Tip Extensions, Winglets, and C-wings: Conceptual Design and Optimization

Conceptual wing design analysis methods are combined with numerical optimization to find minimum drag wings subject to constraints on lift, weight, pitching moment, and stall speed. Minimum drag tip extensions and winglets are compared using nonlinear optimization. The minimum drag trapezoidal tip device depends on the ratio of the maneuver lift coefficient to the cruise lift coefficient, although the difference in performance is small. The importance of accounting for the depth of the wing structural box in the weight model, and including constraints on stall speed are highlighted. C-wings are investigated for their potential to enhance performance beyond that offered by wings with winglets. C-wings are found to be a reasonable design alternative for tailless aircraft with requirements for nose-up pitching moment about the aerodynamic center. This may be especially true for span-constrained or low sweep designs. Optimizing C-wing designs introduces several subtleties into the optimization process including the necessity of evaluating the maneuver load at various lift coefficients, and constraining the wing skin thickness. Finally, the method is applied to wings with active load alleviation. Active load alleviation allows reductions in drag on the order of 15%, however further analysis of the structural dynamics is necessary to more fully quantify the results.