Figure 2-11.  Three types of stability. Figure 2-12.  Trim tabs. in straight-and-level flight it remains on its course heading  even though the pilot takes his or her hands and feet off the  controls. If an aircraft recovers automatically from a skid, it  has been well designed for directional balance. The vertical  stabilizer is the primary surface that controls directional  stability. Directional stability can be designed into an aircraft,  where appropriate, by using a large dorsal fin, a long fuselage,  and sweptback wings.  Motion about the aircraft’s longitudinal (fore and aft) axis  is a lateral, or rolling, motion. The tendency to return to the  original attitude from such motion is called lateral stability.  A Dutch Roll is an aircraft motion consisting of an out-of- phase combination of yaw and roll. Dutch roll stability can  be artificially increased by the installation of a yaw damper.  The primary controls are the ailerons, elevator, and the  rudder, which provide the aerodynamic force to make the  aircraft follow a desired flightpath.  [Figure 2-10]  The flight  control surfaces are hinged or movable airfoils designed to  change the attitude of the aircraft by changing the airflow  over the aircraft’s surface during flight. These surfaces are  used for moving the aircraft about its three axes. Typically, the ailerons and elevators are operated from the  flight deck by means of a control stick, a wheel, and yoke  assembly and on some of the newer design aircraft, a joy- stick. The rudder is normally operated by foot pedals on most  aircraft. Lateral control is the banking movement or roll of an  aircraft that is controlled by the ailerons. Longitudinal control  is the climb and dive movement or pitch of an aircraft that  is controlled by the elevator. Directional control is the left  and right movement or yaw of an aircraft that is controlled  by the rudder.  Included in the trim controls are the trim tabs, servo tabs,  balance tabs, and spring tabs. Trim tabs are small airfoils  recessed into the trailing edges of the primary control  surfaces.  [Figure 2-12]  Trim tabs can be used to correct any  tendency of the aircraft to move toward an undesirable flight  Trim Controls Lateral Stability Dutch Roll Primary Flight Controls Trim tabs 2-11 Positive static stability Neutral static stability Negative static stability CG CG CG CG Applied force Applied force Applied force