Why do turbofan engines keep turning very slowly after the engine is shut down ?

[William Gaatjes]

I noticed that sometimes the fan is turning very slowly after an airplane has landed and the motor is shutdown. I always assumed this is done to keep constant amount of force from the weight of the turbo fan. To keep a constant amount of force on the bearings to prevent deformation causing unbalance which may result in catastrophic engine failure while at high rpm in flight.
Is this the real reason or is there some other reason i am overlooking ?

 

[tynopik]

my totally WAG is low friction, it doesn't take much wind to get them going

 

[PottedMeat]

my totally WAG is low friction, it doesn't take much wind to get them going

yeah you can move them with your finger, a little wind + lots of surface area

I always assumed this is done to keep constant amount of force from the weight of the turbo fan. To keep a constant amount of force on the bearings to prevent deformation causing unbalance which may result in catastrophic engine failure while at high rpm in flight.

they're not that delicate

 

[brianmanahan]

currents in the luminiferous ether

 

[William Gaatjes]

yeah you can move them with your finger, a little wind + lots of surface area

they're not that delicate

Oke, i must be confusing it with something else that is kept moving to prevent deformation because of the weight.
Arghh. That awful "On the tip of my tongue" feeling.

 

[William Gaatjes]

I remember now, wind turbines. The ones that generate electricity, turn very slowly even when not in use. This to prevent deformation of the bearings because of the weight of the rotor.

 

[lsd]

They do not keep turning unless there is wind. The fan section you are able to see is very easily turned by hand, but the high pressure compressor section takes much more force because of all the accessories attached to it (generator, hydraulic pump, etc.)

 

[Red Squirrel]

I could see that, for wind turbines. Would also ensure that the oils don't all settle at the bottom in the various mechanical parts, like gear boxes etc. I recall hearing something about wind turbines actually end up using more power than they produce, when used in inadequate areas. They need to be heated too so the oils don't get too thick and so they don't get snow or ice build up on the blades. Simply turning the blades may be enough to take care of snow too, so maybe the blades themselves don't need to be heated, just the actual generator room. I'm just speculating though, I could be wrong.

 

[Stopsignhank]

??

Why do the turbines not spin at times?
The most common reason that turbines stop spinning is because the wind is not blowing fast enough. Most wind turbines need a sustained wind speed of 9 MPH or higher to operate. Technicians will also stop turbines to perform routine maintenance or repairs.

https://www.we-energies.com/environmental/winddev_qa.htm

http://www.windenergydirect.ie/faqs/

Why do some wind turbines spin, while others next to them do not?
The direction, speed and force of wind changes all the time, even within a short radius of space. That’s why you may sometimes see a wind farm with only a few spinning turbines.

I was near one of those huge wind turbines one time. What impressed me was the sound that the rotor made when it passed by. The rotors are so huge it is incredible.

 

[William Gaatjes]

I could see that, for wind turbines. Would also ensure that the oils don't all settle at the bottom in the various mechanical parts, like gear boxes etc. I recall hearing something about wind turbines actually end up using more power than they produce, when used in inadequate areas. They need to be heated too so the oils don't get too thick and so they don't get snow or ice build up on the blades. Simply turning the blades may be enough to take care of snow too, so maybe the blades themselves don't need to be heated, just the actual generator room. I'm just speculating though, I could be wrong.

Now you mention it, it does sound scary when i would be walking underneath a windturbine and a lot of ice comes falling down.
According to a windturbine explanation about windturbines in the Netherlands, the windturbines that are not heated are actually stopped when the temperature gets below a certain point. This is not only to prevent unbalance and because the rotating rotor blades are then in unbalance, the bearings would be overloaded as well the entrire structure. The other reason is because while the rotor is rotating, it might start slinging massive chunks of ice that have come loose from the blades around and that can be a very dangerous situation.

And of course, when the wind is blowing to hard, the rotor is turned perpendicular to the wind direction to prevent overloading the turbine and the rotor is not or only very slowly moving.

 

[who?]

I don't believe that turbofans have any braking mechanism to stop them from turning so if the momentum is stronger than the drag forces the fan, it's shaft and the turbine at the other end of the shaft will keep on moving.

 

[mindless1]

Isn't it just that the fan itself doesn't compress more than air so it's left to that to slow down and as long as it's moving forward, it has help?

 

[NetWareHead]

I remember now, wind turbines. The ones that generate electricity, turn very slowly even when not in use. This to prevent deformation of the bearings because of the weight of the rotor.

I beleive Ive seen this with large container, dealing with the diesel engine specifically. Talking about a crankshaft that weighs hundreds of tons, a straight-14 cylinder layout. If I remember correctly, even when the engine is shut down. The practice is to keep the crankshaft rotating at some very low speed (like 1 revolution per hour) so it doesnt settle into the bearings and induce flex.

 

[ImpulsE69]

I remember now, wind turbines. The ones that generate electricity, turn very slowly even when not in use. This to prevent deformation of the bearings because of the weight of the rotor.

This isn't that either. I see hundreds of them every day that are not turning.

 

[William Gaatjes]

This isn't that either. I see hundreds of them every day that are not turning.

I read about it once on the website from a windturbine park exploiter. They turn so slow it is hardly noticable.
But then again maybe some windturbines do and some don't.
I will try to find that that article.