I would check servo throws to make sure you are getting equal deflection on both ailerons. Also check radio to see if one side is coupled to high rate and one on low if it has two servos. This is a used plane and you don't know if the first owner set it up properly. Or if a single servo are the control rods set equal distance from the center point of the servo and is the servo centered when the trim is centered. Is there and end point adjustment on the radio that is preventing the servo from rotating equally left and right? Hope this helps and good luck.