I had to mount my 55ax as far forward as possible and the battery is in the front of the wing on the right side. I added a piece of lead on the left wing about 1 ounce to laterally balance it. I too am using 821 servos and an 81 up front for throttle, fly it and see if that is good for you. Mine is still about 1/2 inches behind the recommend CG but it flies good and is very stable.