An airplane in flight is constantly subjected to forces that disturb it from its normal horizontal flight path. Rising columns of hot air, down drafts gusty winds, etc., make the air bumpy and the airplane is thrown off its course. Its nose or tail drops or one wing dips. How the airplane reacts to such a disturbance from its flight attitude depends on its stability characteristics.
Stability is the tendency of an airplane in flight to remain in straight, level, upright flight and to return to this attitude, if displaced, without corrective action by the pilot.
Static stability is the initial tendency of an airplane, when disturbed, to return to the original position.a system is statically stable if, when disturbed from equilibrium, it initially tends to return to the equilibrium configuration;
Dynamic stability is the overall tendency of an airplane to return to its original position, following a series of damped out oscillations.
Stability may be (a) positive, meaning the airplane will develop forces or moments which tend to restore it to its original position; (b) neutral, meaning the restoring forces are absent and the airplane will neither return from its disturbed position, nor move further away; (c) negative, meaning it will develop forces or moments which tend to move it further away. Negative stability is, in other words, the condition of instability. A system is dynamically stable if, when disturbed from equilibrium,
it does eventually return to the equilibrium configuration.
A stable airplane is one that will fly "hands off" and is pleasant and easy to handle. An exceedingly stable airplane, on the other hand, may lack maneuverability.
An airplane which, following a disturbance, oscillates with increasing up and down movements until it eventually stalls or enters a dangerous dive would be said to be unstable, or to have negative dynamic stability.
An airplane that has positive dynamic stability does not automatically have positive static stability. The designers may have elected to build in, for example, negative static stability and positive dynamic stability in order to achieve their objective in maneuverability. In other words, negative and positive dynamic and static stability may be incorporated in any combination in any particular design of airplane.
An airplane may be inherently stable, that is, stable due to features incorporated in the design, but may become unstable due to changes in the position of the center of gravity (caused by consumption of fuel, improper disposition of the disposable load, etc.).
Stability may be (a) longitudinal, (b) lateral, or (c) directional, depending on whether the disturbance has affected the airframe in the (a) pitching, (b) rolling, or (c) yawing plane.
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