Optimization of joined-wing aircraft

The joined wing is an innovative aircraft configuration with a rear wing that has its root attached near the top of the vertical tail and a tip that sweeps forward to join the trailing edge of the main wing. This study demonstrates the application of numerical optimization to aircraft design and presents a quantitative comparison of joined-wing and conventional aircraft designed for the same medium-range transport mission. The computer program developed for this study used a vortex-lattice model of the complete aircraft to estimate aerodynamic performance, and a beam model of the lifting-surface structure to calculate wing and tail weight. Weight estimation depended on a fully stressed design algorithm that included a constraint on buckling and a correlation with a statistically based method for total lifting-surface weight. A variety of "optimum" joined-wing and conventional aircraft designs are compared on the basis of direct operating cost, gross weight, and cruise drag. Maximum lift and horizontal tail buckling were identified as critical joined-wing design issues. The addition of a buckling constraint is shown to decrease the optimum joined-wing span and increase direct operating cost by about 4%. The most promising joined-wing designs were found to 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.