When I change the prop setting on my (simulated) Baron 58, lowering
the prop RPM, my airspeed drops. I thought that for a given throttle
setting, the actual thrust produced by the powerplant was supposed to
remain the same for a wide range of prop settings, because of
automatic pitch changes made when I change the prop RPM. However,
that doesn't seem to be the case. A lowering of the prop RPM also
lowers airspeed, which implies a change in thrust. The fuel flow also
diminishes, which implies a change in power (?).

So, exactly what do I gain or lose by adjusting prop RPM when I'm
cruising along? Why would I want to change it? Some sources I've
read say that the prop makes less noise, which is surely true, but it
seems that I can't lower the RPM without losing airspeed (and thus I
must be losing power, right?).

I can hardly believe that I am reading this thread, much less responding to
it!

However, someone reading all this may very well work their way up to
captain, and I may very well be one of their passengers...

As of 9:50am EST on Jan 16, most of the responses seem to pertain to the
first power reduction after take-off, even though the question was specific
to cruise, and most of the remainder appear to presume a very radical change
in RPM. OTOH, none have mentioned that many aircraft--especdially
twins--have an automatic manifold pressure control (as contrasted to a plane
old throttle plate) connected to the throttle levers. This is (or should
be) universal for turbo-supercharged engines equipped with automatic
waste-gates.

None of these systems function perfectly, but to the extent that they
approxamate a constant manifold pressure, horsepower will thus be directly
proportional to RPM.

In addition; when operating above the critical altitude a normally aspirated
engine will typically operate as though the manifold pressure is
regfulated--but a turbo-supercharged engine (or any engine with a
centrifugal supercharger) will work in reverse of what might be intuitive:
an increase in RPM will result in an increase in manifold pressure and a
decrease in RPM will result in a decrease in Manifold pressure.

The exception to the above occurs when operating an "entry level" conplex
aircraft at low altitude and reduced power. In that case, a reduction in
RPM by means of the prop control will result in an increase in manifold
pressure. There will still be a reduction in horsepower, but not nearly as
much.

Peter