Secondly, if that rule is to apply to the sizes we've just covered a bit here, then it should consider 22" props differently from the 10" ones for example. None of the "rules of thumb" really ever consider that model airplanes have come a long way since Master Airscrew first offered their line of 3-blade props and wrote up their recommendations. Back then, the recommendations on the back of the packaging suggested a 12x6 for a .90-1.08. No mention of 4strokes ('cause there were none) and the power output back then must have been a lot less than nowadays. Because what you see on most OS91FXs today will be a 14x7(3) and that's a prop that's significantly more load on an engine than one with 2" less diameter AND less pitch. Moreover, that comparison is 3-blade versuse 3-blade.

Also, Master Airscrew's line hasn't been increased to include props for 100cc, 150cc, and larger engines, which are fast becoming the popular sizes for a lot of IMAC flyers, both competitive and sport flyers. MAS's recommendations are significantly wrong, and they're printed right beside the "general rule for converting from 2 blades to 3......"

What would you guess for applying that rule to the big engines that're the ones you're most apt to see sporting a 3 blade? "Reduce diameter by 2 and increase pitch by 2"? That actually would be a shortfall in both dimensions if actual power were properly corrected for.

The best advice nowadays is to ask around to find someone using your engine and and a plane like your plane. And better advice would be to take that advice and try anything you can find the next step up and the next step down in diameter while you're trying the suggested one. But expect to hear a lot of confrontational conjecture about taking that advice.