1968-02-01

Vulnerability and Crashworthiness in the Design of Rotary Wing Vehicle Structures 680673

The structural design and material usage of rotary wing vehicle airframe and dynamic components are investigated with regard to reducing vulnerability from small arms fire and improving crashworthiness.
The design flight operating conditions and the relationship to critical structural loadings are investigated. It is found possible to reduce flight loadings to a 50% static level for the airframe, 85% fatigue level for main rotor, and to a 50% fatigue level for the control system.
The airframe structure of the example aircraft is found to be sufficiently redundant to be considered fail-safe in most of the sheet metal areas. However primary frames, major attachment fittings, and areas lacking redundancy could be reduced to 50% remaining damaged strength. The use of material having high fracture toughness can appreciably improve the damaged strength so that an order of 70% residual strength is attained for critical areas.
The dynamic components were found to be fatigue critical. The use of materials having high fatigue crack to fracture time is required for reduced vulnerability. Examples are shown of the relative crack to fracture time, and a ranking for material selection is presented.
Crash data of rotary wing vehicles is reviewed. While the vertical crash mode is found to result in the most severe type of G loading, the forward crash is also found to be significant for design. The example cases taken for longitudinal and vertical crashes show the airframe structural energy absorption, plus some auxiliary energy absorption, has the capacity to reduce crash loadings to within human tolerance. The crashworthy structural design should also include the mode of failure, hydrodynamic impact capability, comparted areas for flotation, and strengthened areas to provide living space.
A schematic structural diagram is presented of the critical structural areas and design features that can reduce vulnerability and improve crashworthiness.

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