A typical power chart for a constant speed prop will limit the RPM to
2400 at all altitudes and power settings. The 75% power table will
just end when it is no longer possible to make enough manifold pressure
to get 75% HP out of the engine at 2400 RPM.

In contrast, a fixed pitch prop will turn faster and faster to make 75%
at high altitudes. I think some Cherokees call for up to 2650 RPM
cruise settings.

Another way to look at it is that a plane with a constant speed prop
may take off with full throttle and full RPM, reach a cruising altitude
of 8000' and then pull back to 2400 RPM (leaving the throttle full forward)
while a fixed pitch prop plane would just accept a few hundred RPM rise
at full throttle and 8000'.

Is the answer that the constant speed prop is slowed down because we
*can* and the fixed pitch prop is just suffering all the ill effects
you'd expect, like higher wear, more noise and frictional losses?

--
Ben Jackson

http://www.ben.com/