Prop sizes are chosen to keep the engine within a specific rpm range. 2 stroke engines develop their power on the high end, so you want your engine to spin at close to a specific rpm.

As a general rule you can change prop sizes by adjusting your pitch based on diameter. For instance, if the manufacturer recommends an 11x7, you might be able to go to a 10x8 or a 12x6. If you go up in diameter by one, you go down in pitch by one. If you go down in diameter by one, you can go up in pitch by one. This doesn't always work, but is usually a good indicator.

I have also heard that you can go by multiples. If the manufacturer recommends an 11x5, you can multiple 11 by 5 and get 55. Then you can divide 55 by whatever diameter prop you would like to use to get an idea of what kind of pitch you can get away with.

Diameter equates to thrust, and pitch equates to speed. The higher the diameter, the more thrust, the higher the pitch, the more speed.

Speed planes generally have pitches from 7+ while most others do not go beyond 6. 3D planes often have large diameters and low pitches, down in the 3-4 range often. 3D doesn't want speed, in fact, most of their planes cannot handle a lot of speed.




There are different props for breaking in for a few reasons. During break in period you are trying to reach a specific set of conditions for that period. The prop choice is to help the engine maintain a certain rpm to aid in the break in.