"Ben Jackson" wrote in message
news:yLqec.18880$rg5.39150@attbi_s52...
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. [...]

What do you mean "typical"? There are plenty of constant-speed
installations that don't limit published power settings to 2400, and of
course there are others for which the aircraft manual doesn't bother to
publish much of anything with respect to power settings. Documentation is
all over the map, so I don't see how you can talk about "a typical power
chart".

For a specific installation, I'm sure there's a specific answer. But
without knowing more about the specific installation, seems to me the reason
could be any number of things. Possibly the manufacturer just didn't want
to flight test higher RPM conditions. Or possibly there's a published RPM
limit for the installation. Perhaps for that installation, you're just not
going to get more than 2400 RPM above the altitude in question and power
really IS limited.

For that matter, your comment about fixed-pitch installations is similarly
overly optimistic in its generalization. I don't doubt that there are some
that can make 75% power at 8000', but I certainly doubt that they all do.
For those that don't, their power charts (assuming the aircraft manual has
one) will also stop showing 75% power at a lower altitude.

Not all engines are the same, nor are all props the same. They all come
with their own variety of operating and performance limitations. If you are
flying behind a constant-speed prop, the aircraft limitations don't say
anything about limiting RPM, the manual doesn't show settings beyond 2400
RPM and 75% power above a certain altitude, but you think you can get 75%
power above a certain altitude simply by using a higher RPM setting, by all
means...go for it.

Pete