A Design Approach of a Fuselage Barrel in Grid Structure

Abstract

In this paper a design method of a cylindrical fuselage barrel in grid structure to fabricate in composite material is illustrated. The approach can be divided in two phases. In the first step, a lattice structure, that is a grid structure with ribs only and without a skin, is sized thanks to an analytical approach. The independent variables to optimize are five: number of helical ribs, number of circumferential ribs, rib height, circumferential rib thickness, helical rib thickness. Some geometric constraints on the dimensions of the ribs are introduced. The best lattice structure is individuated by an analytical theory due to professor Vasiliev of the Central Research Institute of Special Machine Building (CRISM), the leading Russian Composite Center. The theory is based on the calculation of four safety factors for the helical rib strength, axisymmetric global buckling, non axisymmetric global buckling and the helical rib local buckling. Successively, a finite element analysis is performed in order to verify the analytical results. A software tool developed in Matlab environment prepares the finite element model, runs the finite element solver (MSC/Nastran) and reads the results. In the second step, an outer skin is added and the best lay-up for it is chosen; the maximum failure index for the skin laminate is calculated too. At the end the structure with the minimum weight and all margins of safety positive is individuated. The skin is extremely light so as to minimize overall weight. In the finite element model ribs and skin are simulated with beam and shell elements respectively; for the skin a common high-strength graphite/epoxy composite material is used, for the ribs an isotropic material with properties derived from experimental data. A grid-stiffened structure allows to reduce the structural mass of about 20% respect to a metallic reference baseline. Moreover, it can be quickly constructed by using filament winding technology; the process can be highly automated and the manufacturing costs can be reduced of the 30%.