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What are the merits and demerits of adding a gearbox to larger jet engines like the Trent 900. If its about torque, why not use a magnetic gearbox? What are the demerits of using magnetic gearboxes in a geared turbofan engine?

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One major benefit is that gearing slows down the fan tips below supersonic, giving a major reduction in noise and drag. The newer massive turbofan jets are ever improving in efficiency, and, with gearing, are taking a page out of the turboprop book to do it.

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  • $\begingroup$ The goal is actually the opposite. A turbofan engine is unlikely to be designed with supersonic fan tip speeds because of the losses (the odd experimental usage aside). Rather, since the fan tip speed is limited to below the speed of sound, adding a gearbox allows the compressor speed to be increased instead, or maintained as the ratio of fan to compressor diameter increases. $\endgroup$ – XRF Sep 25 '18 at 7:55
  • $\begingroup$ You are are saying the same thing I am, the gearbox allows for slower fan speed. Slower bigger "wings" are more efficient. Geared turbofans were developed by the British for noise abatement as well. $\endgroup$ – Robert DiGiovanni Sep 25 '18 at 8:49
  • $\begingroup$ BTW the British engine was developed from a chinook (helicopter) turboshaft! $\endgroup$ – Robert DiGiovanni Sep 25 '18 at 8:59
  • $\begingroup$ @XRF. Not quite. The turbine needs to spin fast to be efficient, the fan needs to spin slowly. Adding a gearbox enables both to be optimised, rather than both (partly) comprised. So, it’s about the fan/turbine speed ratio, not the fan/compressor speed ratio. $\endgroup$ – Penguin Sep 25 '18 at 11:48
  • $\begingroup$ @Penguin The compressor and turbine are already connected and their common diameter means their speed requirements are similar. The turbine design requirements are a lot easier on aerodynamics than the compressor and if it is any part of the turbine you want to run fast, it is high pressure turbine not the low pressure turbine, and that is because of weight savings not basic efficiency. $\endgroup$ – XRF Sep 25 '18 at 22:37
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The point of the gear box is not about torque but bypass ratio. With modern turbofans part of the means by which efficiency is achieved is to send as little of the air through the engine core as possible. This leads to dramatically different rotation rates required to achieve the proper blade tip speeds in the low pressure compressor and at the fan. Adding a gear box allows both the fan and low pressure compressor to operate at optimal speeds.

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  • $\begingroup$ As I read it, the comment in the OP about torque is about the gearbox being able to handle the torque produced by the engine, not increasing the torque to drive the fan. $\endgroup$ – FreeMan Sep 25 '18 at 14:39
  • $\begingroup$ Low pressure compressor vs fan can be addressed by using seperate spools. Afaict the real issue is turbine vs fan. $\endgroup$ – Peter Green Sep 25 '18 at 19:34
  • $\begingroup$ @PeterGreen Adding more spools adds more weight and may require more turbine stages depending on how well they are able to divid up the power. Turbines are fairly flexible with the angles and speeds you can give them, unlike compressors which are much more prone to stalling. $\endgroup$ – XRF Sep 25 '18 at 22:46

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