Keywords: aircraft, multi-disciplinary optimization, aerodynamics, empennage, panel methods, computer aided design, NURBS, VSAERO.

The preliminary design of aircraft components requires a high quality geometrical model representing all the major structural and functional features with a level of detail amenable for medium to high fidelity numerical analysis. The advent of generic CAD systems based on Bezier, NURBS, B-Rep and other geometric engines has enabled the manual generation of high quality geometry suitable for detailed design and manufacturing of aerospace components. While this technology provides flexibility and accuracy it is computationally expensive and generally not well adapted to the automatic generation of generic geometries suitable for numerical analysis, which generally require manual meshing on the CAD geometry. Many attempts have been made to produce parametric geometry generators based on commercial CAD packages oriented to the automatic analysis. Generally a trade-off has been made between the complexity and flexibility of the geometry and its suitability for simple automatic meshing.

A new approach is presented in this paper where a novel geometric engine has been developed with the objective of enabling the automatic mesh generation of aerodynamic and structural numerical models of aircraft components. The numerical geometry is created using mathematical functions adapted to the functional requirements of the component by controlling continuity in tangency and curvature and driven by a reduced set of non-dimensional parameters which makes use of constraints representing aircraft design knowledge, thereby helping to reduce the dimensionality of the design space while providing sufficient design flexibility. The geometry obtained is of high quality from a mathematical and functional point of view and is characterised by the use of topologically rectangular patches to represent the surfaces of the major components. Additional geometry generation modules provide further flexibility by implementing NURBS geometry, allowing direct compatibility of the three-dimensional models with existing CAD packages used in industry.

The first consideration in the preliminary design of the rear fuselage and tail surfaces of an aircraft is the aerodynamic behaviour, particularly in terms of aerodynamic stability and control derivatives. An automatic structured surface mesh generation adapted to an aerodynamic panel analysis method (VSAERO) has been developed. The aerodynamic mesh generation is performed by the same program which generates the geometry and is therefore not dependent on an external CAD program. Full use is made of the topological definition of the geometric patches. There are several considerations that must be taken into account in order to produce a good quality VSAERO mesh and these are implemented in the code.

Given a set of geometric parameters and flight conditions, the external geometry is generated and meshed automatically and the aerodynamic panels method provides the aerodynamic response, generally in terms of stability and control derivatives. These responses are used to drive changes in the parametric geometry in an optimisation loop driven by an independent optimiser program.

An additional capability is the automatic generation of three-dimensional models suitable for aerodynamic analysis with unsteady Lattice-Boltzmann methods, examples of which are presented in this paper.

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