How many blades on the prop in your picture? 8 or 9? But this plane was happy with 4? I don't think anyone has mentioned one of the main reasons they put more blades on the prop of most full scale for high altitude high speed flight. When flying fast and especially in thin air a smaller higher velocity air stream wins every time. In fact with a large diameter the engine will not have enough power for the air velocity required. If anybody said this I missed it .