Box Wing and Induced Drag: Compressibility Effects in Subsonic and Transonic Regimes

Prandtl introduced the best wing system or Box Wing a century ago and showed its exceptional lift-induced drag performance with respect to wing systems having the same wingspan and lift (Prandtl, L., "Induced Drag of Multiplanes," NACA TN-182, 1924.). In this work, and for the first time, their superior induced drag properties are verified in compressible and transonic regimes. The aerodynamic forces are obtained by analyzing computational fluid dynamics data, and the drag is subdivided in induced, viscous, and wave contributions. This decomposition is provided by a recently introduced aerodynamic force breakdown technique based on the vortex-force theory and allows an unambiguous definition of the lift-induced drag. The methodology is applicable to incompressible, compressible, and transonic flows. This paper will address whether the Best Wing system provides interesting induced drag performance also in transonic viscous flow.