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I understand that the Aerodynamic Centre (AC) is the point in the aircraft where the pitching moment never changes with a change in Angle of Attack (AoA). My understanding of the Centre of Pressure (CoP) is that it is the resultant pressure point in the airfoil and it slides rearward when AoA decreases and vice-versa, i.e. it changes its position with regard to the AoA.
Now the point is, how does the pitching moment (the force arm, i.e. the distance between the CoP and the AC multiplied by the force) remain constant when the CoP itself moves with the change in AoA?