Design it so that it knife edges without coupling at various speeds, rudder angles, and throttle settings. Idle downlines should stay straight down. Knife edge flight should require very little angle of attack. It should exit a snap instantly. In a cross wind it should orientate itself at the proper crap angle. It should be draggy enough that a YS 140DZ at idle coming around the downside of a large loop should have to throttle up to maintain a good flying speed.