Imagine a weight suspended on a string. If you hold the end of the string such that the weight hangs below and you move the weight out to one side and release it.
At first, the weight will swing back towards its initial position. It will overshoot and continue to swing back and forth, but each oscillation will get smaller and smaller in size. This is an example of positive stability.
Aircraft also have varying degrees of stability, and their tendency to return to their original position after a disturbance (such as turbulence) determines their stability. A stable aircraft will resist change and an unstable aircraft will respond rapidly to the pilot’s control inputs. It can be said that stability and maneuverability are opposites.
Static stability describes the initial response of an aircraft after it is disturbed by a gust. For example, if a gust causes the nose to pitch up and the initial tendency is for the nose to pitch down towards its initial position, it is said to have posititive static stability.
If the nose remains in the position is was disturbed to with no further tendency to pitch up or down, it is said to have neutral static stability.
If, after the same gust, the nose continues to pitch up beyond where it was originally disturbed to, the aircraft is said to have negative static stability.
It is desirable for aircraft to have some positive static stability in all axes.
Dynamic stability describes the tendency of the oscillations over time after the initial response to a disturbance.
Consider again an aircraft that is pitched nose up after an initial disturbance. The positive static stability will cause the nose to pitch down towards its original position.
If the up and down pitching oscillations reduce in size over time, the aircraft is said to have positive dynamic stability.
If the pitching oscillations remain the same over time, the aircraft is said to have neutral dynamic stability.
If the pitching oscillations increase in magnitude over time, then the aircraft is said to have negative dynamic stability.
The aircraft must have positive static stability in order to have any dynamic stability.
It is desirable to have both positive static stability and positive dynamic stability in aircraft.