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Stability | |
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Stability can either be static or dynamic. Static stability involves only the return of the disturbed object to its original position. Dynamic stability is concerned with how much time it may take for the object to return to its original position. Static stability can easily be demonstrated with a marble, and a round bowl on a flat surface. Place the marble gently in the bottom of the bowl so that it is not moving (stationary). This is the original or equilibrium position. If the marble is given a light push in one direction, it will try to return to the center of the bowl, and eventually, it will return to the original equilibrium position. This is called a statically stable system. This was the goal of the early airplane designers; that the airplane would try to return to its original orientation (position) after a disturbance, such as a gust of wind. If the bowl is turned over (round bottom up), and the marble placed gently on the top, any slight touch will cause the marble to pick up speed and roll off the bowl. It will not return to its originial position! This is called a statically unstable system. Statically unstable airplanes need constant attention to control to keep the airplane on course, otherwise, like the marble, the plane will move any way the forces push it. Place the marble on a flat surface, such as a table. If it is given a small push, it will roll for a while until the friction causes it to come to a stop. It is no longer moving and will not return to its original position. This is called a neutrally stable system. Airplanes normally do not follow this pattern.
Dynamic stability is concerned with how long it takes an object to
return to its original position. Again, the marble in the bowl can be
used to illustrate this concept. Place the marble at the bottom of the
bowl. After it is moved slightly, watch how long it takes to come back
to its original position. If the object eventually does return to its
original position, then the system is considered dynamically stable. If
it does not, then it is considered dynamically unstable. If the marble
is given too much of a push, it could fly out of the bowl and never
return to its original position!
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