Thanks for the answers guys. I'm going to try it for one flight, just as an experiment. From what you guys are saying, it should be okay as long as I make sure the engine is kept cool.

The engine is an RCV 90SP (the weird one with 2:1 gear reduction to the prop). The prop hub is custom made and features removable and adjustable pitch propeller blades. The propeller blades come from Soloprops ([link]http://www.soloprops.com[/link]).

I've been flying with a 16x11x3-Blade configuration at 5200 RPM. Remember it's geared down... the prop is spinning at 5200 RPM, but the engine is chugging away at 10,400 RPM. Next time out, I'm going to try flying with more impressive (visually) 18" blades set to 10" pitch. I expect to get 4400 - 4500 RPM with that prop configuration. That's more than enough to take off, and it'll fly at half throttle.

Check out this video of this setup on a test stand...
[link=http://www.vimeo.com/1443554]RCV 90SP + 18x10x3-Blade prop[/link]

Thanks,
Juice