not sure either, but here's a 2 and 3 blade MAS K series props on OS91's, for them I use the 14/6 to8 2 blade or 14/6 3 blade depending on the pull I want, both have survived several strikes with minimal dammage, the P-40 in particular is a real tipper it ends up on its spinner if I am not carefull on landing, I now use a 14/8 2blade on it.