FFF-most of my flying is done in machines with a wobbly prop. Obviously in these cases engine rpm is not directly related to prop rpm
Yes, it is. The prop is still bolted straight onto the front of the engine, and will still turn at the same speed as the engine, wobbly or not.
You're flying along at, say, 2400rpm, 24". You advance the prop lever. The prop blades become finer. Because they are now finer, they don't meet so much air resistance as they turn. So they can turn faster. This results in the engine turning faster, too, since the engine turns at the same speed as the prop, even though it still generates (roughly) the same amount of power and the throttle hasn't moved.
I'm trying to think of a good analagy. The closest I can think of would be driving along a rough surface with long grass, at 20mph, in second gear. Then you reach the edge of the grass. Don't move your right foot, and keep driving. The wheels will meet less resistance now that you're on tarmac, and so they'll begin to move faster. Because they're directly connected to the engine (via a diff, a gearbox and a clutch in this case) the engine will speed up too. (Your natural reaction to this would be to ease off the gas - but I told you not to move your right foot!)
FFF
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