Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 100

For cambered airfoils, the CP moves along the chord line when the AOA changes. As the AOA increases, the CP moves forward and vice versa. This movement makes calculations involving stability and stress analysis very difficult from an engineering perspective. There is a point on an airfoil where the pitching moment is a constant with changing AOA, if the velocity is constant. This point is called the aerodynamic center (AC).

The AC, unlike the CP, does not move with changes in AOA. If we consider the lift and drag forces as acting at the AC, the calculations will be greatly simplified. The location of the AC varies slightly, depending on airfoil shape. Subsonically, it is between 23 and 27% of the chord back from the leading edge. Supersonically, the AC shifts to the 50% chord.

In summary, the pitching moment at the AC does not change when the angle of attack changes (at constant velocity) and all changes in lift effectively occur at the AC. As an airfoil experiences greater velocity, its AC commonly moves toward the trailing edge, with the AC near 25% chord subsonically and at 50% supersonically. A more detailed discussion on the aerodynamic center of a wing, and its relation to longitudinal stability, is found in Chapter 12.