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For Turbo Fan Engine CFM56-7, was wondering how come the Fan blades (N1 shaft) turns when I turn the N2 Shaft manually.

I thought N1 shaft is connected to the fan and booster rotor and LPT rotor and N2 shaft is connected to the HPC and HPT rotor and both are on individual spools.

So if the N2 shaft turns, by right anything on the N1 shaft will not turn right?

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    $\begingroup$ If wind blows from the rear the fan sometimes rotates backwards, even with the turbine idling. I've seen this at least once at the airport. $\endgroup$
    – user3528438
    Aug 12, 2021 at 18:00

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Both shafts are not connected directly. But there are still ways to transfer rotation from one to the other. The most obvious one would be the airflow generated by N2 turning the LPT. But also the bearings between both are not totally without friction and so will also transfer some rotation.

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    $\begingroup$ Yeah if he's turning the fan by hand it's going to be just bearing drag. $\endgroup$
    – John K
    Aug 12, 2021 at 23:06
  • $\begingroup$ "bearings between both are not totally without friction", there is only one. On the other hand, N1 is also centered within frames by three bearings. So there is possibly more friction tending to maintain N1 stationary than friction tending to rotate it. $\endgroup$
    – mins
    Aug 13, 2021 at 10:48
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    $\begingroup$ @mins I don't think counting bearings is a good way to estimate friction. $\endgroup$
    – Sanchises
    Aug 13, 2021 at 13:55
  • $\begingroup$ @Sanchises: Sure this is not accurate engineering, but when bearings of the same diameter, and all with balls, are not lubricated, I believe they won't be very different. In addition #1 which is on the fan shaft (rotating with N1) is a large one, likely the one with the most friction when not lubricated. So you're right, but let see if a mechanics can confirm or deny. $\endgroup$
    – mins
    Aug 13, 2021 at 15:31
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    $\begingroup$ On a CFM56, there is an intershaft bearing between the N1 and N2 shafts (#4 bearing at the aft end). So for this engine, yes, some slight friction in the bearings can transfer torque between the shafts. This is also true for some cousins of the CFM56 like CF34-10 and F110. However, for the majority of jet engine designs, bearings connect from the rotor to the stator, not rotor to rotor. So for most engines, bearing friction cannot transfer torque between shafts. $\endgroup$
    – Daniel K
    Mar 7 at 13:05

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