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Windmilling with a tail wind & lubrication
I recently transited through AMS, and went for a bus ride across the airport. It was quite blustery and I noticed many of the aircraft were windmilling pretty good. I noticed that for the aircraft parked tail to the wind, the windmilling rotated the fan in reverse. This led me to wonder, is there an oiling issue when windmilling backwards? Is there even an oil pump on N2, such that N2 rolling backwards would even matter?
Re: Windmilling with a tail wind & lubrication
N2 doesn’t typically windmill. There just isn’t enough velocity to do it. What you’re seeing is N1 or the Fan. I’ve never seen a prohibition against, nor an inspection required for windmilling on the ground due to surface winds.
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- Starlionblue
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Re: Windmilling with a tail wind & lubrication
Even in a strong wind, N1 rotates very slowly compared to when it is operating. You're talking a couple of RPM versus thousands. Not fast enough to cause issues.
The spool isn't connected to any accessories so spinning "backwards" isn't an issue.
The spool isn't connected to any accessories so spinning "backwards" isn't an issue.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
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- Redbellyguppy
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Re: Windmilling with a tail wind & lubrication
I usually see 1.5-2.0 n1 when it is windmilling in a stiff wind which amounts to about a hundred rpm.
Re: Windmilling with a tail wind & lubrication
MatthewDB wrote:Is there even an oil pump on N2, such that N2 rolling backwards would even matter?
Most commercial turbofans have the accessory gearbox and hence oil pumps driven by the high pressure spool which would thus be N2 (GE, P&W, CFM) or N3 (RR), but as pointed out by fr8mech, what you are seeing is the N1 spool.
If N2 or N3 windmilled backwards I suppose things would get interesting as many oil pumps in commercial turbofans are of the gear type, so you would be trying to pump oil backwards through the system if this were possible.
Starlionblue wrote:Not fast enough to cause issues.
Redbellyguppy wrote:I usually see 1.5-2.0 n1 when it is windmilling in a stiff wind which amounts to about a hundred rpm.
I think the main concern would probably be load instead of RPM. During windmilling, there is very little axial (thrust) load on the bearings which is probably the entity of most importance.
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair 
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.Re: Windmilling with a tail wind & lubrication
fr8mech wrote:N2 doesn’t typically windmill. There just isn’t enough velocity to do it. What you’re seeing is N1 or the Fan. I’ve never seen a prohibition against, nor an inspection required for windmilling on the ground due to surface winds.
Cleary I reversed the terminology... Yes, all I could see is the fan and it's the fan that is rolling backwards. I didn't think N1 had a pump hooked to it. Do any designs have any accessories driven by N1?
Re: Windmilling with a tail wind & lubrication
Redbellyguppy wrote:I usually see 1.5-2.0 n1 when it is windmilling in a stiff wind which amounts to about a hundred rpm.
That's about what I'd guess they were spinning at. More than one rotation per second.
Starlionblue wrote:Not fast enough to cause issues.
Redbellyguppy wrote:I usually see 1.5-2.0 n1 when it is windmilling in a stiff wind which amounts to about a hundred rpm.
I think the main concern would probably be load instead of RPM. During windmilling, there is very little axial (thrust) load on the bearings which is probably the entity of most importance.
Regards, JetMech[/quote]
Low load and low speed would meant that heat buildup is not an issue. But rolling dry will kill a bearing regardless of speed. I'm guessing that there is enough retained oil at all times that this isn't an issue.
My experience around turbines has all been with hydro and steam. Both of those use hydrodynamic bearings. Even the tiniest bit of rotation without lubrication on means wiped bearings. Ball bearings are a different beast altogether.
Thanks for the responses.
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- Starlionblue
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Re: Windmilling with a tail wind & lubrication
MatthewDB wrote:fr8mech wrote:N2 doesn’t typically windmill. There just isn’t enough velocity to do it. What you’re seeing is N1 or the Fan. I’ve never seen a prohibition against, nor an inspection required for windmilling on the ground due to surface winds.
Cleary I reversed the terminology... Yes, all I could see is the fan and it's the fan that is rolling backwards. I didn't think N1 had a pump hooked to it. Do any designs have any accessories driven by N1?
I don't think anything has accessories powered by N1. What would be the point? If nothing else, it's a bit tricky to hook anything up since the shaft is inside the N2 shaft (and the N3 shaft if present).
Then again, this being aviation, there always seems to be some weird exception.
(APUs have only one shaft, N, and of course accessories are hooked up to it.)
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
Re: Windmilling with a tail wind & lubrication
MatthewDB wrote:But rolling dry will kill a bearing regardless of speed. I'm guessing that there is enough retained oil at all times that this isn't an issue.
Indeed. Not surprisingly, a big killer of rolling element bearings is when the balls or rollers skid along the races instead of rolling. When changing wheels, you usually torque to a very high value whilst spinning the wheel to ensure everything is seated properly. You then back off all the torque and re-torque to a much lower value as the final, installed torque. Part of this preload is to ensure the rolling elements roll at all times and do not skid.
Starlionblue wrote:Then again, this being aviation, there always seems to be some weird exception.
The Trent 1000 drives the gearbox from the IP (N2) spool.
https://image.slidesharecdn.com/trent10 ... 1415159754
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .
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