Beyond the Elliptical Span Load: Optimizing Minimum Induced Drag Using Enhanced Leading Edge Suction

Abstract

The elliptical spanwise lift distribution is widely regarded as being the most aerodynamically efficient for any planform. Many studies conclude that introducing nonplanar geometry, such as winglets, C-wings, box-wings, etc., may increase the efficiency of a given, base planform. We conducted configuration design studies considering aerodynamic effects in an incompressible, inviscid flow to investigate the influence of basic geometric parameters on induced drag. Planar and non-planar configurations were simulated and compared using a vortex-lattice method CFD code to solve the potential flow. Planar configurations, with varying sweep angles, taper ratios, and aspect ratios, supported conventional belief that elliptical loadings are the most aerodynamically efficient. Nonplanar studies, incorporating both spanwise geometric twist and dihedral angles, revealed the theoretical existence of non-elliptical span loads of much greater efficiency. These span loads exploit the combined effects of sweep, taper, dihedral, twist (geometric and aerodynamic), and leading edge suction. Analysis of high efficiency configurations revealed further reductions in induced drag are possible using airfoil sections that enhance the effects of leading edge suction. The most efficient planar and non-planar configurations are compared, leading to the realization that spanwise twist must be incorporated into the planform preliminary design phase if maximum aerodynamic efficiency is to be achieved. Physically feasible planform configurations and design guidelines for optimal aerodynamic span loads are proposed.