Mash the cg would stay the same its around 3.1/2 in back i liked mine a little tail heavy i installed a OS61 and added a dubro 12oz tank and it flys fine. i added 4oz to the nose at most for balance on my gp balance machine and slowly removed some to help slow her down on landings. and as far as wasting extra money for new control horns like some people suggest just use all theres.........theres nothing wrong with the hardware. infact i like there tail wheel compared to most . rather than spend money on hard ware buy a aluminum nose cone rather than using there plastic one.i just built a [link=http://www.aero-works.net/store/detail.aspx?ID=75]aeroworks[/link] 540T with a OS46FX it was a little tail heavy added no weight and i love the way it flys light as a feather. good luck BILL


CHOPPER DUDE the 91OS weighs the same as there 61 are you balancing the plane upside down?..... as you are supossed to