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  #21  
Old 12-01-2022, 12:53 PM
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rocketbob rocketbob is offline
 
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While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

An alternative would be some sort of brake fabricated to work against the flywheel to stop the prop which would likely further improve glide characteristics.

Seeing stopped vs. windmilling prop glide data would be interesting to see.
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  #22  
Old 12-01-2022, 01:42 PM
scsmith scsmith is offline
 
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Quote:
Originally Posted by rocketbob View Post
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

An alternative would be some sort of brake fabricated to work against the flywheel to stop the prop which would likely further improve glide characteristics.

Seeing stopped vs. windmilling prop glide data would be interesting to see.
BillL said that it only takes 30 psi to push the prop to coarse pitch against the spring when no aerodynamic forces on it. So at coarse pitch, so I think the evidence and practical experience is that the windmilling prop can turn the engine fast enough to generate enough oil pressure to maintain coarse pitch (assuming the governor will control to that low an RPM at all)

Kevin Horton published some prop-stopped glide performance numbers years ago. I think he was planning a flight to a remote island, and wanted to figure out how high he would have to be to be able to glide half way across (or turn around and glide back). IIRC, he found that he essentially had to stall to stop the prop, and the altitude lost in that maneuver was worse than the saving in the glide, unless you were really high and were going to glide for a loooong way.
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  #23  
Old 12-01-2022, 01:51 PM
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Quote:
Originally Posted by scsmith View Post
BillL said that it only takes 30 psi to push the prop to coarse pitch against the spring when no aerodynamic forces on it. So at coarse pitch, so I think the evidence and practical experience is that the windmilling prop can turn the engine fast enough to generate enough oil pressure to maintain coarse pitch (assuming the governor will control to that low an RPM at all)
So if catastrophic engine failure occurs where no rotation is possible how would one get 30psi?

Windmilling is the problem...at higher RPMs there is more far induced drag from the thrust the windmilling prop is producing. Thats why the prop needs to be stopped. Feathering a blade is one way to stop it, a brake is another. Feathering obviously has a bit less drag.
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  #24  
Old 12-01-2022, 02:16 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by rocketbob View Post
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.
Quite a few years ago my 182 partner did this experiment. He really wanted to see how hard it was to stop the prop. He started with a power at idle-prop in coarse pitch glide. Then he pulled the mixture. The prop stayed in coarse pitch.
(He also found, as others have, that it was hard to stop the prop without stalling the plane. But, once stopped, it stayed stopped.)
Obviously things may be different in an actual engine out, where something might be broken, or there is no oil left, etc.
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  #25  
Old 12-01-2022, 02:41 PM
BobTurner BobTurner is offline
 
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Originally Posted by lr172 View Post
We can agree to disagree on that. If your hypothesis is correct, why does it take 150 - 200 amps (1800-2400 watts) to get one of these engines spinning at 200 RPM? In my experience, idle vs WOT makes very little difference in the power required to get them spinning. Cylinders are made to hold pressure at high RPMs. At very low RPMs, they leak like a sieve. I can drain my pancake compressor in a minute or two when doing leak down tests.

Not an engineer, so won't debate the science behind it. Only working off real world experience over the years. My belief is that the majority of the pressure created on the upstroke is consumed by bleeding out of the gaps and therefore very little energy left to push the piston back down.
If you carefully watch your ammeter during start, I think you’ll see that it draws much more current during the first 180 deg of rotation (and is much more likely to ‘hang’) than it does once the prop is spinning. That’s because there’s no other cylinder already full of compressed air on its ‘power stroke’. Yes, at very low RPM the losses past the rings are noticeable. But at 800 rpm or more, they’re quite small. If this was not the case, any prolonged idling would result in your oil becoming very thin - from all the raw gas pushed out on the compression stroke.
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  #26  
Old 12-01-2022, 04:12 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by rocketbob View Post
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.
That aligns with the results of testing we did at Van's with engines fully shut down but the prop. windmilling.
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  #27  
Old 12-01-2022, 04:31 PM
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Dan 57 Dan 57 is offline
 
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Quote:
things may be different in an actual engine out, where something might be broken, or there is no oil left, etc.
Yep, and nothing beats real life experience...
January 3rd, 2008, con rod failure on my Falco #1. An exciting ROD with a windmilling prop... luckily my guardian angels were working overtime, thank you all



Now regularly practising the "impossible turn" with my 3 bladed MT demonstrates a net worth in reduction in ROD if the blue lever is pulled ASAP. Oil P does help indeed.
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Last edited by Dan 57 : 12-01-2022 at 04:35 PM.
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  #28  
Old 12-01-2022, 11:55 PM
Finley Atherton Finley Atherton is offline
 
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Quote:
Originally Posted by rocketbob View Post
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.
Fourteen years ago I tested this with a dead engine and the prop would go coarse just like if the engine was idling. Hartzell, MT governor, O-320.

At 70 kts with the mixture at idle cut off (dead engine) and the prop control full out (coarse):
Oil pressure 50 psi
RPM 900

Fin 9A.
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  #29  
Old 12-02-2022, 10:27 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by rocketbob View Post
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

An alternative would be some sort of brake fabricated to work against the flywheel to stop the prop which would likely further improve glide characteristics.

Seeing stopped vs. windmilling prop glide data would be interesting to see.
From my research, it is the speeder spring that is the big issue and not oil pressure. Once the engine RPM gets below a certain level (around 1600-1800 on a Hartzel S series) there is not enough energy to move the spring that controls the flow of oil out to the prop. Not surprising that different governor models have different designs and therefore different minimum control levels. I suspect feathering governors have a very different design, but not sure.
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Last edited by lr172 : 12-02-2022 at 10:29 AM.
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  #30  
Old 12-02-2022, 07:55 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by lr172 View Post
I suspect feathering governors have a very different design, but not sure.
Most feathering props are designed to work ‘backwards’, e.g., no oil pressure moves them to full feather.
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