The Lifting System of a Prandtlplane, Part 1: Design and Analysis of a Light Alloy Structural Solution

The lifting system of a PrandtlPlane is a box in the front view, made of two swept wings set at different heights and of vertical wings at their tips; one or two fins connect the rear wing to the upper fuselage, and the front wing is clamped to the same; thus, the wing system is over-constrained, and the structural design becomes a challenge. In this paper, starting from a preliminary solution, an optimization procedure is applied to minimize the overall weight with the constraints of maximum allowable stress, stability of compressed structures, minimum aileron effectiveness, aeroelastic effects on load distribution and flutter. The system is approximated as beams with 54 elements. Two models of the wing section are presented: in the first, the wing section is approximated with three parameters (skin thickness, spar web thickness and stringer section total area) for a total of 162 degrees of freedom; in the second one, two other parameters are adjoined in any section (areas of two spar flanges) with a total of 270 d.o.f. According to the first model, the overall weight, including the fins, is about 17.5% of the maximum takeoff weight, while, in the second one, it is reduced to 15.8%.