If the airframe is not warped in any way, the reason a lot of trainers tend to bank right as soon as they lift off is because the pilot is holding right rudder that was used to counter-act the left-pulling tendency of the engine during the takeoff roll. As soon as the model lifts off, you need to get off any rudder you were holding.

If the model has strong banking tendencies in the air, then something's off. A control surface isn't centered properly, the wing or tail is out of alignment, and so on. We've seen a case or two where the wing flaps or wingtip droops were not quite aligned. Any of those things can make a difference.

If a surface is warped, and it can't be straightened by re-shrinking the covering, then please contact us about it.