Hi Rick,I can't find that file Jerry mentioned, so either it was purged previously, or was a forum message. So, let me take a stab at it, then the real pilots can come in and rip me to shreds <g>.
In a typical fixed pitch prop, the engine RPMs and power output are detirmined by the throttle setting. In a variable pitch prop, throttle is used to control the engine power output, but engine RPMs are determined by the pitch of the blades controlled by the prop lever (and/or governor).
This is analogous to the gears in a car. When you first start out, you want a low gear, and high engine power. As your speed increases, you shift to a higher gear, and lower engine power. As you know to go 30mph in 1st gear requires high engine RPMs as opposed to 3rd gear, which results in the same speed but much lower RPMs. This increases fuel economy, and therefore range, and results in much less wear and tear on the engine.
Same on the airplane, but instead of gears, you adjust the pitch of the blades to take a bigger "bite" of air (higher gear) which subsequently requires less RPMs (and power) for a given airspeed. So for takeoff, you would want full engine power (throttle), and high RPMs (prop), as you transition to cruise, you would reduce engine power, and increase the prop pitch to maintain your desired airspeed at a lower RPM setting.
Clear as mud now? I hope it helps a little anyway
--Greenie
**6 miles SSE KSJC**