Is the N2 rotor turning while the fan is windmilling on the ground from wind?

Question

If a high wind is blowing you can see the fan rotating but, what about the N2 rotor? I deal with generators for small turbofan jets. This question stems from discussions about what the generator sees while the aircraft is off and sitting on the ground. An engineer at company c say you could get a small reverse or foward rotation from the accessory drive gearbox due to wind. An engineer at compay p sais the friction of the oil pump would prevent this. What do you know about this?

Answer 1

Yep

the wind blowing through the engine nacelle spins all the stuff thats inside.

As far as the generator turning, to the best of my knowledge it will turn.  And therefore it will generate a small electrical current.  however, the RPM produced by windmilling is so small compared to the RPM that normally happens during engine operation, that the electrical charge built up is not significant enough to create anything that even remotely resembles a spark.  Its grounded anyway once the generator switch is turned off, so the current does not go anywhere.

Answer 2

Nice Q, it made me think and look at my manual for I was thinking when we're doing boroscope at the core section (High Pressure Compressor -N2) we open something to turn the N2 unlike the Low Pressure Turbine, we only turn the Fan blades while doing boroscope on the LPT turbine blades. One more thing only the N1 is fluctuating at the digital gage in the cockpit when it is windmilling in the ground or the crew doesn't really know when N1 is rotating already because the starter drives the N2. So we at ground if we're the headset man, we're obliged to say N1 rotating.

So the answer is NO.for the simple reason when doing boroscope to turn the LPT blades we turn the fan blades, while when we boroscope the core or N2 (HPT blades) we open up an N2 rotor pad and get a 3/4 socket to turn the core blades.

Answer 3

This is a good question and to answer it correctly you would have to go to the peculiar engine maintenance manual.  Depending on the engine design the short answer is YES.  If you go to AC65.12A Airframe and Powerplant Mechanics Powerplant Handbook Chapter 1 Theory and construction figure 1-36 you will see a common shaft connects the N1 and N2.

As the N1 turns so does the N2 the internal gearbox will also turn, but the generator switch in the cockpit should be turned off not allowing residual electrical to be introduced in the system.  Remember each make and model are different and the current maintenance manual should be checked.

Some engines have a dual spools so be careful one answer may be correct for one engine and not the other.

Keep in mind a generator does not care which way it turns to have conduction.  On some engines you will have a starter/generator configuration using one unit for starting and electrical power.

Answer 4

hmm, very interesting question. obviously there would be some resistance since the N2 is connected to the accessory gearbox as you mentioned and the HP compressor/turbine sections are pretty short and stubby unlike the N1 so i'd say the engineer at company c is right. i dont see how the oil pump comes into this.

Answer 5

On a High Bypass engine with N1 windmilling, N2 will not normally rotate. When the air is blown through the fan it is directed down the by-pass. The volume and energy of the air reaching the N2 compressor will be miniamal.

Answer 6

Good question but unfortunately I don't think can be answered with a yes or no. I'm very familiar with the williams FJ44 series engine and I would say on that engine with a "NORMAL" wind NO, obvioulsy if you pump enough air at a high enough speed you will get the N2 shaft to turn. There are forces opposing the shaft from turning (the friction and loading of the accesories: fuel pump, oil pump, hydralic pump, generator and the fact that the spools are counter-rotating so the bearing friction also opposes the N2 from turning in the normal direction). Also on this engine the HP compressor (N2 compressor) is centrifugal not axial. Getting a centrifugal compressor to spin with an axial airflow is a challenge in itself especially when theres a much less restricted path for the air (bypass ducts). So again on this engine I would say NO but there's enough difference in jet engines that I wouldn't be comfortable stating this for ALL engines.

Answer 7

I am a jet engine mechanic

when I was in the airforce larger turbofan engines would have 2x4s shoved in them to prevent them from rotating in the wind.

so yes they will spin the whole engine, you can spin them with one finger on the ground.

thats why its so important on the older engines to have the throttle pulled all the way back after shut down , you didnt want the fuel pump leaking fuel into the mainframe and then giving you a high egt on start, and maybe a fireball out the back

(which looks totally wicked at night)