Comparing the accuracy of VICONOPT to FEM for analysing aircraft wing skin type panels
2014-02-24T14:59:46Z (GMT) by
Modern aircraft wings are thin walled structures composed of ribs, spars and stiffened skins. For civil aircraft, the top skin is subject to compressive forces, during aerodynamic loading, which can cause buckling instability. As a major airframe part, the mass efficiency of these panels has a noticeable impact on the total mass of the aircraft so a significant amount of effort goes into their optimisation. VICONOPT is a program that uses the Finite Strip Method to analyse and optimise axially loaded panels. The speed of this analysis method offers the prospect of a significant reduction in design time if used in place of tools such as Finite Element Modelling. This thesis focuses on comparing the accuracy of VICONOPT against FEM for axially loaded panels that are typical of aircraft wing skin structures. While it is usual to assume that the end cross-section of thin-walled stiffened panels can be modelled by a number of rectangular, joined areas; real panels, by nature of their manufacture, usually have a filleted corner at plate junctions. Validation against PATRAN/NASTRAN show that VICONOPT can conservatively account for the effect on buckling load of these more realistic junctions geometries for most types of buckling mode. The initial modelling of filleted junctions showed the effect of the fillets depended on buckling mode shape, so an FEM investigation was carried out to validate the cause of this. It shows that the change of buckling load is dominated by the increase in rotational stiffness around plate junctions or the increase in moment of inertia, depending on buckling mode shape. FEM models in the first two investigations showed that the boundary conditions used were not completely accurate when modelling an infinite length panel such as the one VICONOPT assumes. When no suitable boundary condition arrangement could be found in literature a set of load and support conditions were derived, tested and shown to be valid for buckling and postbuckling behaviour for almost all aircraft wing type panels. Using VICONOPT's optimisation capabilities, the accuracy of its calculation of a lower-mass panel is compared to FEM, with a geometry typical of aerospace panels. Because panel manufacture intrinsically introduces geometric imperfections, these are included to make the optimisation more robust and mitigate any of their detrimental effects. Results are accurate as compared to an infinite length FEM model and show a benefit to using a pared stiffener combination.