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Old 05-24-2016, 03:37 PM
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rca2000 rca2000 is offline
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Jet engine Question...

On a large turbofan jet engine...I always thought that the front fan was driven by the turbine shaft. BUT...i have seen MANY instances of the front fan just "freewhelling in the wind" on a parked Jet plane that as not even powered at all. If the turbine shaft was connected to the fan..HOW could simple wind make it rotate? It would seem to be WAAYYY too much mass for that. Also...I have also seen pictures of the front fan spinning MUCH slower--it appeared and sounded like--than the jet engine..Maybe 1500-2000 RPM..but jet engine spin at MANY thousands of RPM when running--even at idle--right?

So...is the front fan driven...OR does it just spin by AIR PRESSURE ?
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Old 05-24-2016, 04:20 PM
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Good explanation here:

https://en.m.wikipedia.org/wiki/Turbofan

Concentric shafts for low and high pressure components.

jr
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Old 05-24-2016, 07:12 PM
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STILL no explanation of why the fan can sit and slowly spin just sitting on the tarmac...with a sleeping engine..

BUT maybe it explains why the fans seem to go a LOT slower than the engine ..(gear reduction)..so it does not spin too fast--..or it would be VERY noisy..
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Old 05-24-2016, 10:14 PM
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Why wouldn't a large, efficient fan blade supported by nearly frictionless bearings turn in the wind? My attic fan turns a lot when we we have a bit of east wind.

jr
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Old 05-25-2016, 12:34 AM
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I guess I figured all of the OTHER stuff it was connected through--gears, power shaft, power turbine--would be too much drag on the fan for it to just turn in the wind...UNLESS..there is some sort of a clutch between all of it, which does not engage till the power shaft is spinning sufficiently fast..
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Old 05-25-2016, 12:59 AM
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I may be looking a this wrong, but it looks like there is no direct connection between the low pressure components and the high pressure components. If pushed by wind, the large front fan only has to turn a few very small rotors at the rear of the engine through the centermost of the concentric shafts. I bet you could easily spin that puppy by hand!

jr
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Old 05-25-2016, 02:05 AM
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Quote:
Originally Posted by jr_tech View Post
If pushed by wind, the large front fan only has to turn a few very small rotors at the rear of the engine through the centermost of the concentric shafts. I bet you could easily spin that puppy by hand!

jr
I'm picturing some one yelling "contact" to an idiot sitting in front of the turbine preparing to spin it by hand to start it...
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Old 05-26-2016, 01:37 PM
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Found this video of a small turbofan... it is easily turned by hand:

https://m.youtube.com/watch?v=BEghFV2y2rY

jr
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Old 06-01-2016, 09:11 PM
quaddriver quaddriver is offline
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the hi bypass fans are driven by the exhaust.

on a dead engine they are oft turned by an electric motor to keep oil flowing on colder days
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