Improved Stall Prediction for Swept Wings Using Low-Order Aerodynamics

Abstract

The ability of low-order aerodynamic prediction methods, such as the vortex lattice method, to predict the force and moment characteristics of arbitrary wing geometries, including those with sweep, is well established for pre-stall conditions. Approaches to augment such methods by modeling the flow separation as an effective reduction in camber has allowed extension of the predictive capability to stall and post-stall conditions for unswept wings. Such approaches assume locally two-dimensional flow and use airfoil lift curves for the wing sections to provide the decambering corrections. For swept wings, spanwise pressure gradients cause tipward transport of the separated flow, resulting in modified stall behavior, with attached flow in the inboard regions and excessively separated flow outboard. As a result of this behavior, use of airfoil lift curves in the application of the decambering approach to swept wings results in poor prediction of stall characteristics. This paper explores the use of modified lift curves for the sections of swept wings, with modifications derived from analysis of RANS computational results. When these modified lift curves are used in the decambering approach, the low-order predictions for the swept wings are seen to agree well with RANS CFD predictions. The results indicate that the strip-theory approach is still applicable for swept wings provided the effect of spanwise redistribution of flow separation is correctly taken into consideration. This success provides impetus to develop an entirely predictive version of the low-order method.