Spacemonkey,

Let me add a couple of things to what has been said.

First the 11 stands for the diameter of your prop and the 8 is the pitch. The same thing about the 9X6, it has a 9" diameter and a 6" pitch.

In theory if you have an 8" pitch for each revolution the prop moves the airplane forward 8". A larger propeller requires more power to turn it so a lower pitch prop maybe required to bring the engine rpm into range. A smaller diameter will turn up faster and will require more pitch or bite to control the rpm of the engine.

What propeller you select has to do with a number of considerations. What is the rpm range of the airplane's engine. How much ground clearance do you have to try to avoid prop ground strikes? What type of performance do you want out of the airplane.

Here are some thoughts to keep in mind. If you want takeoff and climb performance a flatter pitch is going to rev-up more quickly getting you off the ground faster, but be slower flying once airborne. The opposite is true with a higher pitch its going to take more bite out of the air each revolution moving the airplane forward faster but have slower climb and takeoff performance. In model aviation most airplanes have more than adequate power so even with a higher pitch prop the performance is most often good.

Its a good idea to reference your engines specifications and use a tachometer to see what the engine is turning at full throttle. Its desirable to be somewhere close to the top end of the engines rpm range for best performance, but not over the rating. Its not a good idea to constantly overspeed your engine, its always a good idea to use power management by reducing power in dives often to idle depending upon the maneuver.

Thats the gist of it! Sometimes it gets to be a balancing act to come up with the right prop to give you the exact performance your looking for.