Aerodynamic Force System
Figure 1.13 illustrates several features of the force system on a moving body that is the focus of considerable attention throughout this book. The aerodynamic forces represent the integrated effect of a continuous distribution of pressure and shear forces acting on all of the exposed surfaces of the vehicle. The shape of the vehicle plays a crucial role in determining both the magnitude of the forces and their line
of action. In particular, notice that the net aerodynamic forces act at a point defined in the figure as the center of pressure. Clearly, to predict the vehicle-flight characteristics, it is necessary to know the precise location of this point relative to, say, the center of mass through which the vehicle weight acts. In level equilibrium flight, a component of the lift force is called on to balance the weight, W, of the airplane, expressed in vector form as W = mg, where m is the mass of the vehicle and g is the gravitational acceleration vector. Recalling the fundamental concepts of statics, it is clear that the balance of moments about the mass center also must be considered. Again, the latter considerations form the subject of stability and control; we concentrate here on the generation of the forces and moments and their relationship to properties of the airflow and vehicle shape.