Unified Approximation for Nonparabolic Drag Polars

TLDR: In this article, the minimum drag of the configuration (airfoil or wing) occurs at zero lift and the contribution of pressure drag (kpC 2 L for the airfoil) or pressure and induced (vortex) drag(kp iC l for the wing).

Abstract: where, by convention (excluding the k term), uppercase subscripts apply to wings and lowercase subscripts apply to airfoils. The first term on the right-hand side is the drag coefficient at zero lift and the second term represents the contribution of pressure drag (kpC 2 L for the airfoil) or pressure and induced (vortex) drag (kp iC l for the wing). For wings, both the pressure component of the profile drag coefficient kpC 2 L and the planform-dependent induced drag coefficient kiC 2 L vary with the square of the lift coefficient. This makes their separation difficult [thus the combination kp i parameter in Eq. (1b)]. Equations (1a) and (1b) are used as an approximation in performance estimates for preliminary design, as well as for interpretation and quantification of experimental and computational data. The form of Eq. (1) implies that the minimum drag of the configuration (airfoil or wing) occurs at zero lift. This is generally true for symmetrical airfoils or wings. For cambered airfoils or cambered section wings, a more representative approximation is of the form