Thomas Borchert writes:

Who supposes that?

The sources I've read. One of them compares pitch to the gearing in a
car, with fine pitch being like a low gear, and coarse pitch being
like a high gear. The implication is that you can go faster in cruise
with coarse pitch (just as you can go faster in overdrive in a car
when cruising), but it doesn't seem to work that way. Decreasing the
RPM supposedly compels the pitch to become coarser, like upshifting to
a higher gear.

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.

The Baron has a CS prop, I know that.

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.

OK, but why does the airspeed drop? In a car, you use the highest
gears (coarsest pitch, hence lowest prop RPM) for high-speed cruise.

It seems that high speed and fine pitch should produce exactly the
same thrust as low speed and coarse pitch, as long as the prop blades
don't stall or reach transonic speeds.

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.

So you're saying that lowering the RPM necessarily means a drop in net
thrust unless the manifold pressure is increased (a throttle
increase)?

I'm reminded of diesel-electric locomotives. These locomotives have a
large diesel engine that drives a generator or alternator, which
provides electricity for traction motors. When you control speed on
these locomotives, you don't adjust the diesel engine speed directly;
instead you adjust the power demand of the traction motors, and a
governor adjusts the actual diesel engine throttle to provide the
necessary power.

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