Design Box The result of Example 4.3 shows that the aerodynamic center for the NACA 23012 airfoil is located ahead of, but very close to, the quarter-chord point. For some other families of airfoils, the aerodynamic center is located behind, but similarly close to, the quarter-chord point. For a given airfoil family, the location of the aerodynamic center depends on the airfoil thickness, as shown in Figure 4.25. The variation of xac with thickness for the NACA 230XX family is given in Figure 4.25a. Here, the aerodynamic center is ahead of the quarter-chord point, and becomes progressively farther ahead as the airfoil thickness is increased. In contrast, the variation of iac with thickness for the NACA 64-2XX family is given in Figure 4.25b. Here, the aerodynamic center is behind the quarter-chord point, and becomes progressively farther behind as the airfoil thickness is increased. From the point of view of purely aerodynamics, the existence of the aerodynamic center is interesting, but the specification of the force and moment system on the airfoil by placing the lift and drag at the aerodynamic center and giving the value of M’c as illustrated in Figure 4.23, is not more useful than placing the lift and drag at any other point on the airfoil and giving the value of M’ at that point, such as shown in Figure 1.19. However, in flight 0 4 8 12 16 20 24 Airfoil thickness, percent of chord (a) NACA 230XX Airfoil 0 4 8 12 16 20 24 Airfoil thickness, percent of chord (b) NACA 64-2XX Airfoil Figure 4.35 Variation of the location of the aerodynamic center with airfoil thickness, (a) NACA 230XX airfoil, (b) NACA 64-2XX airfoil. (continued)  
