Weight Prediction of the Liftig System for an Unconventional Aircraft Configuration

In this paper a tool, previously developed and validated for the prediction of wing box structural weight in very early design stages, is improved to better match effects on panel sizing of stability constraints. It is applied to an unconventional configuration aircraft based on the Best Wing System concept, introduced by Ludwig Prandtl in 1924, to achieve minimum induced drag: such lifting system is composed of two swept wings (fore and aft) connected by vertical wings at their tips and two fins connecting the rear wing to the fuselage. The system is over-constrained to the fuselage and, thus, the structural design, as well as the static aeroelasticity and flutter characteristics, totally differs from a conventional aircraft. An optimization method finds out a suitable prediction of the structural weight by defining the wing span stiffness behavior compatible with a mix of global and local design constraints, but without any claim about a structural design. The link among stiffness properties and structural weight is made by means of simplified models of the box cross-sections, suitable also to supply responses for the approximated evaluation of local constraints.