The theory is that you want to go with the biggest pitch prop that you can find.... without overloading your engine. Reduce the diameter of the prop to compensate for increased pitch.

Pitch is the theoretcal distance the prop will travel forward with one complete revolution. A 10x7 prop will go forward 7 inches with one revolution. It will spin faster than an 11x7 because it is smaller, thus a 10x7 prop will give a better top speed than an 11x7 (and it will spin a lot faster). A 10x8 will make your plane faster than a 10x7, and so on.

With the increas in speed, your plane gets a correlated reduction in "power". The pitch and diameter of the prop can be thought of like gears on a bicycle. A 10x5 prop is like 1st gear, and can really pull your plane strongly, but will not get to a very high speed. A 10x10 prop would be like the top gear, it will make your plane scream, but probably will not be able to get the plane airborne from the ground because it does not produce the right thrust at low speeds.

If you really want speed, go with the 10x8 over the 11x7. Experiment by increasing the pitch some more if you want.

Experiment with different prop brands as well. I have found that APC props give me better performance than Master Airscrew....

gus