Position of the Aerodynamic Center

The position of the aerodynamic center is important in designing the blade to be free of flutter. The more aft the aerodynamic center is, the less weight will have to be installed in the nose to achieve the proper balance. It has been estimated that

moving the aerodynamic center back 2% of the chord can result in a 10% saving in total blade weight. The positions of the aerodynamic center of NACA airfoils are listed in reference 6.1. For the four – or five-digit airfoils, the aerodynamic center is on or ahead of the quarter chord; for the six-series airfoils, the aerodynamic center is behind the quarter chord. For a given thickness ratio, the position of the aerodynamic center is roughly proportional to the enclosed trailing edge angle, as shown in Figure 6.38. The addition of a flat tab to the trailing edge is an effective way to decrease the trailing edge angle and to move the aerodynamic center aft. The measured effect of tabs on several airfoils is as follows: