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I just watched this pretty dumbed-down video about the idea of putting a gearbox in a turbofan so the fan can run at its optimum slower speed while the core runs at its optimum high speed.

And I immediately remembered this answer from Peter Kampf that said basically, a normal gearbox would have huge waste heat.

Then I suddenly remembered an old question of mine about driving a helicopter engine with a diesel-electric system.

So now the obvious 2-and-2 together: Can the fan of a turbofan be driven with an electric motor (powered by the already-existing generator of the engine)? Or the more detailed questions: Can an electric motor be small enough to fit around the fan's axle and still power the fan? How much it would weigh? I'm interested in large turbofans on the orders of 100 to 500 kN, but I think that would make the task easier since there's more room plus weight increases have diminishing returns against a huge airliner.

This would have obvious advantages. The fan:core ratio can be variable without any gearing. Presumably much less maintenance, or at least no more maintenance than the generator requires.

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  • $\begingroup$ Electric transmission is only useful when mechanical transmission is not feasible, e.g. WWI era tanks and WW2 era ships. Currently mechanical transmission has the advantage in terms of weight, space, and energy efficiency. But things becomes interesting if electricity is generated by mechanical generator, e.g. solar panel. $\endgroup$ – user3528438 May 1 '17 at 5:02
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    $\begingroup$ "Can the fan be driven with an electric motor " Yes, in principle, but you are replacing a simple steel shaft with a 50MW electrical generator and a 50MW motor, and except at takeoff both those would only be running at say 20% of their power rating - but you still have to carry all the extra weight around. Plus you need more bearings to support two separate shafts, more control systems to match up the electrical power generated and used, etc, etc. FWIW the P&W geared fan is an engine option for the A320neo, but airlines aren't interested - it's too unreliable. $\endgroup$ – alephzero May 2 '17 at 0:19
  • $\begingroup$ @alephzero Can you explain how you got a value of 50 MW? Seems pretty big to me. Boeing 777 engine does around 500 kN max, so not even 1 meganewton. And remember the fan will be spinning slower than normal to prevent supersonic tip speed. $\endgroup$ – DrZ214 May 3 '17 at 0:43
  • $\begingroup$ @DrZ214: Power (watts) != force (newtons). Some basic math tells me that 500 kN (engine force) * 100 m/s (air speed) = 50 MW (engine power). Power is the product of force and speed. $\endgroup$ – Oskar Skog Jul 13 '17 at 16:25
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There are plans to run a turbofan-like engine with an electric motor rather than the gas generator used today. But the weight of electric engines does not promise an immediate advantage - only when the electricity is generated in a much more efficient way that what today's gas generators offer will the overall propulsion mass be lower. An example is a hydrogen-powered fuel cell somewhere in the fuselage.

We had a similar discussion here and here.

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  • $\begingroup$ Can you cite exactly how much power the fan alone will need during takeoff and during cruise? I'm thinking for a plane like the Boeing 777, which I choose just because it's still got the biggest turbofans of an airliner. If I know what power rating the motor needs, I can look up how big and heavy such a motor will be. $\endgroup$ – DrZ214 May 4 '17 at 3:51
  • $\begingroup$ @DrZ214: No, not exactly. We had a similar question here $\endgroup$ – Peter Kämpf May 4 '17 at 21:41
  • $\begingroup$ RR's book "The Jet Engine" p101 says the fan provides 75% of the engine thrust... so knowing a 777's thrust and the formula that equates lbs thrust to eqv hp, you'll get your answer. $\endgroup$ – Thomas McKelvey May 19 '17 at 9:35

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