Induced Drag Estimation of Box Wings for Conceptual Aircraft Design

Abstract

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.