a la wiki
The
torques or moments acting on an
airfoil moving through a
fluid can be accounted for by the net
lift applied at some point on the foil, and a separate net pitching moment about that point whose magnitude varies with the choice of where the lift is chosen to be applied. The
aerodynamic center is the point at which the
pitching moment coefficient for the airfoil does not vary with
lift coefficient i.e.
angle of attack, so this choice makes analysis simpler
[1].
long story short the point where all aerodynamic forces appear to be acting, will move according to configuration, angle of attack, etc.....