On a typical aerofoil, a point exists about which the resultant aerodynamic pitching moment M is independent of incidence. This special point is called the Aerodynamic Centre (AC). The numerical value of the pitching moment about AC is denoted MAC. For standard aerofoils in subsonic flows, the Aerodynamic Centre lies approximate ¼ of the chord behind the leading edge (i.e. h=xAC/c ≈ ¼). For supersonic flows, the AC moves abruptly aft to around h ≈ ½; see Fig. 2.2-1. Since MAC is approximately constant over a wide range of CL including zero, MAC is sometimes referred to as the zero-lift pitching moment and denoted M0.