Constant Speed Propeller

Dazzler

Well-Known Member
OK so I've finally started my Commercial Training !!!

My instructor wants me to research how a constant speed propeller works. I've found a couple of resources, but neither are all that clear.

Oil, flyweights, governor, overspeed, underspeed, blade pitch, etc. are all involved.

Can someone explain it all to me like I'm a two-year old please?

Thanks!
Dazzler
 

braidkid

New Member
Wow, that's a lot to discuss in a post and I don't know where a good on-line reference would be. The source that really helped me understand it was "The Turbine Pilot's Flight Manual." I'm reading it now and it has a wealth of information on constant speed props as well as various other high performance aircraft systems.
 

viper548

Well-Known Member
On a SE airplane, Compressed gas or a spring pushes a piston which moves the prop into a low pitch/high rpm setting. The other side of the piston has oil trying to press it into a high pitch/low rpm setting. This way, if you lose oil pressure, it goes into a low pitch/high rpm and can still be flown. ME airplanes are different. The spring or compressed gas moves the piston to high pitch/low rpm. If it loses oil pressure it goes in high pitch/low rpm, which has less drag if the engine isn't operating.
 

pilot602

If specified, this will replace the title that
[ QUOTE ]
he spring or compressed gas moves the piston to high pitch/low rpm. If it loses oil pressure it goes in high pitch/low rpm, which has less drag if the engine isn't operating.


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It should go all the way into feather ... to stop all rotation.
 

EatSleepFly

Well-Known Member
[ QUOTE ]
It should go all the way into feather ... to stop all rotation.

[/ QUOTE ]

A prop on a piston twin will go all the way to feather by itself after an engine failure? News to me...

Or am I misunderstanding what you meant?
 

RiddlePilot

New Member
[ QUOTE ]
A prop on a piston twin will go all the way to feather by itself after an engine failure? News to me...

[/ QUOTE ]

I think what he probably meant to say is that it will go to feather if there is an prop governor leak/rupture to allow oil to spill out.
 

EatSleepFly

Well-Known Member
I still don't think it'll go all the way to feather. That would be the same as losing oil pressure- and while that would bring the blades back to a high pitch, low RPM position...it won't feather itself.
 

pilot602

If specified, this will replace the title that
Well there are a couple of conditions ...

If the oil pressure is lost fast enough (say catastrophic) the gas charge/spring will/may push the blade into feather because at the higher RPM range the centrifugal locks are still being held open.

Take, for example, the Apache. Our "new" props won't (or aren't supposed to anyway) feather below 800 RPM. Below that RPM the locks close to a point that prevents feathering (if they were to be held open somehow on the ground the prop WOULD feather ... in fact we had this happen not to long ago) but if the oil pressure were to be lost at some speed ABOVE 800 RPM, and in a short enough period, theoretically the blades would feather. Assuming the aerodynamic force of the rotating prop doesn't overpower the charge/spring.

Or in other words ... the only thing stopping a prop from feathering - on most twins - is the centrifugal lock. If those aren't closed the prop WILL feather (assuming there is no oil pressure either from a leak, loss or non-operating engine).
 

averyrm

Well-Known Member
Also - not all c/s props will do the same thing, or work the same way.

For example, on a Decathalon, the prop will go into high pitch when oil pressure is lost. This is the opposite of most SE aircraft. (The reason for this is when flying knife edge, sometimes the oil won't circulate, and rather then overspeed the engine, it's better to have the prop go into a lower rpm)
 

FL270

New Member
The exact mechanics of the system vary from aircraft to aircraft and engine to engine, but basically, engine oil will circulate through the propeller hub and vary propeller blade pitch to keep the RPMs constant at different power settings. For example, let's say you are cruising at 20" MP and 2300 RPM. You decide to increase power to 23" MP and 2300 RPM. You advance the throttle to 23". On a fixed-pitch prop, advancing the throttle like this would cause an RPM increase, right? Well, on the constant-speed prop, the blade angle will increase to take a larger "bite" of the air without a corresponding increase in RPM ... more power, same prop speed. Reverse the process when you reduce power. Make sense?

