I just finished an Ultra Stick 40 with FS 70. Works best for me with 13X6 K4 prop. Had to add 2oz to the tail even after pushing the battery back as far as i could. I think this was due to the fact that I used the Dubro Anti-Vibe mount which pushed the engine out a little further than anticipated. For some reason I like my engines mounted upright, maybe I'm just wierd !!