Mxsmanic,

But supposedly reducing the RPM
slightly just causes the CS prop to change pitch,


Who supposes that?

We need to be clear about definitions from the start:

There are prop installations (though uncommon) where the pilot can set
prop pitch directly. This is not a constant speed prop. If, for
example, the pilot pitches the aircraft down, that will lead to an
increase in RPM with such a constant pitch prop.

A Constant Speed prop, OTOH, hold RPM (its "speed", hence the name)
constant. If you pull into a climb, the prop RPM will not slow, it will
stay the same. If you push into a descent, RPM will not increase, it
will stay the same. In order to achieve that, the prop will change its
blade pitch continually. With the prop lever, you don't change pitch
(directly), you set an RPM which the CS mechanism will then maintain by
adjusting pitch.

Ok, once this is understood, it becomes much clearer (I hope) that RPM
does of course influence engine power. Think about less air-fuel mix
being burned per minute if the engine does fewer revolutions in that
minute.

What you get with CS, however, is something that is best likened to a
gear-box in a car. You can optimize the engine RPM and the "load" on
the prop to what you are doing. High RPM means the prop is taking
"small bites out of the air" per revolution, low rpm means it's taking
big bites. Hence, high RPM is good for take-off, lower RPM is good for
cruise.

As per the cruise power tables in the POH (you really need to start to
read these things), various combinations of manifold pressure and RPM
will give you the same amount of engine power (often, tables are for 65
and 75 percent of engine power). Which to chose? Well, many people
prefer a low noise setting, that means a combination of low RPM and
high MP. You can't use any combination if you don't want to harm your
engine, but you can use all that are in the POH table.

I know you don't read the stuff you're pointed to here, but for
lurkers: This is all very well explained by John Deakin in his columns
on engine management at www.avweb.com.

--
Thomas Borchert (EDDH)