I had a GP Corsair ARF a while back, and I put an OS 61 fx on it and I still needed 1lb of weight in the nose. I know that sounds a lot but I wished I had put a 91 4stroke on it. It would have flown better and taken off better. This was even a 40, if I had put a 40 size engine on it it would have never gotten off of the ground unless I had a 100yd runway. It flew decent with the 61 but a 91 4stk would have made it better.