hi, i have jsut finished this model with an OS 70 fl (4 stroke) and i am having trouble sustaining vertical flight, like i only get about 3 seconds of vertical, pretty pathetic..., it just stalls, but i was wondering whether changing my prop size would get my vertical performance up a bit, coz at the moment i pull up, and next thing you know your pretty well on ya way back down again from stalling. I am currently swinging a 12x7 prop