FEM analyses of joined wing aircraft configuration

The article describes Finite Element Method (FEM) analysis of a novel aircraft configuration – joined wing, or in other nomenclature Prandtl plane. The aerodynamic configuration promises great induced drag reduction for high lift coefficients. To check the feasibility of the idea project MOSUPS was started. The project includes multidisciplinary analysis, design and building airworthy aircraft. After preliminary analysis it was proved that front upper wing and lower rear wing, which is less commonly used wings configuration, improves aerodynamic characteristics resulting from positive interference of the wings. Although, aerodynamic benefits where the original motivation for the concept of the Prandtl plane, this configuration can also bring structural benefits. Wings structure of the airplane creates closed frame, what is different from classical cantilever wings structure. Loads acting on the wings can be optimally distributed between the wings. Higher stiffness of the closed frame may significantly improve aeroelastic features of the lightweight structure. FEM analysis were done to see how the initially proposed structure works. Three dimensional beam wings model was created. It showed some problems with considered simplifications in the regions of connection of wings to side plate. Problematic regions had high stress values. Additionally, any increasing of the thickness in affected regions significantly change internal loads distribution over whole wings. This makes optimization of the structure more difficult. Not all optimization algorithms assure convergence to solution. To investigate this phenomena more accurate shell model was prepared. Aerodynamic pressure loads, derived from panel method, were mapped on the airplane’s structure. Numerical model for the FEM analysis was simplified, by elimination of the fuselage and the vertical stabilizer. Wings have added ribs and all panels have constant, not optimized thickness. Root profiles of the front and rear wings were fixed. Simple isotropic material was set for the first analysis. After the analysis there were found the same stress concentrations at the arc joints between the wings and the side plates connecting the wings. Possible solutions of this problem are discussed in the article.