In the event of ditching, helicopters are too heavy to float or unstable on water. The designers have to add inflatable buoyancy devices to prevent capsizing. Dimensioning such devices is an essential issue in the helicopter's certification.

The helicopter is represented as a deformable volumic body and the float as an air-inflated tubular membrane. The water is modeled as an incompressible perfect fluid. This study is concerned with the dynamical response of toy cases and the behavior of a selection of coupling schemes.

The structure is solved through a home-made finite element code. The time-stepping is carried out using the Newmark method, where the nonlinear problem at each time increment is solved through a Newton iterative scheme. For lack of an analytical expression for the hydrodynamic water pressure, the tangent matrix coming from the hydrodynamic pressure is only partly computed. The fluid is computed with a home-made boundary element solver using the von~Kàrmàn model. The time-stepping is achieved using the explicit finite difference or the fourth order Runge-Kutta method.

In order to perform the coupling, we consider two Dirichlet-Neumann partitioning schemes: (i) the Conventional Serial Staggered procedure, where the hydrodynamic load is computed once at each time step; and (ii) a Gauss-Seidel procedure, where coupling iterations are performed to ensure the convergence.

The obtained numerical results show which coupling scheme is the most efficient regarding precision and computation speed. Significant quantities to investigate the helicopter's behavior are given, namely the structure deformation, the hydrodynamic load and the internal pressure of floats.