Constant Speed Prop

"Prop governors are gear-driven by the engine, so the faster the engine turns, the faster the
governor turns. Flyweights work against a spring, and the prop control in the cockpit adjusts
the loading on that "speeder" spring. If the engine is turning faster than the desired RPM,
centrifugal force flings the flyweights out against the spring pressure, which moves a valve
that allows oil to flow in the proper direction to move the prop blades to a greater pitch. If
the RPM drops below that desired, the flyweights move in towards the center, moving the oil
valve that allows oil to flow as needed to decrease the blade angle, increasing RPM. When
the RPM exactly matches the setting by the pilot, the flyweights exactly balance the spring
pressure, and shuts off all oil flow to the prop.

Most governors also contain a high-pressure oil pump to boost engine oil pressure (which is
typically around 50 PSI) up to the levels needed to control the prop (often around 200 PSI).
In reality, the flyweights are constantly making tiny movements to keep the RPM at the
desired value, either pumping little squirts of oil to the dome, or letting a little oil out. Once
again, there are many variations here. Some systems will pump oil to drive the blades one
way, or allow that oil to dump into the crankcase, allowing the spring to push the piston the
other way.

For general aviation singles and twins, the very least you should know is what will happen
with a failure. For singles, the prop will generally go flat. For twins, it will probably feather.
Check your POH, and ask your mechanic about the details for the airplane you fly. You can't
know too much about props!"

-- http://www.advancedpilot.com/downloads/prep.pdf   John Deakin