Reduce RPM to "coast" in long descent?

"Stuart Grant" wrote in message
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I think I DID find the answer in my pilot handbook. The range curves
show significant increase in range with the same MP and lower RPM.
Manifold pressure 22" and 1800 RPM for example.

Your handbook is telling you something different. That is, that the engine
is generally more efficient for a given percentage horsepower when the lower
RPM is selected.

That will always be true, but it doesn't mean there's a reduction in
airframe drag, which is what you asked about. As Dale said, if the engine
is driving the prop rather than the other way around, reducing prop pitch
isn't going to change the drag of the prop.

Now, if you have an engine failure, or are gliding with the power completely
reduced, that's a different story and reducing prop RPM will increase your
glide range. But that's not what you asked.

In descent the reduced
power would be made up for by gravity for a more constant airspeed. I
think I will try this.

I don't understand "more constant airspeed". In a stabilized descent, the
airspeed should always be constant. You are certainly right that in a
descent, gravity adds thrust (equivalent to adding power), so you can reduce
power and maintain the same airspeed.

This is, in fact, a technique that is usually taught to every pilot during
their initial training: to descend while maintaining your current airspeed,
simply reduce power. The airplane will remain at (or near) its trimmed
airspeed, and will descend at that airspeed.

Pete