I was talking about fixed pitch props, like we use on models, at varying RPMs and in flight. The best pitch and/or diameter to use is based upon figuring out the efficiency based upon a formula that includes airspeed. You can't have large diameter and a large pitch for most engines, so you have to make a choice. That would be what it is all about in the end. Getting the most out of an engine and airframe. You could call it calculating the pulling power of the prop, or whatever. A lot of people think that only the highest RPMs give the most power.

You can use a smaller diameter prop for faster RPMs and reach a point where it is the most efficient. If you get the tips near the speed of sound your performance will drop quite a bit. You can do the inverse and choose a larger diameter to reach peak prop efficiency. The large diameter wins out in efficiency if used on the same engine, same airframe, etc.