Seneca Propeller Feathering

shdw said:
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).
Click to expand...
Esa's propeller did not come to rest on the stops. If it had, he would have been unable to feather it with the prop lever. The only thing holding his prop out of feather was the residual high-pressure oil trapped in the undamaged path between the governor pilot valve and propeller hub. That pressure was released when he pulled the prop lever to feather and mechanically unseated the pilot valve, allowing the trapped oil to flow out.

I'd be interested to know how you think the pilot is able to retract any propeller stops using only the prop lever. When you pull on that prop control lever, what do you think the other end of the control cable is connected to? How exactly do you assume this pilot-controlled retraction of the stops takes place, mechanically?
 
Berkut said:
Esa's propeller did not come to rest on the stops. If it had, he would have been unable to feather it with the prop lever.
Click to expand...

You are confusing the feather lock pins with the high pitch and low pitch stops which are each different. As was mentioned earlier every constant speed prop has high pitch and low pitch stops. This has been repeatedly confused throughout this topic. Feather locking pins are a function of RPM and are completely different than the stops for high and low pitch which you test on the ground during run up to ensure they disengage and allow feathering.


Edit: Did you notice in Esa's information provided that he said the prop went to a high pitch with the RPM just above the RPM that the feather locking pins would engage? I can only speculate that the designed high pitch stop will, at Vmc or above, yield an RPM just above the feather locking pins specifically to avoid an inability to feather. Again this is only a speculation, but it would seem a necessary design.
 
shdw said:
You are confusing the feather lock pins with the high pitch and low pitch stops which are each different. As was mentioned earlier every constant speed prop has high pitch and low pitch stops. This has been repeatedly confused throughout this topic. Feather locking pins are a function of RPM and are completely different than the stops for high and low pitch which you test on the ground during run up to ensure they disengage and allow feathering.
Click to expand...
:banghead:
 
Berkut said:
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.
Click to expand...

Excellent writeup.
 
Question posed to the hartzell engineers, I will get back to you when I have a verdict from them.
 
Berkut said:
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.
Click to expand...

If the oil transfer system was tight, would the pressure be maintained? Would it stay at the maximum high pitch allowed by the governor instead of feathering?

One other question: is the governor boost pump engine driven or operated solely by high pressure oil from the regular oil pump?
 
matt152 said:
From the Hartzell website: Conventional twin-engine airplanes need propellers having the ability to feather (where the blades arealigned parallel with the direction of flight) when there is a loss of power or a loss of oil pressure.
Click to expand...


Listen bud, you have made up your mind a long time ago so you will read everything to fit your opinion. I on the other hand am still in the air as I don't take your word as the be all end all. I was taught differently and in practice it has proved differently.


For instance you read this: "If oil pressure is lost during operations, the propeller will feather. Feathering occurs because the air charge, spring, and blade counterweights are no longer opposed by hydraulic oil pressure. The air charge, spring and blade counterweights are then free to increase blade pitch to the feathering (high pitch) stop."

All you see is the part I bolded in the beginning. What I read is the part bolded contradicting the part underlined at the end. Notice it says "feathering" not "feathered" high pitch stops, splitting hairs, but contradicting non the less especially given the real world scenario.


For instance number two you read this: "From the Hartzell website: Conventional twin-engine airplanes need propellers having the ability to feather (where the blades arealigned parallel with the direction of flight) when there is a loss of power or a loss of oil pressure."

Again, since you have your mind made up you read that bold, it will feather with a loss of oil pressure. Me on the other hand, I again add in the underlined section and recognize it only says ability to, not that it will feather on its own.


You continue to argue here as though you built the system based on what you read from what an engineer sitting in a lounge chair wrote. You have put all your marbles in one basket and into one source which, itself, contradicts other (the POH/experience/my college training) sources.


Could we all be wrong, sure there is no doubt we could be and maybe esa's system was just plain broken or a different system than many here are familiar with. But you need to come to the realization that you too could be wrong and your one source might be a 20 year old misprint that nobody bothered to correct.

That being said, I will wait to hear back from the engineers and dig to the source of the problem instead of speculate based on one reading. I do hope you don't take this as a personal jab at you, I was much the same way months back coming onto this forum thinking in a world of absolutes. I still find myself with the same issues as I am sure many members here can attest to.

However, as I did, you must realize that in chasing down the truth it's often not as simple as picking up one book and reading the answer. It also must be realized that if you make up your mind early, your opinion from that point on will be biased, like it or not.
 
shdw said:
You continue to argue here as though you built the system based on what you read from what an engineer sitting in a lounge chair wrote.
Click to expand...

You emailed the engineer. When he responds from his lounge chair, please post the response.
 
Honestly guys, I'm not trying to be a dick about this. My reading has given me one impression of the way the system operates.

But if it is different than what I have been led to believe, only good come from clearing it up.
 
Next time I get my hands on a run-out multi hub I'll mail it to you and let you figure it out.

The lesson here is books are MERELY a guide, not a bible.
 
The search for the truth continues, here is what I have from hartzell, two parts are important:

For explaining the stops:

The Hartzell compact series feathering propeller has three mechanical pitch stops - low, high and feather. The low and feather stops are hard limits that the propeller cannot exceed. The high pitch stop is
actuated by centrifugal force and can only be engaged under certain
conditions.
Click to expand...

