Hi everyone,
I really need help from someone in explaining the governor in a controllable pitch propellor. I fly the seminole in commercial training right now and my instructor hasn't really explained it in detail to me, just myself trying to find explanations in books. I understand it provides various oil pressure to maintain a constant RPM but where does the nitrogen come into play? What are the details with the fly weights?

Many thanks and sorry if this is an easy explanation.

Harry

 

Try to search around and find a good simple explanation.

A good simple explanation doesn't tell you which way the pilot valve moves or all the intricate parts, it focuses on the concepts and basic operating principles.

Governor employs speeder spring and flyweights. Flyweights help to maintain constant RPM by adjusting oil flow to and from the hub. As prop slows down or speeds up, flyweights move in or out, allowing oil to flow to/from the hub to adjust the angle and keep the speed constant. Think about it like a fixed pitch propeller and what that will do if you pitch up and down, then think about what the constant speed has to do to avoid this.

Speeder spring keeps tension on flyweights and allow you to set RPM settings. If there was no speeder spring, the flyweights would overpower and the prop would "stick" at one extreme setting. This basically allows you to set the "point" that the prop will try to seek out when it experiences something that slows it down or speeds it up.

Nitrogen usually (in a twin) is used to pressurize the hub and is working against the oil pressure, it's trying to feather the prop. If you lose oil pressure, this is one of the reasons why it will feather, the nitrogen will push the oil out of the hub and twist the blades towards feather (but the counterweights and aerodynamic twisting force are important too).

This isn't the best explanation I'm sure, but be wary of complex diagrams and explanations. I didn't explain which way the oil moves (except for out of the hub when it feathers) or whether the flyweights get closer or further on purpose, because I don't think that's critical to understanding/describing the system and being able to competently operate it. I can explain that no problem and I've honed to a T drawing complex prop diagrams...but in the end I don't think it really helps much. I'd be happy to describe some specifics if you like (tomorrow though).

 

Years ago there was a great book put out by Hamilton Standard called "Prop to Pilot". Did a very good job of describing the constant speed propellor system. Not sure if it is still available.

Google Prop to Pilot and you will find a link to a.pdf version of the book.

 

The exact directions of mechanism movement, oil psi, etc is not relevant to the concept (these vary with make and model)

The RPM is controlled by the position of the flyweight mechanism. The mechanism moves in and out, allowing the right amount of oil to the prop mechanism to control the blade angle.

The position of the flyweight mechanism is determined by a BALANCE between the force applied by the speeder spring and the centrifugal force applied by the flyweight mechanism. If one or the other changes, the mechanism moves, changing the oil supplied to the hub.

This force change can occur due to RPM change, in which case the system will try to correct back to the original RPM. Or it can change if the pilot adjusts the speeder spring which will cause the hub to seek a new RPM.

There is a balance between engine power (indicated by MP) and the blade pitch. If you change MP, the governor will respond to keep the RPM at the set speed by changing the blade pitch to take a bigger or smaller "bite" of air.

Think of it as a balance between forces.

 

Some items I would add for the OP... The propellers you are dealing with on the Seminole are not termed "controllable pitch" that is yet another type of propeller and some were in use as essentially an interim step in the evolution to the "constant speed" propellers used on most airplanes today. Controllable pitch propellers are a pain in the butt, like turbocharger installations with manual wastegates... The constant speed propeller control system uses engine oil as hydraulic fluid. This negates having to make use of a separate, dedicated fluid supply system for the propeller. However the engine oil pressure is insufficient for the purpose of moving the propeller blades in a dynamic situation. To that end propeller governors also incorporate a small oil pump to greatly raise the fluid pressure in the system. For practical purposes the constant speed system in the twin works much the same as that in a single, except backwards as that is the easiest way to "design in" the feathering capability. When you lose oil pressure in a single engine airplane a constant speed propeller will default to low pitch (high RPM) in a multi engine airplane the default is to high pitch, toward feather (low RPM)

 

Google: "mccauley full feathering constant speed propeller"

The first link is an excellent PDF document with pictures that will help illuminate the concepts described on this thread.

The pictures on that PDF really helped me understand its operation.

Although it's not specifically describing the Seminole's Hartzell prop, the principles are the same.

 

You don't have to understand it in detail. All you have to know is what its function is. All i was asked on the multi was what does it do? And that was it. Explain that it uses oil pressure to move the pitch of the blades and thats it. Anything else is serious overkill and if an examiner goes that deep into it i would be shocked. Your main focus should be an EXPLANATION of aerodynamics and not just repeating what the instructor told you. If you know the aerodynamics inside and out know PAST and David Carson Bought A Low Fat Cow the rest is limitations. You will be fine. The multi commercial add on is easier and more laid back than the other tickets. Where you need to go absolutely crazy is on the cfi initial because you could end up getting a 9 hour oral and still fail if you know everything because the inspector wanted to be "thorough"

 

I strongly disagree with the above. Myself and others I know were asked much more than just the very basics of how a constant speed propeller works.

It is better to know it and not need it, than to need it and not know.

 

Hm. Maybe i had an easy examiner because he was way more in depth on aerodynamics than he was on how the governor worked.

 

How many orals/checkrides have you taken with an examiner where this might be a posed question? I'd say my last P135 ride at work had more questions on the governor operation than your experience

TheStranger - thanks for the Goole search clue.
Interesting article and informative reading. I always enjoy reading the different methods for explaining a subject matter - and constant speed props are one of those which I can always use refreshing on!