A Comparative Examination of Some Measurements of Airfoil Section Lift and Drag at Supercritical Speeds

A study was made of the lift and drag characteristics, as determined from wind-tunnel tests, of a number of airfoil sections at supercritical Mach numbers. Semiempirical correlations of supercritical drag data were made for a family of symmetrical airfoils and for several series of cambered airfoils at small and moderate angles of attack. The correlations are of pressure-drag rise per unit chord length as a function of Mach number. For the airfoils considered, there is an essentially unique shape of the drag-rise curve when the angle of attack is that for maximum drag-divergence Mach number. The primary effect of changing the airfoil shape apparently is to change the Mach number at which the drag rise begins. No means have been devised for applying these results to the prediction of supercritical drag characteristics. The lift study consisted primarily of an examination of the separate normal-force components of the upper and lower surfaces of several airfoil sections. One of the most significant observations to be made concerning the lift data studied is that, at moderate positive angles of attack and in the range of Mach numbers for which supersonic flow occurred over only the upper surface, there appeared a marked change in the rate of variation with 1 - M sq2 of the component of the normal force coefficient contributed by the lower surface as the drag-divergence Mach number was exceeded. This change was most abrupt for thicker sections and is the primary cause of the loss of lift at supercritical speeds.