Keywords: aircraft, multidisciplinary optimization, structures, empennage, finite element method, computer aided design, CATIA.

The design and analysis of aircraft structures is a key driver in the multidisciplinary design process, impacting on the weight, cost, manufacturability, operability and aeroelastic response. The major aircraft components are usually designed using lightweight design principles and most structural subcomponents adopt the arrangement of a thin skin stiffened by stringers and frames or ribs. In the preliminary aircraft design process, where early availability of weight data is crucial, the structure is often modelled using simple beam theories. While this approach, when well calibrated and used by experts, provides sufficiently accurate weight and stiffness data, its applicability is limited to conventional slender structures.

Ideally a better representation of the structural layout should be used as soon as possible during the design process. Coupled with a structural sizing method and a means of generating aircraft loads, access to a representation of the structure of higher fidelity would enable the identification of functional behaviour, manufacturing issues and opportunities, weight and balance and drive the design towards lower weight and costs and increased overall performance.

As part of a multidisciplinary design framework developed for the preliminary design and optimization of aircraft rear fuselage and tail surfaces, a complete structural design capability, including automatic high fidelity finite elements model generation and structural sizing has been developed. Based on a parametric geometric modelling module part of the framework, the structural layout is defined using a mixture of dimensional (stringer, rib and frame pitch) and non-dimensional parameters (relative chordwise spar and spanwise rib positions, master frame positions). The structural layout definition is stored independently from the geometry and can be applied, preserving the design intent, to variations of the external geometry. The structural lay-out parameters can be changed independently from the aircraft geometry and the complete three-dimensional structural model is generated and visualised within the framework, exported to a commercial CAD package (CATIA V5) and written as a global NASTRAN finite element model (FEM). Topological details are handled appropriately (e.g., stringers run-outs are modelled following industrial guidelines) and the FEM uses a structured numbering convention and the same elements and idealization used in a manual advanced preliminary design phase.

The complete global FEM generation, including an aeroelastic model, is performed automatically by the program and given a set of flight conditions, NASTRAN generates the external and internal structural loads. A proprietary structural sizing method then sizes the structure for strength and finally a structural weight is obtained, as well as flexible aeroelastic stability derivatives and a preliminary flutter behaviour (after augmenting the mass model with non-structural mass properties).

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £50 +P&P)