I don't have reference other than JAA ATPL Aerodynamics textbooks and some (limited) knowledge from my studies. Anyway, the aerodynamic centre is the point on the airfoil, where the turning moment is constant with change of the AOA. Then you add the fuselage, engines, empenage etc., which MOVE the a.c. of the whole aircraft to a different position - but it still exists and is the point of neutral stability. However, I stand by to be corrected by the more knowledgeable
