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Old 7th Jan 2011, 18:11
  #6 (permalink)  
Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,342
Let my try:

When a number of forces are exerted on different points of a body, the net effect of all those forces on the body (both lineal and angular) is the same as the effect of a single force acting on a particular point.

This resultant force is the sum of all the forces, and the moment that this force creates about any point is the sum of the moments of all the forces about that point.

Each air molecule exerts a force on the airfoil. The resultant of all the forces is Lift, and the point at which Lift acts is the center of pressure.

The relative position of the CP with respect to the CG determines the necessary tailplane lift to trim the airplane.

We use the center of pressure for static situations. But it is better to use the aerodynamic center when it comes to non-static situations.

When there is a change in Lift we can think of the Lift after the change as being the sum of the Lift before the change plus the Lift increment. Understand? think of two vectors, one being the "old" Lift and the other the lift increment. The point at which the lift increment is acting is the Aerodynamic Center.

The relative location of the AC with respect to the CG will determine the degree of stability of the airplane.
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