Seneca Propeller Feathering
- Thread starter bLizZuE
- Start date
bLizZuE
Calling for engine starts en français
We can't quite mash all these different systems into one clump either. I don't know what system the 310 uses.
Seneca POH, "Feathering Procedure":
Loss of centrifugal force due to slowing RPM will actuate a stop pin that keeps the propeller from feathering each time the engine is stopped on the ground.
Obviously in the real world, we would all pull the prop back to feather.
Certainly it will seize eventually. But I am willing to bet that it will run with zero oil pressure for, say 30 seconds before seizing. Check out youtube for Honda engines. People drain the oil and fill the crankcase with water. The things run for minutes before seizing.
No, I don't.
The prop will feather will a complete loss of oil pressure. What else is keeping it from feathering that is described by the POH?
That's the first thing that stands out at me.
Like I've said before in this thread...I'll defer to the guy that actually lost oil pressure in a piston twin, didn't get the prop to automatically feather and survived to tell us about it.
You can continue to make up your own mind and believe the engineers on the subject.
-mini
That's the first thing that stands out at me.
Are you really suggesting the engine knows whether it's in the air or on the ground? Really?
It's not just the engineers. This is how a full-feathering system SHOULD operate. Would you really want a propeller that could NOT be feathered because there was a failure of a mechanical stop? It just does not make any sense.
Maybe esa17' nitro charge was low or something.
bLizZuE
Calling for engine starts en français
I can appreciate the differences between the way it 'should' work and the way it 'does' work. However, how should I teach it?
I don't think there is anyone here that would risk the prop NOT feathering by letting the engine lose oil pressure and say 'Watch this, it will feather all by itself...' Anyone would pull the propeller lever to the feather position.
As far as I'm concerned the system is designed to feather automagically when it loses oil pressure and the RPM is above 800-950 at the time of the oil loss. Based on the experience of others perhaps that won't always be the case. However I have to at least pretend that is what should, and will happen.
I don't think there is anyone here that would risk the prop NOT feathering by letting the engine lose oil pressure and say 'Watch this, it will feather all by itself...' Anyone would pull the propeller lever to the feather position.
As far as I'm concerned the system is designed to feather automagically when it loses oil pressure and the RPM is above 800-950 at the time of the oil loss. Based on the experience of others perhaps that won't always be the case. However I have to at least pretend that is what should, and will happen.
bLizZuE
Calling for engine starts en français
That's the first thing that stands out at me.
Like I've said before in this thread...I'll defer to the guy that actually lost oil pressure in a piston twin, didn't get the prop to automatically feather and survived to tell us about it.
You can continue to make up your own mind and believe the engineers on the subject.
-mini
The only reason a propeller system uses anti feathering pins is to prevent unneccessary feathering during engine shut downs. They are calibrated to be at a very low idle RPM, such an RPM would never be achieved in flight due to induced airflow over the propeller.
Yes. In some ways we are talking academically. It should feather as a result of complete loss of oil pressure. In the real world, we would all "hurry it along" and pull the prop back to feather if we saw oil coating the nacelle.
The problem is, there is no way this can be simulated without an actual engine failure by loss of oil pressure.
Some day when I win mega millions, I'll pay to have someone pop the plug and let the oil out of an engine just to see what happens.
...I'll be comfortably sitting in my lawn chair reading the POH and Hartzell manual telling me what should happen.
We can compare notes then.
How should you teach it? I like real world scenarios.
"Mr. Student pilot...bla bla bla prop bla bla bla governor bla bla bla oil pressure bla bla bla feather. But, here's something to watch out for..."
-mini
...I'll be comfortably sitting in my lawn chair reading the POH and Hartzell manual telling me what should happen.
We can compare notes then.
How should you teach it? I like real world scenarios.
"Mr. Student pilot...bla bla bla prop bla bla bla governor bla bla bla oil pressure bla bla bla feather. But, here's something to watch out for..."
-mini
System knowledge is important. The system should work as designed and described in the POH. This is what should be taught.
But I agree that real world application is certainly more important. This should also be taught.
Agreed! How about a beer to the winner? :beer:
Berkut
Well-Known Member
I think the easiest way to understand how propeller control works is by thinking of it as two separate but related systems:
1) Normal pressure oil, which is what the engine's "main" oil pump supplies, and is what you read on the oil pressure gauge.
2) High pressure oil, which is what the governor supplies to the propeller by means of a small internal boost pump. Most people aren't aware that this boost pump even exists. The propeller is actuated by this high pressure oil, not the engine's main oil pump.
The engine's main oil pump draws oil from the sump and supplies it to the prop governor at normal engine oil pressure. The prop governor then uses its small boost pump to boost the pressure to 180-300psi. This high-pressure oil flows through the governor's pilot valve, then through an oil transfer system, and on through the hollow crankshaft into the propeller.