Hope this helps.

FL270
 

triple7

Well-Known Member
which happens first? to initiate a climb do you pushthe prop forward or the throttle first.....
 

pilot602

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When increasing power, remember: MPT (Mixtures, Props, Throttles)
When decreasing power, remember: TPM (Throttle, Props, Mixture)
 

chris

Well-Known Member
I recommend you go to www.avweb.com, search for the author "John Deakin," and look for his article called "Those Marvellous Props."

It explains it very well and in an easy to understanding manner.
 

Alchemy

Well-Known Member
Ditto what chris says, particularly this article http://www.avweb.com/news/columns/182082-1.htmlp. Also read the POH for your airplane. It should have an extensive section on the operation of the propeller, especially if it's a cessna POH.

Basically there is a cylinder which moves back and forth to control the pitch of the blades via mechanical linkage. The piston moves becaus oil is being squirted on to it. Usually squirting more oil onto the cylinder results in a higher blade pitch. This is why you will se a slight drop in oil pressure when you pull back on the prop control during the runup. The prop governor controls weather oil is being is being fed into or released from the piston. When the prop decreases pitch, oil pressure is relived. The movement toward low pitch is also aided by centrifugal force and internal springs.



If the RPM's drop below (underspeed) or rise above (overspeed) that desired, the flyweights move, in turn adjusting the valve that controls the amount of oil pressure acting on the cylinder. The governor is constantly making changes during flight to insure that the prop turns at the proper speed (the speed selected by you using the propellor control in the cockpit).
 

RiddlePilot

New Member
[ QUOTE ]
When increasing power, remember: MPT (Mixtures, Props, Throttles)
When decreasing power, remember: TPM (Throttle, Props, Mixture)

[/ QUOTE ]

A pretty easy way to remember this is to think in terms of "uphill" and "downhill." Put your right hand to your lower right, just below the throttles. If you're going to be adding power for the climb, move your hand up and to the left, i.e. an "uphill" movement. If you're reducing power, move your hand back down and to the right, i.e. "downhill." This is kind of hard to convey over a message board, but as you can see, these motions allow you to add/reduce power properly.
 

bluelake

Well-Known Member
i dont think that is easy ay all... if I do TPM, which way do I go with the prop and mixture... its not so obvious to those we are training !!!! MPT.. TPM.. uphill.. downhihlll... HELLO. Is there a descent way to train this stuff?? I aint talking about training some gel-haired airline aspirant either.. how about the normal folks who are buying and driving 182's and Arrows, etc...

I decrease power ALL THE TIME and dont change prop/mixture settings.. as an example, in IFR training, thats whats called going from cruise configuration to cruise descent configuration.

With constant speed prop airplanes, I think there is too much emphasis on these memory aids and not enough focus on what one is really trying to accomplish, within the limits of what the POH specifies.

I dare anyone to tell me that you should NEVER have your MP higher than your RPM
:) I am salivating at the offer:)
 

Alchemy

Well-Known Member
Running MP higher than RPM is perfectably acceptable within the POH's limits. If I'm not mistaken (no actual experience in C-421's and the like) geared engines have MP's MUCH higher than the prop RPM's. When you think about it, it's pretty arbitrary to think that Inches of Mercuruy and Rotations per minute are some how related units which must be equal.

You can cause detonation if you have very high manifold pressures and low RPM's in direct drive engines. In these situations peak pressure in the cylinder chamber occurs earlier than it should in the cylinders' cycle. The cylinder does not move "fast enough", causing stress at peak pressure during compression stroke, damaging the engine.




However, most people are more cautious than necessary about always having RPM's "higher" than MP on small, direct drive engines. It takes some pretty extreme settings to really cause problems on those types of engines. Radial engines are more sensitive to this issue, and that's probably a big reason why we're told to operate the engine the way we are....it's a holdover from the days of the radials.
 
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