AIRCRAFT DITCHING NUMERICAL SIMULATION

摘要

Ditching is an aircraft emergency condition that ends with the planned impact of the aircraft with water. Three main phases have to be analyzed during ditching: 1. Aircraft conditions before impact. 2. Structural response during the impact. 3. Subsequent floatation. This paper is mainly devoted to the second phase (i.e. the impact with water). During this phase, the high pressures developed by the impact with water of the sliding aircraft may cause rupture of the structure, jeopardizing the required safe evacuation of crew and passengers. The classical approach to ditching has been the use of model ditching test under several ditching scenarios. By these means, the global behavior of the model is assessed and extrapolated to the real aircraft size in order to define the optimum ditching conditions. In addition, the model is instrumented with pressure transducers that in turn are used to compute loads on the structure. Nevertheless, the increase of computer power and reliability of numerical models makes it possible to perform ditching numerical simulations. For the cases with vertical-velocity only (i.e. helicopters) this is particularly true and large advances has been made in recent years. The use of techniques like Smooth ParticleHydrodynamics (SPH) has proven to be very effective in vertical impacts in which good correlations have been demonstrated with tests including full-scale tests. The cases with combined vertical speed and horizontal speed (i.e. aircraft) are significantly more difficult. The pressures produced by the water on the aircraft structure may be either positive (over pressures) or negative (suction) and the SPH technique with the available constitutive laws is not able to properly represent suction forces. Hence, more sophisticated techniques are needed (i.e. CFD). A hybrid approach that combines model test with sophisticated numerical simulation techniques has been followed at EADS-CASA to address these problems. First a detailed explicit Finite Element (FE) model of the structure is prepared and impacted a water block model. Then a complete series of model ditching tests is used to derive critical load cases in the structure. The FE model, the ditching tests and the procedure to pass from rigid mock-up pressures to real size flexible aircraft will be described in the paper as well as the structural response to these loads. The paper will end with lessons learned and suggested ways of improvement for future ditching analyses.