Aerodynamic center

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 airfoil, 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 .

Forces (lift/drag) can be summed up to act through a single point, the center of pressure, about which sum of all moments equal zero. The center of pressure location however, changes significantly with a change in angle of attack and is thus impractical for analysis. Thus the 25% chord position, or assumed aerodynamic center, is taken about which the forces and moment are generated. At the 25% chord position the moment generated was found and proven to be nearly constant with varying angle of attack. The concept of the aerodynamic center (AC) is important in aerodynamics. It is fundamental in the science of stability of aircraft in flight.

Please note that in highly theoretical/analytical analysis the aerodynamic center does vary slightly and changes location. In most literature however the aerodynamic center is taken at the 25% chord position. This conversely means that if one keeps the assumed AC fixed at 25% chord, that the moment about 25% chord can be 'not constant' depending on the airfoil. A large portion of cambered airfoils have non constant moments about the 25% chord position because the AC does vary slightly. For most analysis the non constant moment about the 25% chord position is not significant enough to warrant consideration, but is important to keep in mind.

For symmetric airfoils in subsonic flight the aerodynamic center is located approximately 25% of the chord from the leading edge of the airfoil. This point is described as the quarter-chord point. This result also holds true for 'thin-airfoils'. For non-symmetric (cambered) airfoils the quarter-chord is only an approximation for the aerodynamic center.

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Wikipedia