THE LIFTING CHARACTERISTICS OF A FINITE WING

A two-dimensional airfoil with its zero lift line at an angle of attack of 10° will deliver a lift coefficient, Ct, of approximately 1.0. When incorporated into a wing of finite aspect ratio, however, this same airfoil at the same angle of attack will produce a wing lift coefficient, CL, significantly less than 1.0. The effect of aspect ratio is to decrease the slope of the lift curve CLa as the aspect ratio decreases. Figure 3.50 illustrates the principal differences in the

lift behavior of a wing and an airfoil. First, because the wing is twisted so that the tip is at a lower angle of attack than the root (washout), the angle for zero lift, measured at the root, is higher for the wing by approximately 0.6°. Next, the slope of the wing’s lift curve, CLa, is approximately 0.79 of the slope for the airfoil. Finally, is only slightly less than C^, in the ratio of

approximately 0.94. These three differences are almost exactly what one would expect on the basis of wing theory, which will now be developed.