Another thing to consider is engine load vs. thrust, both on model engines and full size. If you want a scale prop , an engine at the upper end of the range for your model is much better. A three or four blade prop will provide more thrust at a given rpm than a two blade of the same diameter(and pitch), even given the reduction in efficiency, allowing lower engine speeds for a specific airspeed. In a glow engine the danger is overloading the engine, causing overheating and/or detonation. A four-stroke is more desireable for lg. diameter or multi blade props because it develops its max torque at lower rpms than 2-strokes. Turning larger diameter props at high speed is noisier, too, because the prop tips are running closer to sonic speed, and they lose efficiency that way, also.