%Longitudinal Stability%
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. As we learned earlier, longitudinal stability is the quality which makes an airplane stable about its lateral axis. It involves the pitching motion as the airplane's nose moves up and down in flight. A longitudinally 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. In analyzing stability it should be recalled
that a body that is free to rotate will always turn about its center of
gravity. To obtain static longitudinal stability, the
relation of the wing & 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 moments 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. We have spoken of the airplane's center of gravity and the
airfoil's center of lift in preceding sections. Now let us reexamine the center
of lift or as it is sometimes called, the center of pressure. As previously pointed out, 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.
