flyboyfreye

Hi all,

So here it is. I'm instructing in a Cirrus, getting a bunch of students ready to go on checkrides, and the more studying I'M doing, the more I'm confusing myself with the governor system. I have read through McAuley's document on constant speed vs. full feathering systems (Product Support) and I'm still a bit miffed.

I guess I was under the impression that in the CIRRUS ( a constant speed system ), when you pushed the throttle forward or began a descent, this was an overspeed. According to Section 7 of the SR-20 manual, it says "moving the throttle lever forward causes the governor to meter less high pressure oil to the hub allowing centrifugal force to lower the pitch." To me, this says that in an overspeed, oil is flowing out of the hub and into the sump. And that "reducing the power causes the governor to meter more high pressure oil to the prop hub, forcing the blades to a higher pitch, lower RPM position." So it would seem that in an underspeed, oil is flowing to the hub.

McAuley's document, however, says that during an overspeed, again caused by increasing engine power or starting a descent, the flyweights move out and raise the pilot valves causing "to the prop in a constant speed system."

help.

shdw

Are you positive you quoted this by reading it and typing it right from the manual? The reason I ask is the situation you have given doesn't make sense at all.

Moving the throttle lever forward would do what in a fixed pitch aircraft? It would increase the RPM, an overspeed situation. To correct for an overspeed situation what would the pitch of the blades have to do? Recall the blades are nothing more than wings. To avoid over speeding the blades would increase their pitch (AOA) which would increase their drag just as a wings drag goes up with an increased AOA.

Moving the throttle lever backward (retarding the throttle) in a fixed pitch would decrease the RPM, an under speed situation. To correct for an under speed the blades pitch (AOA) would be reduced to reduce drag and keep from slowing down.


As for oil into and out of the system here let me go get a post I made on another forum and copy and paste. Sorry it was in regards to multi engines but I clarified the difference in red for you between the two and discussed overspeed versus under speed, here you go:


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. Into the system in a single. 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. Out of the system in a single. 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:

Quote:
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 or a combination of the two. Each system acts 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.

flyboyfreye

thanks for the reply...but that's just the thing. I have the POH right here and I've quoted it exactly. Now I did find the following yesterday which seemed to suggest that there are different kinds of controllable pitch props.

OVERSPEED
"Counterweighted Propeller using Pressure to Decrease Pitch (see Fig. 2)
The flyweight toes raise the pilot valve plunger, uncovering ports in the driver gear shaft that permit pressure oil to flow from the propeller pitch changing mechanism. This allows propeller counterweights to take the propeller blades towards a higher pitch. The load on the engine is increased and engine speed is reduced."

UNDERSPEED
Counterweighted Propeller using Pressure to Decrease Pitch (see Fig. 2)
The pilot valve plunger is forced down uncovering the ports in the drive gear shaft that allow pressure oil to flow to the pitch changing mechanism of the propeller. This overcomes the force of the propeller counterweights and decreases the pitch of the propeller blade.

This scenario would seem to match the system as described in the cirrus, but no where do I see anything about it being a "counterweighted prop".

joepilot

I know nothing about your particular airplane, however I do know that manuals are subject to errors. Frequently the engineer that first wrote the manual had his grammar "corrected" by an English major who did not know anything about airplanes.

Joe

shdw

Now those at least make a little more sense...only a little. Here I will show you what I mean with a black bold in this quote and then a requote of your original. For overspeed I did just bold/black and underspeed in bold/black/underline.

Original:

You see in your original the reverse was happening with regards to blade pitch according to your quotes. That is what was confusing, though after reading your book quote... just wow lol.

As for your counterweights I am unfamiliar with any propeller system using separate counterweights. However, I have heard the flyweights called counterweights in a prop governor system. The rest of what is quoted seems controversial, even contradicting itself, and what I would recommend is going and asking your mechanic for the type of prop governor used. If you can post up the brand/type of prop governor here than we can try and track down a manual and get to the bottom of this.

detpilot

Are you confusing yourself on the linkage between the power lever and propeller lever?

flyboyfreye

Brandon whats up man,

help me out here. If you look in section 7 of the Cirrus operator's handbook where it talks about oil flow during and underspeed and an overspeed, does it NOT say that oil flows TO the hub during and underspeed. Read it for yourself, I know you have it. So what's going on here? Is the Cirrus backwards of other constant speed systems? See, in other constant speed systems, it looks as though the oil moves "in the same direction as your hand" so to speak during over and underspeeds as caused by throttle movement.

joepilot

I think what is throwing a lot of us old pilots is that the Cirrus has a single lever power control, no seperate prop control. The computer decides what it wants to have happen. This does not necessarily have anything to do with what most of us are familiar with.

Joe

ZnCrO4

What is giving you trouble understanding prop theory is the way the system works in the cirrus.

Bear with me here as I have been out of Flight Instruction nearly two years but I was a check airmen at WMU.

In a three lever constant speed system, i.e. PA-28R-201 or BE-36(Throttle, Prop Mixture) the McCauley document is correct. Pushing the throttle forward instantaneously causes an over speed condition where the pilot valve will allow oil to flow to the prop hub. Thus causing the blades to take a bigger bite of air and "slowing" the prop to the on-speed condition set by the speeder spring. Where as pulling the throttle back would do the opposite. In this case adding power would be the same as pushing the nose over and beginning a descent and a climb would be like reducing power. Now if you were to move the prop lever forward it would cause an under speed condition, causing oil to flow from the hub and making the prop take a smaller bite and thus “speeding” up the prop.

Not so fast Joe but the Cirrus Two lever system has no fancy computer, or anything close to a FADEC, leave that to the Diamond. The Cirrus has two levers a Mixture and something they call a "Power Lever." Freye the source of your misunderstanding is the "Power Lever." This "Power Lever" is nothing more than a pretty neat way of having almost all of the advantages of a constant speed system with out the addition of a third lever. Notice I said almost I'll get to that later. But basically moving the "Power Lever" from back to front will control the Throttle and Prop in the same fashion as in a three lever system, just the prop control will not be linear. Moving the lever in this fashion will indeed move the throttle from idle to wide open, although the prop as I said will not be in a linear fashion. In fact you can feel this as you push the lever forward you almost feel it click a bit. I don’t remember for sure but I think it is suppose to have steps at 2300, 2500, and 2700 RPM. And you may not find that in the POH but check the performance section I think I remember the tables having those values.

So back to your original question why does the Cirrus book say that pushing the Power Lever forward causes oil to flow from the hub???? Well imagine you are just short of the 2700 RPM step (nearly full forward but at 2500RPM) and you move the lever forward a bit, the throttle will increase slightly although it may not be enough to overcome the 200RPM increase in prop. Meaning the pushing the power lever forward just a bit will cause an under speed condition moving oil out of the hub.

Hope I have not confused you folks too much it has been a while since I have taught.


Oh yeah and the disadvantage of the two lever system is that in order to run a lower RPM say 2300 at high alt, 8K to 10K(in the non turbo version of the cirrus) you will need to sacrifice a bunch of available power. Where if you were in a three lever system you would be able to operate at full throttle and a lower RPM due to the levers being independent not connected.

If there are gross grammatical errors, I apologize our coffee machine is broke at work…

If anyone has any questions I’ll try to answer them, feel free to PM me.

Cubdriver

Nice post Zinc. That does help. I was having trouble remembering how the Cirrus system works- your post cleared it up.

__________________
Turn the heat O-F-F!