Prop Governor

Fly764

Well-Known Member
I know this might be hard since it is such a complex topic, but can anyone, explain how the prop governor works on a light twin in the simplest yet most thourough fashion. Perhaps in a way you would present it to a new student. Thanks.
 
There are a couple different ways that blade angle is accomplished but the most basic answer is that oil pressure is applied to a hydraulic piston and pushes the blade towards high or low pitch depending on the specific system used.

Some systems use nitrogen pressure to push the blade towards low pitch and oil pressure acts against the nitrogen to push towards high pitch. The other common way is that counterweights on the prop shaft and twisting moment push the blades towards low pitch and the oil pressure pushes towards high. So as you can see it is airplane specific. Loss of oil pressure will affect the systems in different ways.

I can describe this much better with a white board and markers though. :D

I hope that helps somewhat. If tgrayson is still around here, he is a friggin whiz at all things CFI related.
 
pp16e.gif


Definitions:

Fly weights: The two weights on the image that are near the top painted in red, on the left bend inward and on the right bent outward. These weights are attached to a spinning shaft which is used the primary way an under or overspeeding situation is recognized.

Pilot Valve: The valve shown in the picture that is directing oil into or out of the system. This valve is attached to the speeder fly weights and therefore moves up or down based on the fly weights.

Speeder Spring: The spring on the top of the picture, adjusted by the pilot through the prop lever. The tension of that spring is what the pilot is setting.



Multis Versus Singles

Think about it logically, what would you want to happen if you lost oil pressure in, for instance in the event of an engine failure. If it was in a single engine you would like the prop to keep windmilling giving you a chance to restart it. In a multi engine you have another engine running so going to feather would be the optimal result.

For the above reason in a single engine aircraft oil into the system is used to increase the blade angle so in the event of oil pressure loss the blades go to the flattest pitch. On the other hand in a multi oil into the system puts the blades to a flatter pitch so in the event of a loss in oil pressure the blades go to feather.



Scenarios

Here is what will happen in an overspeed situation: the flyweights will go outward due to excess centrifugal force, the pilot valve will be lifted, and in a multi engine the oil will be allowed to flow out of the system. The result will be the blades pitch increasing preventing the RPM from going up and the pilot valve and flyweights returning to their neutral position.

In an under-speed situation the flyweights fall inward, the pilot valve is lowered, and in a multi the oil will flow into the system. The result is the blade pitch decreasing, keeping the RPM from decreasing and again the pilot valve and flyweights return to their neutral position.



Feather Lock

I am unfamiliar with other multis asside from the Seminole as that is what I flew, so this description will be copied from their manual:

A feather lock, operated by centrifugal force, prevents feathering during engine shutdown by making it impossible to feather any time the engine speed falls below 950 RPM. For this reason, when airborne, and the pilot wishes to feather a propeller to save an engine, he must be sure to move the propeller control into the feather position before the engine speed drops below 950 RPM.



Unfeathering

This can be done through a variety of systems. The two I am familiar with is stored oil pressure and nitrogen systems. Both systems act in the same manner, when the pilot pushes the prop out of feather the stored oil pressure or nitrogen is used to take the prop out of feather for restart.
 
Unfeathering

This can be done through a variety of systems. The two I am familiar with is stored oil pressure and nitrogen systems. Both systems act in the same manner, when the pilot pushes the prop out of feather the stored oil pressure or nitrogen is used to take the prop out of feather for restart.


IIRC, the stored nitrogen/oil is all one system stored in an unfeathering accumulator. The cyl fills with oil when things are normal(2qts I think), and then when you shut down/feather the engine, a valve closes off the cyl. When you decide to un-feather the engine, the valve opens and the nitrogen forces the oil back into the prop hub and un-feathers the engine.
 
IIRC, the stored nitrogen/oil

Not in the seminole, " The accumulators store engine oil under pressure from the governors." This may not be the case in other aircraft, but the seminole only uses stored oil. The nitrogen system I was referring to was only discussed in class for me almost 3 years ago now, so I don't recall its operation. From what I remember it was used in the same manner except with nitrogen and not engine oil. Is the manual just leaving certain things out on this? It wouldn't surprise me since our POH is often far less accurate than the individual systems manuals for simplicity purposes, like they do with the electrical system sometimes not giving all volt/amp information.



Also I missed this before in my description, "Pitch is controlled by oil and nitrogen pressure. Oil pressure sends a propeller toward the higher RPM or unfeather position; nitrogen pressure and a large spring sends the propeller toward low RPM." Not that the understanding of what gas is used has any bearing on the operation of the the governor system. It is the same as caring about what paint is used on your propeller in my mind.
 
Not in the seminole, " The accumulators store engine oil under pressure from the governors." This may not be the case in other aircraft, but the seminole only uses stored oil. The nitrogen system I was referring to was only discussed in class for me almost 3 years ago now, so I don't recall its operation. From what I remember it was used in the same manner except with nitrogen and not engine oil. Is the manual just leaving certain things out on this? It wouldn't surprise me since our POH is often far less accurate than the individual systems manuals for simplicity purposes, like they do with the electrical system sometimes not giving all volt/amp information.



