In this question, in the accepted answer, there is a comment by alephzero that says one can actually rotate a large turbofan's blades with one finger! Is that true, can someone elaborate? As an example, does this apply to my google search for strongest turbofan GE9X too?
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1$\begingroup$ @Bianfable Thanks for the input. I am mostly concerned about the easiness/resistance of the blades turning, not security or anything like that, so having to climb is no problem for this question. $\endgroup$– George MenoutisSep 29, 2020 at 9:29
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$\begingroup$ I would imagine that it depends on how recently and how well the engine has been lubricated. $\endgroup$– MD88FanSep 29, 2020 at 16:36
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8$\begingroup$ When you see planes parked in an airport, notice that the blades are turning with any light wind, which means very small torque is needed to turn them $\endgroup$– hdrzSep 30, 2020 at 6:30
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$\begingroup$ what a great point @hdrz $\endgroup$– FattieSep 30, 2020 at 11:59
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4$\begingroup$ The fan is huge. Torque is force multiplied by the radius at which it acts. It is trivial to generate a large torque with a small force if you apply it at a large radius (think breaker bar, etc). Jet engines operate at 10k rpm+, so any friction is deadly - both to performance and to the mechanical components, so we also expect that it should turn with extremely low friction by design. $\endgroup$– J...Sep 30, 2020 at 13:22
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2 Answers
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Yes, large turbofans can be turned by hand without too much force. On smaller aircraft this is regularly done as part of the preflight walkaround. You can see an example in this YouTube video of an A320 walkaround:
Admittedly, the pilot turns the engine with four fingers here, but you can see that not much force is required. Turning it with a single finger would easily be possible.
On a larger aircraft like a Boeing 777 you cannot reach the fan when standing on the ground in front of the engine. Spinning the fan by hand is therefore not usually done during the preflight.
This video shows a GE engineer turning the fan blades of a GE90 (the predecessor of the GE9X) engine by hand:
Again, it is not one finger, but he is not using much force to turn it, so it should be possible with one finger.
answered Sep 29, 2020 at 9:51
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23$\begingroup$ Supplementary detail: an illustration such as en.wikipedia.org/wiki/File:Turbofan_operation.svg shows that in many cases the fan is powered directly by a turbine at the other end of the engine, it's not geared to anything which would provide significant resistance when the engine isn't running. (I suggest not trying to see if the fan can be stopped by hand when the engine is running.) $\endgroup$ Sep 29, 2020 at 20:35
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1$\begingroup$ "I suggest not trying to see if the fan can be stopped by hand when the engine is running." You wouldn't even be able to make the attempt - you'd just be sucked into the engine and turned into a paste with the consistency of chunky salsa. I've seen pictures; they aren't pretty. $\endgroup$ Oct 1, 2020 at 7:59
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Turning the engine with one finger is not necessarily recommended, because the leading edges of the fan blades are quite sharp. But it is possible.
For some experimental vibration measurements, it is preferable to keep the rotors turning slowly to avoid the bearings "sticking" in one position and confusing the results. A common way to do this is simply to put an ordinary office desk fan in front of the engine. That tiny amount of air flow is enough to keep the fan slowly turning.
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1$\begingroup$ Some engines have a mechanism to slowly turn the shaft(s) during cool down . I understand there is a risk of the shaft getting a slight deviation from straight if allowed to cool down in one position. $\endgroup$ Sep 29, 2020 at 21:10
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10$\begingroup$ @blacksmith37, different turbine vendors have different cool-down procedures. I'm not a pilot, but have worked with steam and gas turbines a lot. In a cooling steam turbine, the dripping condensation makes the bottom of the turbine much cooler than the top, and the shaft contracts on the bottom, causing the it to bow upwards ("hogging"). This lasts until the temperatures even out. To prevent this, the turbine shaft is slowly rotated when cooling. I haven't seen this procedure on gas turbines; many vendors feel it's not necessary for gas: the temp differentials are much smaller. $\endgroup$– JasonSep 30, 2020 at 6:26
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1$\begingroup$ If you look at the turbine fan, they often have a spiral painted on the center ; I have often seen them slowly turning. $\endgroup$ Sep 30, 2020 at 15:03
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$\begingroup$ Yep, I've heard of gas turbines that have a slow rotation requirement or recommendation during cool-down, but I've never worked on one myself. Opinion seems to be divided on whether this is worthwhile. $\endgroup$– JasonOct 1, 2020 at 14:31
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6$\begingroup$ @blacksmith37 More often than not, the slow turning you'll see on the intake fan of a turbofan jet engine is just the engine windmilling. It doesn't take much wind at all to windmill it. It takes it quite a while to completely stop after it's shutdown, not because anything is driving it to continue spinning slowly, but just because the bearings have to be really good to prevent it from burning up at full power and, thus, there's not much friction from the bearings to stop the rotation once the engine is shut down. $\endgroup$– reirabOct 1, 2020 at 15:29