Prop governors and oil flow

Xcaliber

Xcaliber

El Chupacabra
This should be a relatively easy question to answer, but it's actually more difficult to find than I thought. In a constant speed prop, we all know that the prop angle is changed by oil pressure to maintain a constant rpm. My question is, in an on-speed condition, is the oil flow through the hub completely stopped (plugged up), or is there a constant flow of oil at a consistent pressure through the hub?

The question stems from me wondering if there was a need to exercise the prop every once in a while in cruise to prevent oil from cooling/congealing in the hub. I suppose it wouldn't hurt anything to do it, but is it necessary?
 
I was always taught on-speed, there is no oil entering or leaving the hub. However, that governor is always moving to adjust. And, no has ever told me I should exercise the prop during cruise. I guess I have never looked into it too deep though.
Ryan
 
In a perfect on speed condition there would be no oil flow but like FS said... the governor is always moving a little and so some oil is still going in and out of the hub cylinder. There is no need to cycle the prop in cruise flight.
 
It seems I read somewhere that most props have small oil drain holes in the oil cylinder to keep oil flowing. I don't remember where that was, and therefore don't know how accurate it might have been. My awesome props book from A&P school has gone missing.
 
The prop only has one oil line, so there can be no flow through the prop.

There will be a constant movement of oil, as the prop adjusts to maintain rpm, but only in an imperfect world.
 
Roger said:
It seems I read somewhere that most props have small oil drain holes in the oil cylinder to keep oil flowing. I don't remember where that was, and therefore don't know how accurate it might have been. My awesome props book from A&P school has gone missing.
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Small oil drain holes in the cylinder would make the cylinder inefficient.
 
subpilot said:
Small oil drain holes in the cylinder would make the cylinder inefficient.
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*shrug* Like I said, it seems to me I read that somewhere. I'll see if I can find it again, and if it was a reputable source. Wish I was still working at the Skool o' Flite, we had props in pieces there for the A&P program that I could look at.
 
It has always seemed to me the answer is that you want there to be some movement to keep the oil in the hub from getting thick and ineffective ;).

If there is leakage in the governor the pitch will change and the flyweights will do their thing to keep things honest.
 
Found it: http://www.mccauley.textron.com/von_klip_tip_cs_propeller.pdf page 10.

It's been a few years since I took aircraft systems, and the little diagram-in-my-head had shifted a bit to more of a loop system, with a pressure line and a relief line. Which then brought about the question of whether oil was continuously flowing out of the relief line. Fix the lapse in knowledge, fix the problem. Recurrent training complete :)

EDIT: Well, not quite. It still doesn't answer the question of whether cycling is necessary. It's blocked up in the hub, so there's no movement of oil (in a perfect world). And even if the prop was exercised, would there be enough movement of the oil to keep it from losing its viscosity? Wouldn't the oil that's in there just stay in there (except for maybe the stuff right near the governor), even though the pressure is changing?
 
subpilot said:
Small oil drain holes in the cylinder would make the cylinder inefficient.
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efficiency my not be the goal though. Keeping RPM is though. I have never seen one taken apart, but I'd reason to bet it's similar to the governor in an automatic transmission. There is always fluid moving in a trans governor. I don't see why there wouldn't be in a prop hub.


Edit: Looks like I spoke too soon. Nice find ExcalibeR
 
In my book "Aircraft Systems For Pilots," by Dale De Remer, there are diagrams/cutaways of a prop governor on- speed, overspeed, and under-speed, and at the bottom of the onspeed diagram is the caption "on-speed positoin of the governor flyweights. All flow is blocked."
 
Adrock said:
In my book "Aircraft Systems For Pilots," by Dale De Remer, there are diagrams/cutaways of a prop governor on- speed, overspeed, and under-speed, and at the bottom of the onspeed diagram is the caption "on-speed positoin of the governor flyweights. All flow is blocked."
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The problem with drawings is that you can't always trust them to be completely truthful, because they tend to be accurate on a conceptual level, but not on an engineering level.
 
It stands to my reasoning that there should be drain holes; but if oil constantly cycles, why would exercising the prop be necessary on run up? My curiosity is piqued...
 
No drain line/port. Look at an engine with a prop off sometime. Just a big honkin hole in the crank/output shaft. The gov is constantly moving a little bit so some oil is going in/out.
 
XcalibeR said:
The question stems from me wondering if there was a need to exercise the prop every once in a while in cruise to prevent oil from cooling/congealing in the hub. I suppose it wouldn't hurt anything to do it, but is it necessary?
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What does the POH say?
 
XcalibeR said:
I suppose it wouldn't hurt anything to do it, but is it necessary?
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The only time (that I've been taught) that it is in your advantage to cycle the props is if you enter icing without prop ice protection, or it fails. Prevents buildup etc.
 
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