Preliminary design optimization of joined-wing aircraft

The joined wing is an innovative aircraft configuration that has a its tail connected to the wing forming a diamond shape in both top and plan view. This geometric arrangement utilizes the tail for both pitch control and as a structural support for the wing. Several researchers have studied this configuration and predicted significant reductions in trimmed drag or structural weight when compared with a conventional T-tail configuration. Kroo et al. compared the cruise drag of joined wings with conventional designs of the same lifting-surface area and structural weight. This study showed an 11 percent reduction in cruise drag for the lifting system of a joined wing. Although this reduction in cruise drag is significant, a complete design study is needed before any economic savings can be claimed for a joined-wing transport. Mission constraints, such as runway length, could increase the wing area and eliminate potential drag savings. Since other design codes do not accurately represent the interaction between structures and aerodynamics for joined wings, we developed a new design code for this study. The aerodynamic and structural analyses in this study are significantly more sophisticated than those used in most conventional design codes. This sophistication was needed to predict the aerodynamic interference between the wing and tail and the stresses in the truss-like structure. This paper describes these analysis methods, discusses some problems encountered when applying the numerical optimizer NPSOL, and compares optimum joined wings with conventional aircraft on the basis of cruise drag, lifting surface weight, and direct operating cost (DOC).