The simple answer is that the manufacturer does not want you to exceed
the maximum sustained horsepower rating at any given rpm/mp/altitude.
For example if you run the book MP during a climb however increase the
prop rpm because you are heavy, hot, etc then you are exceeded the
maximum rated hp for the climb.

I got a chance to witness this years ago with with turbopropeller
driven airplane: The Captain on this particular Regional was having
difficulty climbing during the Summer months with a full load
and looking at the charts kept the maximum torque (a setting used for
turboprops) setting which was designed for a maximum climb HP
at a specific rpm. To expedite the climb he pushed the propeller rpm
up which helped, however this combination exceeded the maximum rated
horsepower of the engine (limited by the reduction gearbox), not the
turbine and the gearbox exploded resulting in several injuries to
the passengers. Follow the book.

Have a great one!

Bush

On Mon, 12 Apr 2004 06:34:06 GMT, (Ben Jackson) wrote:

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?