the questioner also asked about the jet exhaust contributory aspect, which I did not directly answer. But I gave a practical, extant example, with video. Commented Dec 30, 2020 at 2:42
• I'd say a Large Fan moved by an Electric Engine in a Turbofan won't need batteries at all, Hybrid ICE-electric systems may need just an small battery as buffer or emergency safety backup, if the weight of generator needed to supply power to electric engine, it can be turned by the power section shaft of turbine, adds no weight to te current reduction gears or specific turbine to move the fan, and overall efficiency is good, it can be considered, but this needs calculations and wind tunnel/ engine testing. Blessings + Commented Dec 30, 2020 at 12:54
• Original question said to ignore issues of battery / power source, so all discussion of efficiency, weight, capacity etc. are IMO not germane to answering the original question. Commented Dec 30, 2020 at 19:43
• Long range will not use batteries. avalanchefusion.com/tech - fusion reactor, 100kW, sitting on a desk, so small. THAT is your power source - using 0.15g fuel per day on 100% load. Practically unlimited range on any small plane. Commented May 24 at 10:36