If you have a loss of engine oil pressure, the governor's boost pump will no longer have its supply of oil and will be unable to maintain the pressure the prop needs to maintain a fine pitch. The prop will start moving towards feather, but won't quite make it there yet because there is still some residual pressure trapped in the oil transfer system between the propeller and the pilot valve. At this point, two things can happen:
A) If there are any small leaks in the oil transfer system (and there probably are) then the pressure will gradually bleed off and there will be nothing left to oppose the nitrogen/spring pressure, so the prop will very slowly feather on its own. This will probably take more time to bleed off than you have, so most people will choose option B...
B) ...which means they will pull the prop lever back to the feather position. This mechanically opens the pilot valve and vents off all the residual high-pressure oil, allowing the propeller to snap immediately to the feather position.
Unfeathering accumulators are connected in the oil path between the pilot valve and the propeller. The accumulator is charged with oil during normal operation by the boost pump. When the prop lever is moved out of feather, it seats the pilot valve and opens the valve from the accumulator, allowing the stored oil to partially repressurize that path enough to take it out of feather.
1) Normal pressure oil, which is what the engine's "main" oil pump supplies, and is what you read on the oil pressure gauge.
2) High pressure oil, which is what the governor supplies to the propeller by means of a small internal boost pump. Most people aren't aware that this boost pump even exists. The propeller is actuated by this high pressure oil, not the engine's main oil pump.
The engine's main oil pump draws oil from the sump and supplies it to the prop governor at normal engine oil pressure. The prop governor then uses its small boost pump to boost the pressure to 180-300psi. This high-pressure oil flows through the governor's pilot valve, then through an oil transfer system, and on through the hollow crankshaft into the propeller.
If you have a loss of engine oil pressure, the governor's boost pump will no longer have its supply of oil and will be unable to maintain the pressure the prop needs to maintain a fine pitch. The prop will start moving towards feather, but won't quite make it there yet because there is still some residual pressure trapped in the oil transfer system between the propeller and the pilot valve. At this point, two things can happen:
A) If there are any small leaks in the oil transfer system (and there probably are) then the pressure will gradually bleed off and there will be nothing left to oppose the nitrogen/spring pressure, so the prop will very slowly feather on its own. This will probably take more time to bleed off than you have, so most people will choose option B...
B) ...which means they will pull the prop lever back to the feather position. This mechanically opens the pilot valve and vents off all the residual high-pressure oil, allowing the propeller to snap immediately to the feather position.
Unfeathering accumulators are connected in the oil path between the pilot valve and the propeller. The accumulator is charged with oil during normal operation by the boost pump. When the prop lever is moved out of feather, it seats the pilot valve and opens the valve from the accumulator, allowing the stored oil to partially repressurize that path enough to take it out of feather.
I don't drink....and I'm too cheap to buy.
-mini
You can afford it if you win mega millions.
As for the rest here I have finally taken the time to read through this painstakingly long thread and have but one question and two comment.
Question: Esa, how long did you sit and wait to see if the propeller would feather itself?
Comment: As for prop windmilling versus not versus feather drag, I have done it in a seminole by shutting down one engine, idling the other (at 8000 feet or so) and slowing till the prop stopped turning. We then increased speed specifically and throttled the live engine to test this, I can say that the difference between stopped and feather is maybe a couple pounds of rudder pressure. The disk theory seems to stand true in my experience.
Comment 2: I wouldn't want my students to rely on a system that has been shown here to maybe not work. I would explain that the system may be designed to do this but in practical operations it may not and tell them to stay on their toes for any engine failure. I wouldn't want them confirming oil pressure loss and putsing around fat dumb and happy to Vmc roll over when it didn't feather.
esa17
Well-Known Member
If I had to guess, I'd say less than a minute. When the feather was commanded there was a violent yaw as the prop stops were released. Think of all the times you've feathered one during a training flight, always with oil pressure behind the prop. The feathering there is smooth and not at all instantaneous. Without the oil there it was shocking how violent the yawing was when those props feathered. Once they feathered that prop stopped in two blades.
The single most impressive thing of the whole event was feeling that prop slam back once those stops released. A close second is the glory of that beer as tall as I was after we landed.
Sounds like it was more than enough time for it to feather on its own if it was going to then. So as with mini I will differ to the gentlemen with the actual experience. It is too easy to make a simple grammatical error when writing up a manual and would seem pointless to have auto feather systems if all these systems auto feathered already. Furthermore we were taught at my college that it would go towards feather and stop at the high pitch stops just as occurred in this scenario (piper seminole). Thanks for the pictures and such esa, that must have been a fun day.