Case Study

Composite floor panels for military aircraft

Hexcel Composites used LUSAS Composite to analyse honeycomb sandwich floor panels designed to support Land Rover wheel loads in military aircraft.

Panel displacement contoursHoneycomb sandwich panels are used extensively for flooring in both military and commercial aircraft. Typically, commercial aircraft flooring is about 1cm thick and is made of glass or carbon fibre reinforced epoxy skins with a Nomex honeycomb core. Commercial aircraft floors are designed to withstand high compression loads, particularly those induced by high heeled shoes. Typical floor panels weigh about 3 kg/m2. For this military aircraft floor design, Hexcel Composites proposed using a panel with aluminium skins and an aluminium honeycomb core simply supported between the seat rails. The panel is required to support a Land Rover at vertical load factors of 1g and 7.8g. With all types of FE analyses, it is always best to start with less complex solution methods before progressing to more advanced solutions. This case was no exception. Hexcel Composites conducted the analysis in stages progressing from a simple linear analysis to a full nonlinear analysis.

The panel rests on the supporting edges, and is only restrained vertically at the bolt positions. Therefore, nonlinear joints were used along the edges to allow the corners of the panel to "curl" up. For loads up to 7.8 g, a geometrically nonlinear analysis was conducted to determine the maximum deflection and load for the un-reinforced panel. For the 7.8 g load case, Hexcel Composites were also interested in the effect of a reinforcing plate placed on top of the aircraft floor panel. It was assumed that the reinforcing plate and floor panel would adopt the same deflected shape. Therefore a layer of beams was used to separate the plate and panel. Beam elements with no shear stiffness were used allowing the two panels to deflect without developing composite action. The calculated deflection compared well with that estimated by manual calculations. Stress levels in the honeycomb sandwich panel and reinforcing plate were also found to be satisfactory.

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