Hi ffdg,
I am also flying this Hellcat. I am using an OS 1.08 two-stroker turning a Graupner 14X7 three-blade prop. The Graupner page at Hobby Lobby shows the 14X7 for the OS FS 120 as well-
http://www.hobby-lobby.com/grprop.htm#3%20Blade
The Graupner props are really hi quality, and take black paint nicely.
Good Luck,
Jim