RE: "Unloading a prop"
Typically, when someone is talking about "unloading" the prop, what they are referring to is that the engine will pick up additional RPM in the air over what the ground RPM. How much unload is dependent upon factors like engine timing, tuned exhaust, and aircraft drag. For instance, an exteme example of unloading would have been Formula One pylon ships that were powered by .40 sized engines with open exhaust. Needless to say, they were quite loud (at about 115 db). On the ground, they were prop'ed to run at about 22,000 RPM, but in the air they would unload to around 27,000 RPM. Most sport airplanes/engines unload far less, typically in the 1000-1500 range. Part of the reason that you set an engine rich of peak, so you have the fuel to allow the unload.