I would recommend the YS 91FZ. I am using it now on my UCD3D. I saw one fly with a OS 91FX. The OS has more HP but it did not fly the plane as well as the YS, especially hovering. I thought about buying and putting an OS 91FX on mine. But now that I have seen the plane fly with the OS 91fx, I am sticking with the YS. I am running a APC 15X6 and 30% Cool power Heli fuel. I have no problem hovering this plane. I added 2 washers for right thrust as well. I added a great deal of tail weight. I must change the tail wheel to a better one.