Flight Controls

Rudder The rudder controls movement of the aircraft about its vertical axis. This motion is called yaw. Like the other primary control surfaces, the rudder is a movable surface hinged to a fixed surface in this case, to the vertical stabilizer or fin. The rudder is controlled by the left and right rudder pedals. When the rudder is deflected into the airflow, a horizontal force is exerted in the opposite direction. [Figure 6-15] By pushing the left pedal, the rudder moves left. This alters the airflow around the vertical stabilizer/rudder and creates a sideward lift that moves the tail to the right and yaws the nose of the airplane to the left. Rudder effectiveness increases with speed; therefore, large deflections at low speeds and small Figure 6-16. Beechcraft Bonanza V35. deflections at high speeds may be required to provide the desired reaction. In propeller-driven aircraft, any slipstream appropriate amount. The control system for the V-tail is more flowing over the rudder increases its effectiveness. complex than the control system for a conventional tail. In addition, the V-tail design is more susceptible to Dutch roll V-Tail tendencies than a conventional tail, and total reduction in The V-tail design utilizes two slanted tail surfaces to perform drag is minimal. the same functions as the surfaces of a conventional elevator and rudder configuration. The fixed surfaces act as both Secondary Flight Controls horizontal and vertical stabilizers. [Figure 6-16] Secondary flight control systems may consist of wing flaps, leading edge devices, spoilers, and trim systems. The movable surfaces, which are usually called ruddervators, are connected through a special linkage that allows the control Flaps wheel to move both surfaces simultaneously. On the other Flaps are the most common high-lift devices used on aircraft. hand, displacement of the rudder pedals moves the surfaces These surfaces, which are attached to the trailing edge of differentially, thereby providing directional control. the wing, increase both lift and induced drag for any given AOA. Flaps allow a compromise between high cruising When both rudder and elevator controls are moved by the speed and low landing speed because they may be extended pilot, a control mixing mechanism moves each surface the when needed and retracted into the wing’s structure when not needed. There are four common types of flaps: plain, split, slotted, and Fowler flaps. [Figure 6-17] Yaw The plain flap is the simplest of the four types. It increases the airfoil camber, resulting in a significant increase in the coefficient of lift (C ) at a given AOA. At the same time, it L greatly increases drag and moves the center of pressure (CP) aft on the airfoil, resulting in a nose-down pitching moment. CG Left rudder forward The split flap is deflected from the lower surface of the airfoil and produces a slightly greater increase in lift than the plain flap. More drag is created because of the turbulent air pattern produced behind the airfoil. When fully extended, both plain and split flaps produce high drag with little additional lift. The most popular flap on aircraft today is the slotted flap. Variations of this design are used for small aircraft, as well Left rudder Aerodynamic fo rce a si s g f n o if r i c la a r n g t e ly o m n o es re . S th l a o n tt e p d la f in la o p r s s i p n l c it r e fl a a s p e s . t h O e n l s if m t a c l o l e a f i f r i c c r i a e f n t t , the hinge is located below the lower surface of the flap, and Figure 6-15. The effect of left rudder pressure. 6-8