Directional Stability and Control of Canard Airplanes
Vertical tail length, or the distance from the center of gravity to the vertical tail aerodynamic center, is typically short for canards as compared with tail-aft configurations. Plan-view drawings for two Beech aircraft illustrate this (Figure 17.1). The Super King Air B200 and Starship 1 have similar gross weights (12,500 to 14,000 pounds) and have fuselage lengths of about 44 feet. The canard Starship’s vertical tail length is 18 feet; that for the tail-last King Air is 25 feet, or about 40 percent greater. The tail-last configuration would have better directional stability and control, assuming equal vertical lifting surface effectiveness for both aircraft.
Note however that the canard Starship’s vertical tails are at the wing tips. In that location, vertical tail effectiveness is not increased by fuselage end plating. The fuselage-mounted vertical tail for the King Air tail-last configuration benefits from fuselage end plating to the extent of about a 50-percent increase in lift curve slope and in effectiveness.
Lower directional stability and control levels in canard configurations can be corrected by large vertical surfaces, at the expense of higher weight and cost. The original tip-mounted vertical tails of the Rutan Vari Eze were found to be too small in NASA wind-tunnel tests (Yip, 1985). Low directional stability levels are associated with adverse yaw in rolling and poor lateral control. Low directional control power leads to control problems in takeoff and in landings in crosswinds or with asymmetric power.
According to Professor Jan Roskam of Kansas University, directional stability on the Piaggio P.180 Avanti business airplane, which has a canard surface and a relatively short vertical tail length, was improved greatly at high angles of attack with strakes located at the fuselage rear.