22 hours ago, J35OE said:

Huh?  How can engine RPM differ from prop RPM???

The prop is not always connected directly to the engine's crankshaft, but is sometimes driven via a gear reduction system, and prop speed is controlled by either a mechanical or hydraulic governor to maintain a selected speed by varying the pitch of the blades.

The engine spins (just like a car engine) based on fuel flow and the air mixture through the venturi, or fuel pressure controlled by the injector system in the case of a fuel-injected engine. Rather than measuring engine RPM, instead engine power is measured/reported in terms of manifold pressure.

See: http://www.boldmethod.com/learn-to-fly/aircraft-systems/how-a-constant-speed-prop-works/

 

1 hour ago, n4gix said:

The prop is not always connected directly to the engine's crankshaft, but is sometimes driven via a gear reduction system, and prop speed is controlled by either a mechanical or hydraulic governor to maintain a selected speed by varying the pitch of the blades.

Even if the prop is not always connected directly to the engine's crankshaft and a reduction gear is involved, it's still a fixed ratio!

To include geared engines it looks like I have to rephrase my statement:

Propeller RPM can not be controlled independently from engine RPM, regardless if it's a constant speed or fixed pitch propeller.