Diablo-rcu, with my limited experience. When a prop unloads it is a sign of improper throttle mangement and not having the correct prop which then is ineffecient.
A friend of mine has a 21x10 Menz S on a Taurus 3.2 and I've seen him fly his plane, do a loop carrying full power through out the entire loop and never heard the prop unload. The sound of the engine and prop never changed. I thought that was quite interesting. But I have heard other props unload so I know what it sounds like.I'm sure a stiffer blade is more efficient. And that may be the composites. I don't have an engine large enough for a 26" prop, not yet.