Hi T,
Yup--your right .The smaller O.S. 40,46,&61 FX were non -ringed,but the O.S.91 FX ,which makes almost 4 hp,is a ringed motor,and they are all listed as Sport flyers .
I have seen several flyers at the field using Heli motores on their models,and they seemed to work fine.The one thing I noticed was --the heli motors had a lot more power. If you were building from scratch,I doubt if I would spend the extra money for a Heli motor,but if you already have one--I would use it ..