Hi Basketball,

When the pilot wants to increase the RPM they move the prop lever forward in the cockpit. This then adjusts the tension on the speeder spring. The governor now wants equilibrium at a higher RPM. The old RPM is now considered to be an underspeed condition and the governor corrects accordingly.

The new tension in the speeder spring fights against the forces generated by the spinning fly weights at the old RPM. This new speeder spring tension wins out and moves the pilot valve into a position which lets more oil into the hub, forcing the blades to finer pitch. This results in less prop torque and the RPM increases.

Another case is if the pilot does not change the position of the prop lever but the RPM starts to increase because, for example, the pilot has started a descent. In this case the flyweights spin faster and overwhelm the tension of the speeder spring. The pilot valve then moves into position to allow oil to drain out of the hub and back into the sump. The drop in pressure in the hub allows the blade angle to coarsen and the increasing prop torque works against the increasing RPM.

If we increase RPM and then power with a CSU this is what happens:

Step 1) Prop lever set for higher RPM - blade angle decreases to allow for the higher RPM.
Step 2) Power increased - the added engine torque now starts to overwhelm the prop torque generated with the finer pitch so the governor coarsens the blade angle again to compensate.

The eventual TAS you get will depend on the thrust/torque ratio you are getting out of the prop for the RPM airspeed combination you have.

You normally apply finer pitch for take-off, when climbing and as a precaution just before landing. The thrust/torque ratio is closer to optimal with finer blade angles when operating at lower speeds e.g. take-off, climb, landing (in case of go-around).

If you were in cruise at the manufacturer's recommended cruise settings and you applied fine pitch you would get a drop in TAS. This is because at cruise speeds the finer pitch and higher RPM means the prop is no longer working as efficiently as it was before and thrust drops away.


Cheers,

Rich

Thanks for the great explanation Richard

Just wanted to clarify one thing.

If we changed RPM only and left the power constant, then regardless of if we increase or decrease rpm, the resulting true airpseed would be lower, assuming we were operating at the optimum rpm/power combination before? This is because the propellor is not operating as efficiently for the given rpm/power combo? The blade would still be operating at the best angle of attack, just that the fine pitch/rpm/power combo does not give the best overall efficiency?

Thanks again.

The thing to remember is it is the RPM and the TAS which determine the angle of attack the blade is making with the airflow and there is only one angle of attack which gives the optimum thrust/torque ratio and this occurs at different RPMs for different airspeeds. You can prove that using the diagram 2.6 on page 2.3 in the textbook.

When you set an RPM, you are telling the governor to maintain that RPM. It has no idea if that is an efficient RPM for the TAS or not. That's your job. You are supposed to use the recommended figures provided by the manufacturer.

By the way those figures don't just take into consideration the prop but also the engine efficiency too. They should give you the overall best bang for your buck so to speak.

So, back to your question: if you are flying at the optimal RPM/power setting and you change the RPM setting you will most likely get a drop in TAS. As you said, the propeller is not operating as efficiently as it could because the thrust/torque ratio is now wrong.
No, the blade has been moved out of the best angle of attack as the governor changes the blade angle to try and maintain the new RPM setting. As it changes the blade angle the angle of attack with the airflow will be different and the prop efficiency suffers.

To summarise:

RPM and TAS determine the blade's actual angle of attack.

Best prop performance is determined by blade angle of attack so therefore prop performance is dependant on RPM and TAS.

The governor controls the RPM whereas Power affects the eventual TAS the system stabilises to.

More power increases engine torque and the RPMs will increase. The governor makes the blade angle greater to maintain the RPM set by the prop control lever.

Less power reduces engine torque and the RPMs will decrease. The governor makes the blade angle finer to maintain the RPM set by the prop control lever.

Optimal settings are provided by the manufacturer. These will give expected fuel flows and TAS for various RPM/Power combinations at various altitudes and temperature ranges for various profiles (e.g. maximum cruise power, recommended cruise power).

Cheers,

Rich

Thanks again Richard, Makes sense now!

No worries, glad to help

Hi- question: When the Prop Control lever is moved does it basically tighten or loosen the speeder spring so that it actually puts the flyweights in a initial position (simplified I know)-a tilted position, like up or down-which acts as a lever arm to lift up the pilot valve or let it down?
Also, is the housing for the pilot valve rotating with the engine and in turn turning the flyweights (pilot valve is stationary)?