Originally Posted by gkamysz
Elevation has nothing to do with engine RPM on a given prop. As elevation increases the air density decreases both the engine and prop work with the reduced density. Engine power output does drop, but so does the required torque to drive the prop. The result is virtually no change in RPM with air density.
For an electric motor, this is true. Fans (props) are constant volume devices. Not for internal combustion engines.