17x6 it the go to prop for 20's here. Keeps enough load on the engine to keep rpm's in check for starters. Plenty of speed, awesome vertical, helps keep speed in check on down lines (to prevent critical parts from blowing off), and it will slow down to landing speeds very quickly. You can't beat this size prop for 3D. For sport planes with clean lines, like the Great Planes Revolver for instance, 16x8 prop will push 100mph......