Multifunctional Design Concepts for Energy Absorbing Composite Fuselage Sub-Structures

The paper describes design concepts for aircraft fuselage sub-structures which combine the dual functions of structural integrity under normal flight loads with crashworthiness under impact loads. The design concept proposed is based on the assembly of modular composite elements into energy absorbing (EA) subfloor structures, which are fabricated by automated processes to keep costs down and improve quality. The paper considers in detail the development of beam webs based on a composite sandwich concept for aircraft keel beams, and a modular subfloor box concept for helicopters, based on interchangeable standard floor beams, with cruciform node elements. The design, fabrication, numerical simulation and test of the structural elements are described in detail in the paper. Dynamic crush tests on the composite sandwich beams verified the EA concept; however the EA beams did not function as well as expected when tested in a fuselage section due to the presence of relatively stiff seat rails. Tests and simulations on the helicopter subfloor boxes showed the important role played by the cruciform node elements, which need to be correctly triggered to keep the subfloor peak crash loads within safe limits. Analytical modelling is an important tool in the design of these subfloor structures, and dynamic structural simulations, using both finite element and hybrid codes, were conducted and validated through correlation with test data.