Full scale AC use multi-blade props to permit the transmission of available engine power without resorting to very long diameters (no space available) & to avoid the necessary reduction gearing to keep tip speeds of a giant 2b subsonic. FS aircraft have the advantage of constand speed propellers, so the 3b with the same diameter will be at a lower pitch angle than the equivalent 2b & will consequently have greater initial acceleration & climb. however, at cruise, or maximum speed, the 2b will outperform the 3b simply because it is consuming less power for the same thrust. That is why the Schneider Cup racers of the 20's & 30's used 2b props -- to wring every last ounce of speed out of the available power. That is also why the fastest RC models use 2b props & the fastest CL models use one-blade props -- not a single 3b to be seen anywhere when performance really counts.

A very well designed & manufactured 3-blade prop can be 90% - 95% of the diameter of the reference 2b, for the same pitch & power load. If it is not well designed, or is manufactured of a material that doesn't minimize blade thickness, the 3-b diameter will be more like 80% for the same power load. In the example that jeepindog gave (14-6 2b as the reference), the closest equivalent 3b size is 12.5-6 (90% equivalency) or a 13-6 @ 95% equivalency, for the same power load. However, he used a 13-8 as the 3b, & that is directly comparable to a 14-8 2b, or a 15-6 2b @ 95% equivalency, all other factors being equal (same material, same blade configuration, & preferrably, same manufacturer). In other words, the 14-6 2b is a much "smaller" prop than the 13-8 3b as far as the engine is concerned. The difficult part comes when he claims that both props turned at the same 9700 rpm. There is clearly more to the story.

If for example the 2-b prop was a wooden type (eg. Zinger), & the 3b was a nice sharp narrow-blade APC or Brolly GRE prop, that could explain part of the observed difference (the thick 2b blade was absorbing relatively more of the engine power), but it doesn't account for all of it. Similarly, was the 2-b a low aspect ratio nylon prop & the 3-b a narrow blade GRE prop? He didn't say, althought that could account for part, but not all of the difference..

jeepindog observed 9700 rpm for both props -- is he tuning & throttling his engines for 9700 rpm because he wants a safe margin to redline, or was the engine similarly tuned & throttled & able to run freely? He didn't say.

If the engine will turn a 13-8 3b @ 9700 rpm, it will turn a similarly designed 14-8 or 15-6 2b at the same RPM -- & there is no doubt which combo will pull harder.

What information is missing?