Structural and aerodynamic optimization of joined-wing aircraft

The joined wing is an innovative aircraft configuration with a rear wing that is attached near the top of the vertical tail and sweeps forward to join the trailing edge of the forward wing. This study evaluates the performance of joined-wing aircraft and demonstrates the use of numerical optimization in aircraft design. Initially, a parametric design study that considered a single cruise condition indicated an 11% savings in trimmed drag for a joined-wing with the same lifting surface area and 23% longer wing span than a conventional configuration. These results encouraged further study of the joined-wing concept and motivated the development of a computer program that uses numerical optimization to design both joined-wing and conventional configurations. This design program uses a vortex-lattice model of all aircraft components to calculate aerodynamic forces and a beam model of the lifting-surface structure to calculate wing and tail weight. Weight estimation depends on a fully-stressed design algorithm that includes a constraint on buckling and a correlation with a statistically based method for total lifting-surface weight. This fully-stressed sizing routine produced joined-wing structures that are nearly identical to minimum-weight structures designed using numerical optimization. A variety of "optimum" joined-wing and conventional aircraft designs were compared on the basis of direct operating cost, gross weight, and cruise drag. Maximum lift and tail buckling were identified as critical joined-wing design issues. The addition of a buckling constraint was shown to decrease the optimum joined-wing span by 8% and increase direct operating cost by 4%. Although aeroelasticity and dynamic stability were neglected during the design study, separate analyses showed that the optimum configurations have sufficient damping for good handling qualities and flutter speeds that are well above the design dive speed. The most promising joined-wing designs have a joint location at about 70% of the wing semispan, a fuel tank in the tail to trim, and a flap spanning 70% of the wing. These designs are shown to cost 3% more to operate than a conventional configuration designed for the same medium-range mission.