Catalogs of experimental airfoil data can be useful in airplane design. It is difficult, however, to understand the physical behavior of airfoils and the relationships between airfoil aerodynamic performance and airfoil geometry simply by studying such data. There clearly is a need for an analytical method that allows the straightforward prediction of airfoil behavior with satisfactory accuracy. Such a theory would allow the role of the airfoil shape parameters to be studied. For example, for a given maximum camber, is it better to place it near the leading or trailing edge of an airfoil to achieve the largest increase in lift coefficient compared to a symmetrical airfoil at the same angle of attack? The answers to this and related questions are derived readily from thin-airfoil theory.
Thin-airfoil theory is an approximate inviscid-flow theory that relies on an assumption of small thickness ratio (i. e., 10 to 12 percent or less) at moderate angle of attack (i. e., several degrees or less). Within this framework, the theory adequately predicts lift and moment for arbitrary thin airfoils. It does not yield information on drag because the D’Alembert’s Paradox interferes, which is a consequence of neglecting viscous-flow effects in constructing a simple theory.