The centre of pressure (CP) is defined as the point about which the resultant moment is zero; the net effect of the flow in terms of forces and moments would be the same as if the resultant force vector were located at the CP. The problem with using the CP in the analysis of aircraft stability and flight dynamics is that the CP is a function of incidence α and moves around considerably; indeed it is not always even within the aerofoil.
The flow-field around an aircraft results in a pressure distribution that in turn gives rise to a normal pressure force and a surface shear force distribution. Just as with any system of forces, these complex force distributions can be represented by a simple equivalent force system that provides: 1. An identical resultant force; 2. An identical moment. (Identical about any point.) Furthermore, the resultant aerodynamic force can be resolved into lift, drag, and (in asymmetric flight) side-force components; see Fig. 2.1-1. The flow also produces a resultant moment about any given point, e.g. ‘a' in Fig. 2.1-1. This is denoted Ma. The moment about a different point would or course be different.