1 A relative Assessment of existing Potential-Methodologies to accurately estimate the Induced Drag of Highly Non-planar Lifting Systems Julian C. Schirra * RMIT University, Melbourne, Victoria, 3000, Australia FH Aachen University of Applied Sciences, Aachen, North Rhine-Westphalia, 52064, Germany Jonathan H. Watmuff † RMIT University, Melbourne, Victoria, 3000, Australia and J.-Michael Bauschat ‡ FH Aachen University of Applied Sciences, Aachen, North Rhine-Westphalia, 52064, Germany Induced drag estimates are provided for a biplane, a box wing and a swept box wing configuration by means of selected potential-methodologies. Based on parametric studies, the influence of wake modeling on associated key design parameters is evaluated. The geometric relation of the effective height-to-span ratio and the angle of attack is considered. Its impact on the estimation is investigated separately. For any non-zero staggering, non-linear wing-wake interactions are found to have considerable influence on the induced drag of both box wing configurations. To enable accurate induced drag prediction in this case, the rolled-up wake shape should be resolved explicitly. In contrast to the biplane, an entire substitution of the force-free wake is not feasible. An estimation involving higher angles of attack is found to be problematic. The ability of existing potential- methodologies to accurately estimate the induced drag cannot generally be presumed. Nomenclature CD = drag coefficient CL = lift coefficient D = drag L = lift M = Mach number St = staggering b = span c = chord e = span efficiency factor q = dynamic pressure h/b = height-to-span ratio ɑ = angle of attack Λ = aspect ratio λ = taper ratio η = relative span σ = interference factor * Ph.D. Student, School of Aerospace, Mechanical and Manufacturing Engineering, julian.schirra@student.rmit.edu.au. $ Research engineer, Faculty of Aerospace Technology, Department of Flight Systems, Flight Guidance and Control,. $ schirra@fh-aachen.de. † Senior Lecturer, School of Aerospace, Mechanical and Manufacturing Engineering, jon.watmuff@rmit.edu.au.. ‡ Professor, Faculty of Aerospace Technology, Department of Flight Systems, Flight Guidance and Control, . bauschat@fh-aachen.de. ϕ = sweep angle Abbreviations: BF = body-fixed EP = Eppler FF = force-free FSF = freestream-fixed HPM = higher-order panel method LE = leading edge LSM = lifting-surface method MLM = multi-lifting line method NP = non-planar OPT = optimum TR = Trefftz plane VLM = vortex-lattice method