The prop you choose will depend not only on your plane but also your flying style.

If you want to fly smooth pattern type aerobatics then a 14x6 should be perfect. It'll give you *lots* of thrust for those uplines and good braking on the downlines.

However, if you want more speed (ie: you really wanted a pylon racer :-) then a 13x8 or even a 12x10 will be more to your liking.

I have a P51 Mustang with a TT91FS and found that the 14x6 gave me unlimited vertical but the plane flew too slow to be scale. The 13x8 was better but still too slow for my liking

The 12.5x10 I'm using now is nice and fast, but I don't have the unlimited vertical I got from the other two props.

The MAS K-series props look good and sound sweet but to be honest, they're not as efficient as the APC or even the Master Scimitar series. My 12.5x10 is actually a 13x10 Scimitar that I've taken a 1/4" off each tip. It's the *perfect* size for this engine in this plane.