Constant speed propeller

[code0]

Greetings!

in a constant speed propeller in a piston engine fixed with a manifold pressure gauge and a RPM tacho how do you detect a prop governor malfunction from these two gauges and the philosophy behind it? or any other interments you rely on?

Thanks in advance!

Code0

[Wizofoz]

The RPM will either not change when commanded to, or WILL change when NOT commanded to.

[Atlas Shrugged]

The Manifold Pressure Gauge will not tell you anything other than the amount of air available to the engine.

[Caboclo]

Assuming no throttle movement, the MP will vary inversely with RPM as the engine uses more or less of the available air pressure. At least I think that's correct, been a while.

[Wizofoz]

^^^^^^^^

Yes, true, but that's the least of your problems. Most CSUs have springs to return them to the fine pitch stop if oil pressure to the CSU is lost, so a sudden increase in RPM is the most likely sign.

[barit1]

Could go either direction, depending on the prop design. I once found a good description of the McCauley prop and control and was impressed with the astuteness of its balance of forces and torques in the prop hub.

EDIT: per http://www.mccauley.textron.com/von_klip_tip_ff_constant_speed_propeller.pdf, in the McCauley prop, oil pressure decreases blade pitch - like the following models.

But among other prop designs, a counterweight type will probably slew to the high pitch (aka coarse pitch, low rpm) stops. This is by design intent, to prevent overspeed in case of loss of control oil pressure. Older H-S and Hartzells as well as Dowty are this way. Thus, for takeoff/climb, oil pressure is used to drive blades to low pitch.