Application of improved Lagrangian techniques for helicopter crashworthiness on water

Abstract

This thesis concerns the crashworthiness of helicopters onto water and presents a comparison between test and simulation for the impact of a typical helicopter sub floor section onto both hard and water surfaces. The experimental campaign was extended to incorporate a fully instrumented WG30 helicopter drop test onto water, which allowed a comprehensive assessment of the predictive capabilities of the non-linear code LS- DYNA3D to be performed. Validation data was supplied from specific drop tests, which permitted a complete frarne-by-frame analysis to be performed and compared both quantitively and qualitatively with the numerical results. The conclusions from this work enabled a assessment of the validity of the component and full-scale simulations with respect to one another, together with the design changes that could potentially improve the level of crashworthiness currently offered with the current design. Modelling the compressive behaviour of a fluid using a Lagrangian approach is difficult, due to the inherent mesh problems associated with large definitions. Sensitivity studies were performed, which led to the development of a tuned water model that was capable of recreating the impact of various rigid shapes onto water. Alternative techniques to water modelling are also presented in a attempt to minimise the stability problems that arise between fluid and structure boundary, where the definite elements attempt to form a splash. To complete this review of the capabilities of the code, a assessment with respect to capturing joint failure was also performed, through comparison with joint coupon tests. As no methodology concerning the simulation of fluid-structure interaction problems exists within the literature, this thesis addresses this issue by discussing the contributions made to the SAFESA approach (SAFE Structural Analysis), in identifying potential sources of error that are relevant when performing these types of analysis. A discussion of the sources of idealisation, procedural and formulation errors will be performed, along with techniques and recommended practices that have been developed to minimise their affects. The methodology has been extensively tested to be a robust and reliable approach that will greatly assist engineers working in this field. The culmination of this research is the application of the validated simulation tools in developing a potential solution for improving the water crashworthiness response. The concept of maximising skin defection through the purposeful collapse of the interconnecting frames is presented. This would allow the skin to form a continuous curve, as opposed to several inter frame defections. The numerical results verify that this hypothesis could be of benefit in reducing the magnitudes of the accelerations and raises the question of whether next generation designs should concentrate on developing energy absorbing characteristics for each individual cell, or whether a coupled, multiple cell configuration is more preferable.