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Sheep Guts
21st Jun 2003, 23:43
Well this may have been covered before. Where I work at the moment all Aircraft even ones that overnight the N1 fans are left to rotate. Is this good,?I know in a Turbo it aint and hence Prop chocks. Also I remmember when I was a Tech in the RAAF in Richmond Sydney and seeing C-5 Galaxies overnight, their CF6 GE s had the N1 Fan chocked to stop rotation in the wind.

Regards
Sheep

P.S. they dont care about it in the Carib :(

lomapaseo
22nd Jun 2003, 00:47
Nobody likes blowing sand to get in their engines, it wears things.

regarding windmilling fans in the wind. It all depends on whether they have those loose shrouds/clappers/snubbers or those funny thingies that are partway out the blades and touch each other.

If they are present (most new designed engines no longer have them) then the fact of their looseness (at ground windmill conditions) makes them clatter when they fall over at the top and bang into their mates. If they are very loose then the clatter is quite pronounced and can actually wear their surfaces the bang. hence the need to keep them out of the wind.

Even the new engines with none of these thingies partway up the airfoil may rattle at the joint with the hub that they are mounted in. Too much rattle = too much wear.

411A
22nd Jun 2003, 11:35
In actual fact, it's when theydon't rattle that should be of concern.
Sometimes guys have to climb up in the intake and use a big rubber mallet to knock 'em free...works good.

Anthony Carn
22nd Jun 2003, 16:40
Based upon my understanding/teaching....

Gas turbine engine bearings are designed for high speed running. The rolling elements "ride up" on a thin film of oil, promoted by the high speed. Pressure feed of the oil keeps the flow up and that flood of oil promotes the "riding". No metal-to-metal contact. Bearings happy.

Windmilling engines do so at zero oil pressure. This means zero oil feed to bearings. No big deal, I hear, there's oil left in there from the last run. Correct, but there's a risk of the rolling elements having insufficient speed and oil to "ride". No "riding" equals metal-to-metal contact and bearing wear. It's slight - the speeds and loads are low.

Rolling elements are more prone to major failure once their surfaces become scratched/scuffed/fretted/whatever. Any small surface damage can spoil the "riding" and lead to metal-to-metal contact whilst the engine is running, ie during a period of high loads and speeds, leading to accelerated wear/failure. Hence the need to prevent the initial, slight wear.

Previous answers also, correctly, direct the avoidance of wear at the blade roots and snubber contact surfaces - another good reason for preventing windmilling. Scuffing/scratching at the roots can lead (eventually) to stress concentrations - and boy, are those roots stressed enough to begin with !)

lomapaseo
23rd Jun 2003, 00:09
411

The silenced-rattle you refer to is a much rarer phenomena and somewhat unique with engines that normally tinkle or rattle why windmilling on the ground. It's called blade lockup which is a stick/slip sliding friction of the snubbers/clappers that has forgot how to slip at low RPM.


Anthony

The bearings in engines can run damn near forever on the little oil that's left in them at windmill speeds. They only need the oil quantity that you decribe at highly loaded conditions.