I have a Somethin' Extra that has been modified with larger tail feathers and a K&B 61. It does the most perfect knife edge possible. There is no coupling at all, at any speed, and any rudder deflection. It has the stock flat wing tips, and wings rock like crazy going into a hover, but my cg is way fwd right now. It won't harrier because of the wing rock issue. I am moving the cg before I fly again, just to see if I can make it better. It's basically the same plane as the Twist.