Structural design of a ULM PrandtlPlane wing system

Chapter1 Introduction In the last years,the increasing cost of fuel and the need for a reduction in air traffic pollution has led civil aviation to move its attention toward the construction of more efficient aircraft. In order to achieve this goal, the aeronautic industries have usually tried to improve aerodynamics and to build lighter structures but, recently several solutions have been proposed in order to lead a radical change in the standard aircraft configuration. Among them, the attention of the aeronautic community has been focused on the Blended Wing Body and the multiplane configurations and currently, NASA and Boeing are developing the first one. This thesis aims to examine one of the multiplane configuration, which is called PrandtlPlane . In particular the present work faces the problem of finding the minimum weight solution for the main wing structures of a n ultralight PrandtlPlane . In the last years, Pisa University Aerospace Engineering department has started this project following the idea of Best wing System, an alternative aircraft configuration elaborated by L.Prandtl in 1924. 1.1 Best Wing System The Best Wing System idea was presented by L.Prandtl in the TN 182 NACA report, in which Prandtl showed that once wingspan and total lift are given, as the number of wings used increases, the induced drag decreases. Then, for instance a biplane is more efficient than a monoplane, triplane is more efficient than biplane and so on. In this respect the best result available is a box-shaped wing, shown in Figure 1.1, then called Best Wing System. The main characteristics of this configuration, as shown in [1], are an equal lift on upper and lower wing and a butterfly shaped (lateral) forces with nil resultant on the vertical winglets (Figure 1.2). Even though such conditions are necessary to minimize the induced drag, they aren’t suitable for practical use, since 1