no problem man. the pitch of a prop has something to do with how far the plane moves forward with each rotation and with a smaller prop you need mor pitch for the engine to run the same rpm. like a 9 x 9 inch prop on a gms 47 with rpm of over 18,000 will be very very fast. by putting a longer prob with less pitch like a 12 x 4 the plane will have more control at low speeds because more air going over the control surfaces than a 9 x 9 at low speed. being in control at low speeds is good for 3d. low, slow and in control. the longer prop you get the more air will be over your control surfaces at low speeds but to get a longer prop you need to drop some pitch or the engine will not spool up. i have a ys110 and i run a 16 x 6. it is a bit faster than i like so i may change to a 17 x 4wide. which will make it fly slower and tourquier ( not real word but with more torque anyway ).