Towards a Methodology to Design Water Impact Crashworthy Structures

Water landings in emergency are likely to have tragic consequences for helicopters. Most of the safety devices developed to enhance helicopter crashworthiness have been designed referring to ground impacts and they might be not effective in case of ditching. At the crash lab of Politecnico di Milano water impact drop tests were carried out to investigate the collapse of helicopter aluminium alloy skin panels. Experimental tests from several drop heights on panels with different thicknesses were performed measuring accelerations and acquiring high-speed video to analyze the impact dynamics. Moreover a new energy absorbing solution for helicopter subfloors was evaluated. The main idea was both to absorb the impact energy and to avoid the panel collapse through the controlled failure of riveted joints. The joint force-displacement curve was obtained performing tensile tests on suitably realized specimens. In the second part of the research, the tests were numerically reproduced adopting ALE and SPH formulations to model the fluid region and a further innovative technique: a SPH-Lagrangian combined approach. The numerical results were compared to the experimental tests in terms of impact dynamics and data acquired. As a result, a satisfactory correlation was achieved and the crashworthy solution was applied on a simplified subfloor model in order to evaluate its effectiveness in terms of energy absorption capability.