To answer partially the question:

The counterweight is designed such that its centrifugal force overcomes the blade's tendency toward flat pitch, so that the blade/counterweight assembly wants to rotate toward the feathered position. Governors used with feathering props supply oil to decrease pitch, so the propeller will feather if oil pressure is lost. During the feathering process, the RPM decay results in less centrifugal force, so feathering props also have a spring to complete the feathering process and hold the propeller in that position.
Click to expand...

Since loss of centrifugal force occurs as the prop starts to feather and the spring is needed to complete the process that likely explains why the prop didn't completely feather in esa's case. I would assume this means the prop wouldn't completely feather in certain cases/conditions where pressure is lost which leaves me confused as to the bold statement made.

However, at the end of the e-mail I was instructed to contact McCauley as, "Certain models of the Seneca use our governor with a McCauley propeller." So I will be shooting them an e-mail later on today and will let you know.
 
shdw said:
For explaining the stops:
Click to expand...
Looks like I wasn't the one confused about the stops after all.
However, at the end of the e-mail I was instructed to contact McCauley as, "Certain models of the Seneca use our governor with a McCauley propeller." So I will be shooting them an e-mail later on today and will let you know.
Click to expand...
Read this while you're waiting:
http://www.mccauley.textron.com/prop/prop-tech/pg04feather.html
Matt152 said:
If the oil transfer system was tight, would the pressure be maintained? Would it stay at the maximum high pitch allowed by the governor instead of feathering?
Click to expand...
In a perfect, leak-free path between the governor and prop, yes. If a perfect system maintained residual pressure in that path, the propeller would not feather until the pilot manually released the trapped oil by moving the lever to the feather position. Real world systems might be good enough that it would take many minutes or even hours to leak down. A really leaky system that would bleed down enough to allow feather in less than a minute would have probably already been written up for RPM control problems.
One other question: is the governor boost pump engine driven or operated solely by high pressure oil from the regular oil pump?
Click to expand...
The governor boost pump is engine driven by the same shaft on which the flyweights are mounted.
 
Berkut said:
Looks like I wasn't the one confused about the stops after all.Read this while you're waiting:
Click to expand...

I hope you feel better that you got to point that out, I am sure nobody here noticed that till you did. :)

Thanks for the link bud.

Esa's case was about as leaky an oil loss will ever get as I think you would both agree. I am shooting that e-mail over now and will get back to you. But I have a question in the mean time, if residual pressure in his occurrence prevented full feather, what must happen for there to not be any pressure and actually have the prop feather on its own?
 
shdw said:
Esa's case was about as leaky an oil loss will ever get as I think you would both agree.
Click to expand...
Yes, but he didn't lose it from the area that keeps the prop from feathering. Think of the wording as reading, "The propeller will feather on a loss of engine oil pressure to the propeller." The rest of the engine can lose all its oil, but as long as oil is trapped in the propeller it will not feather.
But I have a question in the mean time, if residual pressure in his occurrence prevented full feather, what must happen for there to not be any pressure and actually have the prop feather on its own?
Click to expand...
If you're asking what would cause it to immediately feather on its own, the answer is a failure of the propeller, the governor, or the path between the two that results in a loss of pressure to the propeller. Even if the cylinders blew off and the sump fell from the engine, the prop will not feather until the path between the prop and the governor loses oil pressure.

shdw said:
I hope you feel better that you got to point that out, I am sure nobody here noticed that till you did.
Click to expand...
I apologize, I wasn't trying to be a jerk but I see how it looks that way.
 
Berkut said:
If you're asking what would cause it to immediately feather on its own, the answer is a failure of the propeller, the governor, or the path between the two. Even if the cylinders blew off and the sump fell from the engine, the prop will not feather until the path between the prop and the governor loses oil pressure.
Click to expand...

Dammit why didn't you say that 400 replies ago!!! Well that clears up everything in this entire thread and every argument. A loss of oil pressure will not result in the engine feathering unless that loss occurs from the prop governor itself.

So essentially, of the probably 100 different ways you can loose oil pressure (thus having the engine fail) only one way will result in the prop actually feathering in any reasonable amount of time. The other 99 ways would eventually feather, but take so long that it isn't practical to wait around when you can just do it yourself. Quite an evasive answer to this problem but makes perfect sense. It also explains the need for an auto feather system.



Also I got a reply back trying to clear up what that spring was for, it is not controlled in any way by the pilot. As has been said the pilot only relieves pressure from the system. The spring is in fact always in play and is pre-loaded into the system during assembly to ensure it has sufficient force. Its purpose is to complete the final step of the feather process of the, "now non-rotating propeller."

To understand the need of the spring you need to understand that a spinning prop has a component of centrifugal force that tries to twist the prop to a flatter pitch. This force is what the counter weights are designed to overcome. Once RPM is gone the counterweights no longer have any effect, nor does the centrifugal force that was fighting those weights, but the prop still isn't completely feather. This is where the spring comes in and completes the process.


If McCauley gives me any new information I will post it, other than that I am pretty sure this answers all the questions. Thanks for shinning the light on that important area we all missed berkut, and no worries on the comment, we are all here to learn.
 
You must log in or register to reply here.
Back
Top