Structural optimization for joined-wing synthesis

Abstract

The joined wing is an innovative aircraft configuration with a rear wing, or horizontal tail, that is attached near the top of the vertical tail and sweeps forward to join the trailing edge of the main wing. This study evaluates two structural design methods for application in an aircraft synthesis code and quantifies the differences between these methods for joined wings in terms of weight, stress, direct operating cost, and computational time. A minimum weight optimization method and a fully stressed design method are used to design joined-wing structures. Both methods determine the sizes of 204 structural members, satisfying 1020 stress constraints and five buckling constraints. Monotonic splines are shown to be a very effective way of linking spanwise distributions of material to a few design variables. Five beam buckling constraints for the horizontal tail are included in both design methods. Without this constraint on buckling, the fully stressed design is shown to be very similar to the minimum weight structure. Adding a beam buckling constraint for the horizontal tail increased the structural weight by 13% and produced a fully stressed design that is 0.9% heavier than the minimum weight structure. Using the minimum weight optimization method to design the structure and to save 0.9% in weight required 20 times the computational time. Furthermore, the minimum weight structure produced only a 0.02% savings in direct operating cost. This study suggests that a fully stressed design method based on nonlinear analysis is adequate for a joined-wing synthesis study. The same joined wing considered in this study was shown, in an earlier study, to be slightly more expensive to operate than a conventional configuration designed for the same medium range transport mission. Since the same fully stressed design method was used in this earlier study, this work supports the comparisons of joined-wing and conventional aircraft performance presented in the earlier study. Of course, a different set of mission specifications and design assumptions may produce joined wings that perform significantly better.