In designing an airplane, a great deal of effort is spent in developing the desired degree of stability around all three axes.
But longitudinal stability about the lateral axis is considered to be the most affected by certain variables in various flight conditions.
Longitudinal stability is the quality that makes an airplane stable about its lateral axis. It involves the pitching motion as the airplane's nose moves up and down in flight. Alongitudinally unstable airplane has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall. Thus, an airplane with longitudinal instability becomes difficult and sometimes dangerous to fly.
Static longitudinal stability or instability in an airplane, is dependent upon three factors:
- Location of the wing with respect to the center of gravity;
- Location of the horizontal tail surfaces with respect to the center of gravity; and
- The area or size of the tail surfaces.
To obtain static longitudinal stability, the relation of the wing and tail moments must be such that, if the moments are initially balanced and the airplane is suddenly nosed up, the wing moments and tail moments will change so that the sum of their forces will provide an unbalanced but restoring moment which, in turn, will bring the nose down again.
Similarly, if the airplane is nosed down, the resulting change in moments will bring the nose back up. The center of lift, sometimes called the center of pressure, in most unsymmetrical airfoils has a tendency to change its fore and aft position with a change in the angle of attack. The center of pressure tends to move forward with an increase in angle of attack and to move aft with a decrease in angle of attack. This means that when the angle of attack of an airfoil is increased, the center of pressure (lift) by moving forward, tends to lift the leading edge of the wing still more. This tendency gives the wing an inherent quality of instability.
