An object, such as an aerofoil, moving through a fluid disturbs the surrounding flow field. The fluid velocity varies along the aerofoil surface creating a variation in pressure as indicated in Fig. 7. The figure shows the pressure coeficient cp(x) where cp = (p-p∞)/q with q as dynamic pressure and p∞ as undisturbed pressure far away from the aerofoil. Integrating the pressure coeficient along the object's surface gives the aerodynamic loads of lift, drag and moment.
Pressure coeficient around aerofoil NACA 4412 at Mach 0.3 and αr = 1.0o.
The centre of pressure is defined as the average location of the pressure where the resulting forces act. This is similar to defining the centre of gravity as the average location of the weight. The aerodynamic moment at the centre of pressure is zero. This point varies with the angle of attack, while for an aerofoil at subsonic speeds it is close to the quarter chord point.
Similarly, the aerodynamic centre is defined as the point along the chord where the aero-dynamic moment is independent of the angle of attack. For an aerofoil at subsonic speeds, the aerodynamic centre is close to the quarter chord point, while at supersonic speeds the points shifts to the mid-chord location.