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Manually Rotating Turboprops

Wed Apr 25, 2001 12:51 am

At my local airport, I've noticed sometimes that the pilots of turboprops manually rotate the propeller. What is the purpose for this?
Jack @ AUS
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 1:43 am

Direct drive turbo prop engines sometimes require this to remove residual heat from the engine because once the prop stops turning, so does the engine. When the pilot turns the prop, depending on the gear ratio, he is actually spinning the engine very fast. This helps move some of the heat out of the engine. This procedure is not necessary for turboprop engines with fluidic couplings because the engine is not attached to the prop, and therefore can spin down normally even after the prop has stopped turning.
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 2:45 am

The propeller is rotated during the outside check, which involves checking the leading edges and the surface of the blades for damage. As far as heat removal is concerned, as Latechpilot mentioned, the engine manufacturers prescribe certain cool down times at idle thrust, which normally is sufficient.

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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 3:29 am

We are required to do this both before and after flight to check for damage on the Saab 340, I know of the Jetsream 31s requiring it for heat removal. Just depends on the manufacturer.
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 4:35 am

A C-130 type told me that the reason that props are rotated after landing is to position the prop oil system drain plug "up", to avoid possible leakage. Any Herc guys out there to confirm this?
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 1:37 pm

I'm not a Herc guy specifically, but I've worked enough of them to confirm JohnM. The military will also turn them to "dress" the props, which is an old tradition that comes from the days of wooden propellers. They were turned so that they were an "X" or vertical to help water runoff and prevent warping and rotting. Later the tradition was kept just to keep a neat and orderly appearance.

"Never trust a clean Crew Chief"
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 2:03 pm

Prop inspection is part of a normal pre-flight. The blades are looked at for wear, condition, security in the hub, oil leaks, abnormal play etc.. Rotating the prop through a few times can also indicate hot section problems like turbine blades rubbing the case, depending on the type of engine, of course.
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 2:12 pm

The purpose for the C-130 is as stated. The propellor hub has a vent that is adjacent to the #2 blade of the propellor. Under normal circumstances, this valve doesn't leak when the prop is static. However, as it wears, it will leak, so the #2 blade is placed on top to prevent the prop oil from leaking out of the propellor. When the prop is rotating, cetripigal force keeps the oil slung against the outside of the hub and therefore won't let it leak out of the vent. This is true of any aircraft running the Alison 501... ie Electra, CV-580, etc.

The purpose of turning props on Garret 331 powered aircraft is twofold.. 1 to cool the turbine... 2 to scavenge hot oil from the #4 bearing to keep it from coking. Should pull at least 24 blades through right after shut down for proper scavenge.
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 4:58 pm

One more thought on dressing the props.....

In the Metro service manuals(If I remember correctly) it states that you place the props so that the blades are in a '+' position. That way if it rains any water that might accumulate will drain throught the opening for the prop blade on the bottom.

I remember reading that on the wartime Martin B-26 bomber you placed the props in an 'x'. That was in case the nosegear strut collaped the prop wouldn't strike the ground. Of course aircraft nowadays are supposed to be certifed with enough clearance so that a prop will not strike the ground in the case of a flat strut and tire on the landing gear.
T prop
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RE: Manually Rotating Turboprops

Wed Apr 25, 2001 5:46 pm

On the Garrett TPE331's we were trained to spin the props after shutdown to get cooling air flowing through the engine, this was done right after shutdown. Half an hour after we did the prop spins we had go back out to the aircraft and rotate the propellers one blade width. Moving the prop one blade width rotated the engine one half turn, this was to prevent the shaft from sagging as the engine cooled causing turbine blades to rub.

T prop.

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