A Hybrid Modelling Technique for the Energy Absorption Capabilities of a Crashworthy Helicopter Structure

The design of modern helicopters is highly influenced by crashworthiness requirements aiming at increasing the survivability of the occupants in case of an accident. In the event of a crash impact, the helicopter structure must preserve a minimum survivable space and limit the forces and accelerations transmitted to the occupants and the payload. In order to limit the overall weight of the structure, this requirement has to be taken into account ever since the beginning of the design, introducing in the helicopter structure energy absorbing systems, mainly in the landing gear, the cabin subfloor and in the seats. Numerical analyses represent a valuable tool to support and guide the design process. Finite element techniques as well as multibody models can be used to address this issue. Both these approaches present advantages and drawbacks. A hybrid modelling technique has been validated conjugating multibody and finite element schemes: the multibody technique allows to obtain the overall behaviour of the structure while modelling relevant parts adopting a finite element method permits to gather more detailed information. This is the case of the lumbar spine of the anthropomorphic test device (ATD) used to evaluate the severity of an impact condition. The hybrid model of an ATD has been derived from an existing finite element model and it has been validated in different reference cases comparing the results obtained by the numerical analyses to the experimental evidences of the tests. The hybrid model of the ATD and of a crashworthy seat have been adopted to investigate the consequences of the introduction of a typical subfloor structure. A pure vertical impact has been considered and an experimental test has been performed. The subfloor structure has been modelled adopting a hybrid multibody/finite element approach and the typical force vs. stroke curve of the intersection elements of the subfloor has been derived from experimental tests. The lumbar load time history, obtained via numerical analyses, has been compared to the experimental curve. The hybrid modelling technique presented proved to be capable of obtaining the desired information in terms of deformation of the structure and of effects of the impact with respect to the biodynamics of the occupant. The most appealing features of this technique are represented by the reduced computational costs required to perform the analyses, the possibility to be adopted from the early stages of the design and to provide guidelines to design effective energy absorbing devices of the subfloor and of the crashworthy seat.