Longitudinal Balance, Stability and Control
A longitudinal stabilizer is used to maintain the aircraft in longitudinal balance, or trim: it exerts a vertical force at a distance so that the summation of pitch moments about the center of gravity is zero. The vertical force exerted by the stabilizer to this effect varies with flight conditions, in particular according to the aircraft lift coefficient and wing flaps deflection which both affect the position of the center of lift, and with the position of the aircraft center of gravity (which changes with aircraft loading). Transonic flight makes special demands on horizontal stabilizers, since the crossing of the sound barrier is associated with a sudden move aft of the center of lift.
An other role of a longitudinal stabilizer is to provide longitudinal static stability. Stability can be defined only when the vehicle is in trim; it refers to the tendency of the aircraft to return to the trimmed condition if it is disturbed. This maintains a constant aircraft attitude, with unchanging pitch angle relative to the airstream, without active input from the pilot. Since obtaining static stability often requires that the aircraft center of gravity be ahead of the center of lift of a conventional wing, a stabilizer positioned aft of the wing is then often required to produce negative lift.
The elevators serve to control the pitch axis; in case of a fully movable tail, the entire assembly acts as a control surface.
Read more about this topic: Stabilizer (aircraft)
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