Helicopter and Aircraft Ditching

 

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Aircrafts and helicopters often travel above open waters and thus must prove a safe water landing under emergency conditions. This is particularly true for fixed-wing aircrafts certification, but also for helicopters as they are strongly used to support marine tasks (e.g. serving offshore wind-parks, supporting S&R actions etc.).

The specific challenge for the design of airborne vehicles is to minimize the risk of injury to persons on board during the whole water landing and to give chance for safe evacuation of the occupants. Accordingly, the motion of the aircraft/helicopter along with the forces acting on the structure is studied for controlled water impact during the design phase of an aircraft. Additionally, the subsequent floatation and the evacuation process must be considered.

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The considered situation has close links with crash simulation, but also distinctive features. Examples of distinctive aspects refer to hydrodynamic slamming loads on airborne vehicles and complex hydromechanics – partially at very large forward speeds – as well as the close interaction of multi-phase fluid dynamics (comprising air, water, and vapor phases) with mechanics of elastic and deformable structures.  

In general, ditching can be investigated by accident analysis, by experiments and numerical simulations. Each of these methods individually contributes to the performance assessment and the methods are often combined during the analysis. Physical ditching tests are possible using either prepared full-scale aircraft (rarely) or scaled models (predominantly). Model-scale ditching tests are the traditional way of ditching performance analysis of aircraft designs.

The level of complexity of numerical ditching simulations ranges from (semi-)analytical water-impact formulas to numerical simulation methods considering the complete aircraft and the near field of the surrounding fluids. While the ditching accident analysis is confined to a few well-documented specific incidences, ditching tests and numerical simulations of ditching allow investigating the ditching performance during the design phase considering various parameter influences. The increase of available computational resources has prompted the use of high-resolution methods for the prediction of hydrodynamic loads, for helicopter simulations that do not feature large forward motion. The most successful high-fidelity method presently used is perhaps the Smoothed-Particle Hydrodynamics (SPH) method, which is designed to simulate violent flow phenomena.

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