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Numerical Analysis on Water Impact

J. Majamäki
The helicopter impact on rigid ground has been on focus for several years and principals from Aircraft Crash Survival Design Guide, or FAR as minimum, are integrated more and more into late design concepts. No doubt, the rotorcraft survivability has been positively influenced by the measures. Introduction of the crashworthy seats or crashworthy fuel system are factors of primary significance improving the survivability of a modern helicopter. While the requirements or the guidelines derive from ground impact, water impact of a helicopter has started to receive more attention. It has been acknowledged that impact on water has significant differences in the physics. The different physics (fluid-structure interaction) and longer duration of impact sequence place new requirements for analytical means. Reliable in-depth analyses of a water impact are extremely resource hungry and difficult to execute, not to mention that a validation with a test is equally complex - particularly in a case of simplification to a component level. The purpose of a study made in Eurocopter was initially to define typical impact scenario including speeds and attitudes for a typical helicopter water impact scenario. At the same time, analytical methods for the simulation of a water impact were compared. Results from drop tests of generic shapes on water were obtained in correspondence with AG15 Garteur study group. These tests were performed by LaST crash lab of Politecnico di Milano. With these results it was possible to validate the analysis, compare available methods and define pros and contras. The primary difficulty was the balance between complexity and computing performance. Whereas the structural model can be produced from old experience/knowledge, necessary verification of the water model needs to be performed. It was necessary, for example to assess following primary questions: -) Necessary size of the water "pool", effect of wave reflections; -) Necessary discretion level of the water mesh (element size); -) Usable material laws; -) Analysis method: Lagrangian impact on Lagrangian Water, SPH water model, Lagrange Structure impact on Eulerian Water model. From Eurocopter Germany side the study has now mostly concentrated on applying Eulerian method, whereas Eurocopter France works more with SPH approach. Since the Euler-Lagrange calculation method is rather new in the used analysis code and still developed, part of the problem is defining different parameters for fluid-structure interaction and intensive verification work required. For both approach inside Eurocopter the explicit simulation code Radioss was used. In the end, after establishing the modelling guidelines for the water impact simulations and verifying the water model the knowledge was applied to a practical problem. A verified simulation model, from a real-life drop-test of a composite transport helicopter cabin on rigid ground, was taken as basis and completed with a verified water model. The analyses were performed and a comparison was made between structural impact on rigid ground (as in drop test) and on water considering rigid structure and deformable structure.
35th European Rotorcraft Forum 2009,
Conference Paper
21,0 x 29,7 cm, 7 Seiten
DGLR-Bericht, 2009, 2009-03, 35th European Rotorcraft Forum 2009 - Conference Proceedings; S.1-7; 2009; Deutsche Gesellschaft für Luft- und Raumfahrt - Lilienthal-Oberth e.V., Bonn
in getr. Zählung;
Stichworte zum Inhalt:
crashes, survival, rotorcrafts

Dieses Dokument ist Teil einer übergeordneten Publikation041:
35th European Rotorcraft Forum 2009 - Conference Proceedings