This presentation gives straight advice to calculate induced drag of box wings for the conceptual aircraft design phase. Common for passenger aircraft is a Box Wing Aircraft (BWA) with negative stagger: The forward wing is the low wing. As such the aft wing can use the vertical tail structure for highest h/b ratio. This configuration could use slight positive decalage (more angle of attack on the upper wing) to adapt the upper wing to the downwash from the front wing. However: Positive decalage can lead to separation already at lower angle of attack and hence reduced maximum lift coefficient. A conservative design should do without decalage. An unequal lift share (between forward and aft wing) – as required by static longitudinal stability – does not necessarily lead to increased induced drag at (typical) negative stagger. Wind tunnel measurements and Vortex Lattice Method (VLM) calculations lead to a proposal of "k-values" for the "box wing equation" not far from Prandtl's results.
