Rudder  V-Tail  Secondary Flight Controls  Figure 6-15.  The effect of left rudder pressure.  Figure 6-16.  Beechcraft Bonanza V35.  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  deflections at high speeds may be required to provide the  desired reaction. In propeller-driven aircraft, any slipstream  flowing over the rudder increases its effectiveness.  The V-tail design utilizes two slanted tail surfaces to perform  the same functions as the surfaces of a conventional elevator  and rudder configuration. The fixed surfaces act as both  horizontal and vertical stabilizers.  [Figure 6-16]  The movable surfaces, which are usually called ruddervators,  are connected through a special linkage that allows the control  wheel to move both surfaces simultaneously. On the other  hand, displacement of the rudder pedals moves the surfaces  differentially, thereby providing directional control.  When both rudder and elevator controls are moved by the  pilot, a control mixing mechanism moves each surface the  appropriate amount. The control system for the V-tail is more  complex than the control system for a conventional tail. In  addition, the V-tail design is more susceptible to Dutch roll  tendencies than a conventional tail, and total reduction in  drag is minimal.  Secondary flight control systems may consist of wing flaps,  leading edge devices, spoilers, and trim systems.  Flaps are the most common high-lift devices used on aircraft.  These surfaces, which are attached to the trailing edge of  the wing, increase both lift and induced drag for any given  AOA. Flaps allow a compromise between high cruising  speed and low landing speed because they may be extended  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]  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$_{L}$) at a given AOA. At the same time, it  greatly increases drag and moves the center of pressure (CP)  aft on the airfoil, resulting in a nose-down pitching moment.  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  as for large ones. Slotted flaps increase the lift coefficient  significantly more than plain or split flaps. On small aircraft,  the hinge is located below the lower surface of the flap, and  Flaps  6-8  A e r o d y n a mi c f o r c e Left rudder  CG  Left rudder forward  Y a w 