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. For conceptual design studies root bending moment is often used to assess wing weight. However, this is often a poor indication of wing weight, especially for nonplanar configurations. The weight model used not only retains simplicity, but also accounts for the depth of the wing structural box and variations in area-dependent weight. Stall speed constraints are introduced to move beyond simple trade studies and allow planform optimization without unrealistically small tip chords. The problem of tip extensions vs. winglets is revisited with nonlinear optimization. The choice of a tip extension or a winglet depends on the ratio of the maneuver lift coefficient to the cruise lift coefficient. Design studies are also performed for wings with maneuver load alleviation, and C-wings for tailless aircraft. 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. C-wings may be beneficial for tailless aircraft designed with nose up pitching moment about the aerodynamic center.