Controllable-pitch propellers

[PILOTO]

hello everybody! question for all you experts, how's the oil system and the controllable -pitch propeller works? thanks in advance.

 

[tarp]

Single or Multi-engine airplane?

They have different systems and reasons for it.

 

[avbug]

Piloto,

A controllable pitch propeller (variable pitch, or constant speed) works using blades that rotate in a cuff built into the propeller hub. These blades are geared at the end, and are moved by a piston inside the hub which works on a helical principle. The piston moves forward, and twists internally. As it does so, the gearing attached to the piston moves the gearing on the propeler blades, causing them to rotate, simultaneously. This is easier to see in a disassembled prop than to draw a picture with, using words (obviously).

The point here is that a mechanical arrangement inside the prop hub causes the blades to twist or rotate together.

Several forces are used to move this piston in actual operation. Inside the propeller hub, generally a large spring exists, or in some cases large geared counterweights, and in many cases, a nitrogen charge. In most cases, these forces are arranged to push the piston to move the blades to the low pitch stops, or the high RPM position.

Assisting these forces in flight is aerodynamic twisting force (ATF), which attempts to move the blades to the most streamlined position, at or near the low pitch position (varies with airspeed and prop load).

Opposing these forces in most cases is engine oil pressure. It's routed through a propeller governor. Inside the governor, which is driven from the engine on an accessory shaft, is a small assembly with little weights on it. As the shaft spins, the weight move outward, and as they do, they move a pilot shaft with a valve on the end via either leverage or gearing. The pilot valve or shaft ports oil under pressure from the engine oil pump (or in some cases a dedicated prop oil pump) to the propeller. Most comonly this is done through a hollow crank shaft.

The weights in the prop governor are called "flyweights". As the engine spins faster, the weights move faster, and centripital force spins them outward. They move out, and up, and through gearing and leverage, move the attached valve down, moving the shaft to allow oil to be ported through drilled holes to the desired path. Again, better to see a picture. In any case, the oil is most often ported to the rear of the propeller piston, sometimes to both sides, and sometimes to the front.

By varying the oil pressure on the back side of the propeller piston, the piston is moved forward and aft. Most light airplane propellers move the piston forward using engine oil pressure supplied through the crankshaft via the propeller governor, and move it aft using nitrogen and spring pressure, and ATF in flight. Some systems use constant positive pressure on the back of the piston to drive the propeller into feather in the event oil pressure is lost; auto feather systems. Some systems also incorporate accumulator's, or charged cylinders or spheres that store prop fluid or engine oil under pressure to assist in bringing a prop out of feather.

For these specifics, it's best to refer to the aircraft system you're learning, and the easiest way to do this is through the aircraft maintenance manuals. The aircraft flight manuals and pilot operating handbooks are essentially idiot manuals; the real information is in the maintenance publications. If you want to get to know a system, study these. Good luck!

 

[Hovernut]

I agree with avbug, the AFM, and moreover, the service/maintenance manual is the place to gather more detailed technical information. The mx manual may also be in microfiche form also....lots of information!

In the C-172RG I periodically teach in, the governor is driven by the camshaft, which spins at 1/2 the crankshaft speed. But, the bevel gear that spins the governor spline shaft off the camshaft steps the speed back up to crankshaft speed. So, the governor flyweights as mentioned by avbug travel at the same RPMs as the prop.

The governor also has a speeder spring inside that acts in opposition to the flyweights. In an "on-speed" condition where the prop is happy in its current pitch, no oil is allowed past the governor to or from the propeller hub, thus the prop is "hydraulically locked" in its present position.

The prop will change pitch to match the commanded RPM as set by the propeller control. Move the control in and out, and that changes the speeder spring tension, and thus allows the pilot valve to move up or down and allow governor oil to flow to, or dump from the prop hub allowing the prop blades to change pitch.

There's always a tug-of-war going on between the speeder spring and the counter weights. When on-speed, the forces are equal and opposite, thus no movement on the pilot valve and thus hydraulic lock on the prop hub.

When the airplane is pitched over, the engine begins to race, and the flyweights in turn spin faster....more centrifical force....overcoming the speeder spring.....pilot valve movement....governor oil moves one way or another to increase the blade pitch to slow the engine back down to the commanded RPM. As the engine slows back down, the increased centrifical force seen on the flyweights is dissipated and the speeder spring returns the pilot valve to the on-speed condition. Actually, there's a little positive dynamic dampening going on, but I won't get into that without being able to draw the speed curves.

If you move the prop control, lets say back to a lower RPM, you just unloaded the speeder spring and the counterweights take over and move the pilot valve. In the 172RG, this allows governor oil to be pumped onto the prop hub, moving the hub piston and thus prop blades to a higher pitch, lower RPM. Now you can see as the prop slows down, the counterweights no longer have the "umph" they just had a second ago and they eventually lose all the advantage they just had as the engine slows to the new commanded rpm. Now the spring and counterweights have the same opposing force and the hydraulic lock on the prop hub is back in place.

This constantly happens in very small incriments. The prop constantly "hunts" around for the commanded speed. Mostly the changes are not perceptible to us. But just forget to put the prop forward on a go-around and hear it jump around for a few seconds while it "hunts" for the right blade pitch.

I hope this helped; it's a lot better if I can draw out the different conditions. I'm going to make a 3-D animation one of these days to explain what's going on in the governor and prop hub. Their operation seems to be one of the more difficult subjects for new commercial pilots and CFIs.

Regards,
John, MEI CAA

 

[tarp]

How about this:

http://www.mccauley.textron.com/pro-sup/prosupframeset.html

Under McCauley's support on the web, there is a beautiful explanation of the operation, options and care for propellers.
Enjoy. (PS. if you get lost, go to home page, pick product support and look at tech guides.)

I printed this in color and give to my commercial students. I have never found a better treatment of the subject.

By the way, from my earlier post - twin engine prop systems are designed to force the prop to go to feather in the advent of an engine failure. This is because you have another engine hanging on the other wing and you want to feather or reduce drag. Single engine prop systems are designed to force the prop to high RPM (low pitch). Even though this causes MORE drag, the hope in a single engine is that you can get it restarted. The prop will go to high RPM to help with this restart. In case you DO get it restarted and need to do a go-around, the prop is in the high RPM position giving you a "climb" prop which is the safest default setting.

Happy trails!