Sportrider:

The factory recommended propellers for your engine are 11x6 and 11x7.

http://www.osengines.com/engines/la.html

These load the engine and avoid over-revolutioning and operation at inefficient regime.

In flight, the 6” pitch not always produces the speeds you have calculated, and the generated thrust should be bigger than the static.

If 5” is your conclusion for best pitch, then you should keep the engine loaded increasing the diameter to 12, for which your model has enough ground clearance, I think.

The Cub manual recommends 10x6, 11x4 and 11x6 for two-stroke engines in the range of 0.40-0.51.

Here you can read four interesting articles about matching propeller, engine and frame:

http://findarticles.com/p/articles/m...g=content;col1

http://findarticles.com/p/articles/m...g=content;col1

http://findarticles.com/p/articles/m...g=content;col1

http://www.lcrcc.us/id9.html

The ultimate selection is made in the air, though.