Mxsmanic wrote:
OK, but why does the airspeed drop? In a car, you use the highest
gears (coarsest pitch, hence lowest prop RPM) for high-speed cruise.

Because the RPM drops. Ceteris paribus, the same amount of fuel/air mixture
enters each cylinder with each intake stroke, and each cylinder imparts the
same amount of torque with each power stroke, but because of the lower rpm,
you have less power strokes per time, thus, the engine yields less power.
If you reduce the RPMs by 20%, c.p.* the engine power will drop by 20%,
too. Hence, you loose airspeed.

And the gearbox-metaphor is not very well suited because most cars don't
have a contiously-variable transmission but rather distinct gears, also,
you don't specify the desired engine rpm but rather the desired gear ratio.
That is not the case with the constant-speed prop, you select a desired
rpm, and the prop governor adjusts the load on the engine (by varying the
prop pitch) to maintain that rpm, regardless of the actual power output of
the engine (within the limits of the prop's abilities, of course).

Anno.

*) and that's a pretty strong c.p.