Helicopter crashworthiness on soft soil: crash cases study, structure elements tests and numerical simulations

摘要

A first Research & Technology program on helicopter crashworthiness underlined through a crash cases study that nearly half of helicopter accidents occurs on soft soils, that is to say earth, sand, snow and water. Therefore, a second program was launched by DGA1 to study helicopter crashworthiness on soft soil, involving industry, laboratories and official services. This paper aims at presenting three different phases of this program that all show, at the end, a comparison between standards used to design helicopters and the crashworthiness of as-built rotorcrafts on soft soil. The first phase is a study of real crash cases on soft soils. Accidents of civil and military helicopters were studied so as to find the rotorcrafts speed and attitude at the impact in survivable and non-survivable cases. Then these data are compared to the crash survivability envelope defined in MIL-STD2, to check their convenience towards soft soil crashes. The second part presents the drop tests performed at DGA TA~3 (former CEAT~4) to evaluate both soil and structure. Indeed, rigid specimens were dropped so as to get data on soil dynamic behaviour (earth, sand and water were used). Then, structural elements were tested to check their behaviour when impacting soft soil, which is particularly important for absorbers. The third and last part deals with the evaluation of the explicit FEM~5 code RADIOSS to model soft soils behaviour and their interaction with the dropped specimens (rigid and flexible). Different methodologies are implemented and finally evaluated by comparison with the tests performed at DGA TA3. Numerical simulation results are confronted to experimental data in terms of deceleration profile and maximum soil penetration. Different methodologies were evaluated: in a first step, a macro-modelling method is investigated, in which the interaction with the soil is modelled by non linear calibrated springs whose properties are identified from experimental load-displacement curves. Simulations prove that the method leads to satisfying results, but still remains unpredictive as the spring identified properties are only applicable for the specific tested configurations. To cope with this issue, FEM~5 modelling methodologies are then developed, involving Smooth Particles Hydrodynamics and Lagrangian formulations. A Drucker-Prager material law which combines elastic-plastic behaviour with hydrodynamic coefficients is selected to model the soil behaviour. Bibliography analysis as well as influence studies on the element formulation and material parameters permit to define an appropriate dataset, which leads to convenient correlation with tests results. The paper finally concludes on the suitability of standard specifications for soft soil crashes as well as the applicability of structural concepts developed mainly for crash on concrete. Besides, it addresses the feasibility of each simulation methodology, in terms of industrial application, and their capability to simulate correctly the physical phenomenon involved in soft soil impacts. It also raises their limitations and the complementary developments likely to assess the influence of horizontal velocity and friction effect.