I have a OS 55 AX with a 13 X 6 APC prop in mine, and have the CG set at 90mm and it flies super and lands perfectfully, no boucning on landing like some here have mentioned. I chop the throttle to idle on aprroach and she glides right in smooth as silk. The only thing I noticed is that while flying inverted it seems a tad nose heavy. The magnum weighs 15.70 and my OS weighs 14.30 (if that helps any). I installed a 10 Oz tank as well and have a 1500 NiMh battery. The battery adn receiver are strapped in just in front of the servo tray with the battery as close to the tray as possible. I'm thinking if I set the CG at 100, the nose heavy inverted flight would be okay. but I'm happy with the way it flies and lands other than that, so I think I'll leave at 90mm for now.