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Stability | |
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Stability can be defined as either static stability or dynamic stability. Each is concerned with an object's ability to go back to the way it was before a disturbance affected it. 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, a smooth bowl, and a flat surface. Place the marble gently in the bottom of the bowl so that it is not moving. It is stationary; this is the original position, defined by engineers to be the 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 after a disturbance such as a wind gust. If the bowl is turned over, 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 original position! This is called a statically unstable system. Statically unstable airplanes need constant control intervention to keep the airplane on course, otherwise, like the marble, the plane will move any way the forces on it dictate. An unusual situation can be demonstrated using the marble on a flat surface like a table. Place the marble gently on the table so that it is stationary. If it is given a small push, it will roll for a while until friction causes it to come to a stop. It is no longer moving, like its initial orientation, but its position has changed. This is called a neutrally stable system, and it is extremely rare for airplanes. Static stability is concerned solely with the initial tendency of the object to return to its original orientation. Dynamic stability is concerned with the time history of the object's motion as it attempts to return to the equilibrium 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 to rest in the bottom of the bowl. If the push was a simple movement to the side, the marble will travel back and forth (oscillate) over the original position. Eventually, friction and the gravitational attraction will cause it to come to rest at the original spot. If the push also had a sideways force to it, the marble may also spiral in to the center of the bowl. If the object eventually does return to its original orientation, then the system is considered dynamically stable. If it does not, then it is considered dynamically unstable. If the marble in the bowl was given a push up the side of the bowl, it would travel down across the bottom of the bowl and up the other side. If it hit a grease slick on the other side of the bowl, so that the friction force was decreased and the velocity didn't decrease as fast, it could run up the side of the bowl and fly out. That would be a dynamically unstable system! The marble would not be returning to the original position at any time.
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