Stability is a condition whereby an object disturbed has a natural tendency to return to its first and normal position. Example: a weight suspended by a cord.
Instability is a condition whereby an object disturbed has a natural tendency to move as far as possible away from its first position, with no tendency to return. Example: a stick balanced vertically upon your finger.
Neutral Instability is a condition whereby an object disturbed has no tendency to move farther than displaced by the force of the disturbance, and no tendency to return to its first position.
In order that an aeroplane may be reasonably controllable, it is necessary for it to possess some degree of stability longitudinally, laterally, and directionally.
Longitudinal Stability in an aeroplane is its stability about an axis transverse to the direction of normal horizontal flight, and without which it would pitch and toss.
Lateral Stability is its stability about its longitudinal axis, and without which it would roll sideways.
Directional Stability is its stability about its vertical axis, and without which it would have no tendency to keep its course.
For such directional stability to exist there must be, in effect,[16] more "keel-surface" behind the vertical axis than there is in front of it. By keel-surface I mean everything to be seen when looking at an aeroplane from the side of it—the sides of the body, undercarriage, struts, wires, etc. The same thing applies to a weathercock. You know what would happen if there was insufficient keel-surface behind the vertical axis upon which it is pivoted. It would turn off its proper course, which is opposite to the direction of the wind. It is very much the same in the case of an aeroplane.
