In this work four nonplanar wing configurations were studied using the vortex-lattice method: the wing-winglet, the C-wing, the biplane, and the box wing. It has been shown that linear twist, which is more practical in aeronautical construction, is more than adequate to achieve the higher values of span efficiency factor obtained by a completely optimized twist distribution along the span camber, the latter being in general highly varying and thus not very practical. It has also been shown that moderate sweep can slightly increase the span efficiency factor and further reduce vortex drag. The increase is more important for the C-wing, the wing-winglet, and the biplane than it is for the box wing. For the C-wing and wing-winglet configurations, it was found that staggering the vertical extension longitudinally backwards to around two-thirds of the wing tip chord had a non negligible positive effect on the span efficiency factor. Consequently, combining stagger and moderate sweep gave these two configurations a slight edge over the box wing in terms of the value of the span efficiency factor. The latter configuration is known to have the higher value of this important aerodynamic parameter, but only when no sweep and stagger are applied.

When comparing the overall lift-to-drag ratio, the biplane and then the box wing configuration have the highest values. Reduction in vortex drag can be viewed as more important than obtaining a higher lift-to-drag ratio for the wing configuration since vortex drag takes a much higher percentage of the overall drag of an airplane than does the profile drag of the wings alone.



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