'Idle' curiousity
 

Often you see planes on the apron and the engine fans are turning slowly. I've always believed that is to prevent the turbine blades from distorting as they cool. Is that correct? But my real question is, how do the fans turn? They are not idling (as in engine running slowly), so what drives them? I can only imagine that some geared electric motor engages with the power shaft somehow, but I can't see how that would be achieved. Thanks in advance.

Do you mean the front fan? The turbine blades is another issue, they are towards the rear and out of sight.

If it is the front fan then they are simply "windmilling".

No power applied to the fan whatsoever. They turn very easily and even a fairly gentle breeze will start the fan turning. In more windy conditions they go like the proverbial clappers!

And, if they don't, expect big time trouble on departure.
Excessive vibration.
Fan blades are meant to move, if they do not, lubrication is very necessary.
In sandy desert locations, many times done every day.
Ours (RB.211) are lubed every turnaround...and with 800 meters vis at JED (as it was in early January), with blowing sand, very necessary, for continued satisfactory ops.
RVR on two days at JED went down to 300 meters for takeoff, landings aborted, takeoffs only...blowing sand is a mess.
Many diversions.

Thanks for the answers everyone, that clears that up for me!

OK I confess my total ignorance.
What then is the point of the front fan if it is "freewheeling".?
What is it connected to?

Freewheeling while the engine is not running.

The front fan is driven by the low pressure turbine at the rear.

When I was at the De-Havilland museum in St. Albans they had a few engines there and I couldn't resist trying to turn a Ghost (centrifugal flow) engine.

I was surprised but it wasn't actually that hard to move.
I could tell that there was a lot of inertia to overcome but once turning (even slowly) it was easy to spin faster.

Id imagine that the front bypass fans of some smaller jet engines are easier to turn (less inertia and weight because they are axial and not centrifugal).

Hope it helps.

btw, I tried the Gyron engine and had no chance of turning that engine !

hmm, I wonder if someone could manually turn the front fan of a RR Trent..... probably not

You would have no problem hand turning a large fan like the trent.

John R,

Simple - gotta check the blades in good, undamaged condition, much like on any prop aircraft. Damaged blades = potential catastrophic component failure.

He was practicing for "Wheel of Fortune" ;)

Didn't you see him looking up at the flight deck window and shout "Contact!" ? ;)

As mentioned earlier the fan at the front of the engine is driven by the low pressure turbine at the rear of the engine and is a free spinning rotor assembly. If you could see up the back of the engine you'd be able to see the turbine spinning at the same time the fan is.

If the wind is down the front of the engine the fan/turbine will spin in the normal direction but if the wind is from behind the engine the whole lot will spin backwards.

In all multi spool jet engines the inner or high pressure compressor and turbine and burners etc is the bit that does the work that generates the hot gas which drives it, and the low pressure turbine and fan. Accessories such as the generator and hydraulic pump are gear driven from the high pressure rotor.

Regards,
BH.

Windmill
 

So easy to compare the fan of these high bypass engines, to the toy fans of kids.
The wind blows, and it makes the fan turn.
If it blows from the front, the fan will turn it the proper direction.
And pilots know with a tailwind, the fan will turn in the "wrong direction"...
Tailwind limitations, gentlemen... Dont get a hot start...!
xxx
You kids dont know this, but in the 1950s, air forces had fighters with large nose inlets.
The F-84s, the F-86s, the F-100... and you guys with the RAF, the Lightning.
All it took was to put a few airplanes nose into the tail of preceeding plane.
In the Korean War, they started the first airplane of the line-up...
And its exhaust would turn the engine of the second plane, and so on.
Only one start unit needed. Save starters too. Just windmill needed.
I have seen some F-84s getting started that way.
xxx
One day I had problems starting one JT3D on my 707 in Sana'a, Yemen.
So, we got the plane on 3 engines to the runway.
Got it to high speed on the runway, and started the remaining engine.
Was by windmill + crossbleed. It worked. We got careful not to get a "hot start".
Apparently, crossbleed was not sufficient, but "supplemental" windmill helped.
Then taxi again, back for takeoff, we left the gear down for a while.
Certain we had hot brakes after that "not in the book" procedure.
Hope our friend 411A will not criticize we ex-UACI guys.
xxx
:D
Happy contrails