Static Margin
If an aircraft in flight suffers a disturbance in pitch that causes an increase (or decrease) in angle of attack, it is desirable that the aerodynamic forces on the aircraft cause a decrease (or increase) in angle of attack so that the disturbance does not cause a continuous increase (or decrease) in angle of attack. This is longitudinal static stability. Static margin is a concept used to characterize the static longitudinal stability and controllability of aircraft and missiles. In aircraft analysis, static margin is defined as the distance between the center of gravity and the neutral point of the aircraft, expressed as a percentage of the mean aerodynamic chord of the wing. The greater this distance and the narrower the wing, the more stable the aircraft. Conventionally, the neutral point is aft of the c.g., although in rare cases (computer controlled fighter aircraft) it may be forward of the c.g., i.e. slightly unstable, to obtain quickness of response in combat. Too great longitudinal stability makes the aircraft "stiff" in pitch, resulting in such undesirable features as difficulty in obtaining the necessary stalled nose-up pitch when landing.
Horizontal Stabilizer
At the rear of the fuselage of most aircraft one finds a horizontal stabilizer and an elevator. The stabilizer is a fixed wing section whose job is to provide stability for the aircraft, to keep it flying straight. The horizontal stabilizer prevents up-and-down, or pitching, motion of the aircraft nose. The elevator is the small moving section at the rear of the stabilizer that is attached to the fixed sections by hinges. Because the elevator moves, it varies the amount of force generated by the tail surface and is used to generate and control the pitching motion of the aircraft. There is an elevator attached to each side of the fuselage. The elevators work in pairs; when the right elevator goes up, the left elevator also goes up. This slide shows what happens when the pilot deflects the elevator. The elevator is used to control the position of the nose of the aircraft and the angle of attack of the wing. Changing the inclination of the wing to the local flight path changes the amount of lift which the wing generates.
