Rotaryphile

Slotted flaps are aerodynamically more efficient, and thus should produce less turbulent airflow over the tail feathers at a given airspeed. Slotted flaps can also produce much higher lift than plain flaps, enabling the airplane to fly slower. As the airspeed lowers, the horizontal tail, which does not normally have the benefit of slots, will have a tougher and tougher time developing the necessary stabilization force. From this viewpoint, slotted flaps could reduce stability and controllability. Yaw stability also suffers, since the vertical tail will also have less effect at lower airspeeds, and the airplane will be more prone to stall and snap roll. I would increase at least the vertical tail area considerably for use with slotted flaps, unless the airplane had a very generously proportioned vertical tail, a rather rare occurence.