18X10 would probably do best on a 30cc engine. A 38cc or above would do better with a 20" prop, but your going to have to make the choice. Sometimes the "best" prop for a plane with a particular engine will require experimentation. Xoar Simitars in those sizes are'nt that expensive to experiment with, but in general, the 18" prop should be fine for a 30cc engine.