Research On Dynamic Problems Of Helicopter Water Ditching

With the widespread use of helicopter,maritime missions of helicopter become more day by day.Thus the accident rate of waterditching increased.Since the twenty-first century,the accident of water ditching and crash increased year by year.The simulation and experiment of helicopter water ditching have become an extremely important issue.Aviation management departments from foreign and domestic countries have considered the regulations of water ditching safety as one of the main contents to get the aircraft certification.It is necessary to research on the attitude angle variation,acceleration variation and pressure distribution of helicopter water ditching,in order to ensure the water ditching of helicopter successfully when encounters emergency.The water ditching of helicopter is a fluid-solid coupling problem,which involved highly nonlinear dynamic behavior.It involves the gas-liquid-solid coupling when the emergency airbags of helicopter are considered,which is difficult.As the experiment of water ditching of helicopter is expensive,so the simulation of water ditching of helicopter is welcome which is more economical.The water ditching of helicopter was research by SPH method(smoothed particle hydrodynamics method),and a water ditching analysis method was established,which provided some theoretical guidance and technical support for the water ditching of helicopter.The main content of this thesis includes:(1)The theoretical algorithm of SPH method was introduced.The equations of fluid dynamics in discrete form of SPH were derived.The fluid boundary truncation region of particle support domain was supplemented with virtual particle.This method could effectively simulate the boundary conditions of complicated shape.A neighbor particle search and particle reorder algorithm was developed.This method combined the methods of Verlet-list/Cell-list and Morton Key/Z-curve.So the computational efficiency of software improved greatly.The kernel function regularization and density reconstruction were used to improve the robustness of software.(2)A software on water ditching of helicopter based on SPH method was designed and developed.The core modules of this software were coded.The data flow of the program was designed.The GUI was also coded.A numerical wave module was developed,which made up the limitation of commercial software.The wave height and length,the velocity and pressure field could be determined directly.The initial state was a stable state.It didn’t need to wait for the wave developing into a stable state.So it can reduce the computational time and improve the efficiency of software greatly when making waves.(3)Water ditching simulation of helicopter was carried out by the software.Different angle of pitch,center of gravity and inertia of helicopter water ditching was simulated and analysed.The effect of wave and ditching position of helicopter was also simulated and discussed.The simulation results of this software were compared with the experiment results.It showed that the difference of the peak acceleration along the Z direction was small.It was proved that the software is effective,and the theory used is feasible.(4)The model of the helicopter with emergency airbags was established.The effect of emergency airbags on helicopter water ditching was discussed.Due to the existence of the emergency airbags,the displacement of helicopter became less.Main displacement caused by the airbags.Water splashing was more obvious.It became more stable when ditching with airbags.The simulation results were compared with the experiment results.It showed that the difference of the peak acceleration along the Z direction was small.(5)The water ditching of deformable helicopter was established via the nonlinear finite element method based on SPH method.Different attitude of helicopter water ditching was simulated and analysed.Pressure measuring points were arranged in the belly area of helicopter.The pressure curves were given.The contours of stress and the maximum pressure distribution also could be achieved.The peak acceleration obtained by deformable helicopter was in good agreement with the results of rigid model.The peak value of the flexible model was smaller,because of energy absorption of helicopter deformation.