Also I missed this before in my description, "Pitch is controlled by oil and nitrogen pressure. Oil pressure sends a propeller toward the higher RPM or unfeather position; nitrogen pressure and a large spring sends the propeller toward low RPM." Not that the understanding of what gas is used has any bearing on the operation of the the governor system. It is the same as caring about what paint is used on your propeller in my mind.


You are 100% right about the hub haveing nitrogen and oil/nitro fighting each other to control pitch (nitro on one side of the hub, oil on the other). But how does the oil get out of the acumulator? Like I said, I think it's nitrogen that forces it out. I'll dig through my stuff and see if my jello brain is just jigglin.

BTW, I think that Piper uses the same acumulators for all their engines, but it may be jigglin again.

Edit: I can't find my seminole supplement, but I think the pressure in the acumulator was 60psi. Same as the hub.

Edit;Edit: How does the oil come out of the acumulator? Magic? Woulden't that be cool.
 
How does the oil come out of the acumulator? Magic? Woulden't that be cool.

No dummy you have to do a snap roll! But interesting point I look forward to seeing that information you're looking for, or someone else jumping in to unjiggle your brain and enlighten me.
 
No dummy you have to do a snap roll! But interesting point I look forward to seeing that information you're looking for, or someone else jumping in to unjiggle your brain and enlighten me.


Try it and take a video:crazy:. Here is a link from Mcaully (I think PA44's use hartzell, but they both steal technology from each other). See the lower part of the page.

http://www.mccauley.textron.com/prop/prop-tech/pg04feather.html

"The unfeathering accumulator option permits a feathered propeller to be unfeathered in flight for air-starting the engine. With this option, the governor is modified to provide an external high-pressure oil outlet through a check valve, as well as a device for unseating the check valve. The external outlet is connected to an accumulator. One side of the accumulator is filled with compressed nitrogen and the other side with oil. This allows the oil to be stored under high pressure, as it is during normal flight. (Fig. 13) When the propeller is feathered, the check valve maintains oil pressure in the accumulator. (Fig. 14) When the propeller control is moved from feather to low pitch, the check valve is unseated, permitting the high-pressure oil in the accumulator to flow to the governor pilot valve. With the governor control lever and shaft in low pitch, the speeder spring forces the pilot valve down so that the oil flows to the propeller and moves the blades to low pitch. (Fig. 15) "
 
LOL pretty sure he said new student. It got way technical.

As I said it is airplane specific. Oil pressure on one side and something opposing it on the other. Whether its Nitrogen, Oil pressure, Couterweights, or magic the basic concept remains the same.

Hartzell props on the twin comanche use the nitrogen in front, oil pressure in back method, centrifugal locks stop the prop from feathering when you shut down. The figure is much lower that the 950rpm mentioned on the Seminole. If memory serves me correctly it was more like 500rpm, but same concept. Loss of Nitrogen pressure will result in an overspeed condition and low oil pressure will result in underspeed.
 
Ok you want quick and dirty? This is NON airplane specific, and probably Single engine specific. Haven't looked at a multi's systems since Bush's first term.

On speed, over speed, and under speed. On speed the flyweights are neutral, and there is no change in oil pressure in the hub, thus the prop blades don't change angle.

Over speed the prop is spinning so fast centripetal force overcomes the force of a spring pushing down on the flyweights and they separate allowing oil pressure to decrease pitch of the blades.

Under speed, the centripetal force is NOT enough to overcome the tension of the spring and the flyweights are close together, allowing oil pressure to increase the blade angle.

When you move the prop control lever all you are doing is controlling the tension of that spring pushing on the flyweights.

How was that? basic enough?

I tried to make that as generic as I could. not single or multi specific.
 
LOL pretty sure he said new student. It got way technical.

There is no such thing if he is going for his complex sign off he is at least through his private. :)

If the man can't handle the concept of 3 devices working in conjunction with each other than complex training isn't for him in my opinion.
 
There is no such thing if he is going for his complex sign off he is at least through his private. :)

If the man can't handle the concept of 3 devices working in conjunction with each other than complex training isn't for him in my opinion.

All complex equipment works by PFM, that's all I know. :D
 
but you NEED to know stone cold how all those complex things work...otherwise you won't have any clue about all the things you can do with that blue, little lever.

Doesn't matter what you know or say, someone else will always disagree with or expand on or condemn what you say anyways. I taught the material enough and have a high enough pass rate that I am pretty confident I know what I know. Like I said its all airplane specific so get out the POH and Maintenance manual and go crazy.
 
Doesn't matter what you know or say, someone else will always disagree with or expand on or condemn what you say anyways. I taught the material enough and have a high enough pass rate that I am pretty confident I know what I know. Like I said its all airplane specific so get out the POH and Maintenance manual and go crazy.
lol sorry i forgot the :sarcasm: tag. Everything you just said I COMPLETELY agree with. IN fact i've even used a similar line myself. My students know what they know (that I taught them) and so far i've even had a few positive comments from the FSDO.

I also use PFM a bit myself.
 
There is no such thing if he is going for his complex sign off he is at least through his private. :)


Ohhh, carefull with that. I worked for a guy when I was a wrench that got his PPL in a V35(granted he was the exception to the rule).
 
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