i used to have a ST 50 a while back, i initially had a 61 fx on it and it flew great, would hover at about half throttle, i then installed a saito 91, it flew greeat, but started losing power("needs to be overhauled"), after that i tried a super tigre 90 man it was fast. I ended up taking it off because i didnt like te look of the stock exhaust, and in the end i would up putting the 61 fx with bisson pitts back in it, and flew the right winf off it.