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I was reading about bleedless-787 power system but found out it still requires a gearbox.

What if you put coils around the "tube" part of the jet and tiny magnets in the end of the fan/compressor blades, this would generate an electric current that could be useful for powering the aircraft's electrical systems. With the right control system, it could perhaps also be used for precise corrections for vibrations/rpms resulting in better reliability and higher efficiency. Finally, it might be useful to assist the startup process by spinning the blades from 0 to some initial speed (ideally, replacing the starter mechanism but thats probably a stretch).

I know jet blades are subject to extreme mechanical and thermal stresses, so implanting a ferromagnet is probably not possible. But given the tremendous angular velocities, it probably doesn't require much magnetic material to induce a useful current. Perhaps a chemical spray deposition would suffice.

Has such a system been considered before? Are there any other obvious reasons this wouldn't work? Obviously there's no free lunch, and this system would steal angular momentum from the fan blades and require more fuel. But if it could somehow replace the generator and/or starter, it would yield tremendous weight savings.

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    $\begingroup$ The need for a gear box is to stabilize the shaft rpm. Aviation electricity is AC of a fixed frequency. If airplanes used DC system like a car, or have chosen to use a inverter to stabilize the frequency like a home gasoline home generator, then you could certainly skip the gear box. $\endgroup$ Sep 6, 2022 at 6:33
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    $\begingroup$ So you propose replacing a relatively compact generator with a generator many times the size (you'd have to put stator coils all around the fan or the LP compressor). How exactly do you imagine this could save weight? $\endgroup$
    – TooTea
    Sep 6, 2022 at 6:54
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    $\begingroup$ @TooTea: I have never seen the generator on a turbofan, so I dont know how much either would weigh. But I believe fewer moving/contacting parts is generally preferred in any mechanical design, however. $\endgroup$ Sep 6, 2022 at 14:32
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    $\begingroup$ "tiny magnets in the end of the fan/compressor blades" even if very tiny, they are going to a marked increase in the momentum of inertia of the fan ---> bad. The aerospace industry is trying to make the blades as light as possible because of that, not because of the weight itself $\endgroup$
    – EarlGrey
    Sep 7, 2022 at 12:21
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    $\begingroup$ The currie point of iron is roughly 1000C. The operating temperature of a typical turbofan is closer to 2000C. No, you aren't going to be able to keep magnets in a jet engine. $\endgroup$
    – Aron
    Sep 8, 2022 at 8:49

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The power generation method you propose is used on almost all motorcycle and small lawnmower engines: magnets pressed into the rim of the flywheel, with stationary coils nearby in which current is induced to flow.

These motors run at speeds much slower than the fan disc speeds in a turbine engine, so stuffing the flywheel rim with magnets does not compromise its structural integrity the way it would in a highly-stressed fan disc.

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    $\begingroup$ ...Moreover, a the whole point of a flywheel is having a large moment of inertia. If it didn't have the magnets, it would have to have an equivalent amount of ballast mass. Very different story with turbine blades! $\endgroup$ Sep 7, 2022 at 19:00
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Today's commercial jet engines can reach temperatures as high as 1,700 degrees Celsius (that's 3,092 degrees Fahrenheit) (see here).

Every magnet type has a specific Curie temperature above which its magnetic properties are lost forever; the most resistant is cobalt, whose Curie temperature is 1,130 Celsius, so any permanent magnet inside a jet engine will quickly become a lump of non-magnetic metal.

A different option, currently under investigation, is to use the temperature difference between the inside and outside of the jet engine to generate current, via the thermoelectric effect.

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    $\begingroup$ Welcome to Av.SE! The temperatures you mention are at the hottest part of the engine, inside the combustion chambers. The tips of the fan and compressor blades, where the OP suggests putting the magnets, don't get nearly as hot as that, since they're upstream of where fuel is added to the compressed air. $\endgroup$
    – Ralph J
    Sep 6, 2022 at 7:45
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    $\begingroup$ @RalphJ Yes, that is true, but the demagnetisation curve still creates an issue. As long as the temperature isn't even there will be an unbalanced load added by the magnets. Even worse, the pull and the imbalance could change dynamically depending on the season, microclimate, the flight phase, the flight leg and the time of day. $\endgroup$ Sep 6, 2022 at 9:58
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    $\begingroup$ I totally agree that magnets on the end of the finely balanced fan blades would be extremely problematic for all sorts of reasons. My comment was simply to highlight that temperatures upstream of the combustion chambers are nothing like those inside or just past them. $\endgroup$
    – Ralph J
    Sep 6, 2022 at 14:14
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    $\begingroup$ To be specific, this is the Seebeck Effect $\endgroup$ Sep 6, 2022 at 21:24
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Another case of yes, but why would you want to? The starter generator assemblies on most jet engines does the same thing at far less weight and complexity.

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Can electricity be harvested from magnets in jet engines?

Yes, it could be. There would certainly be engineering challenges to overcome in order for it to be a stable design, but it would be possible.

The main reason it's not done was raised in the comments by @user3528438. Expanding on that a little bit: generating power in this manner will result in AC current, which is what the plane uses, but the frequency will be dependent on the speed of the rotors. Since the rotor RPM changes throughout flight the frequency of the AC current generated will change, which is not good for the airplane's electronics. Hence why there is a gearbox to regulate RPM to the right level for the airplane's electronics to work properly.

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