OS-T: 3200 Design of a Composite Aircraft Underbelly Fairing

Composite materials have become popular in the application of aircraft structures. The need for innovative designs has posed a great challenge. In this tutorial you will perform an optimization-driven design approach of a composite aircraft underbelly fairing using OptiStruct.

The design takes a three-phased approach:
Phase 1: Reference Design Synthesis (Free-Size Optimization)
Concept design synthesis Free-size optimization identifies the optimal ply shapes and locations of patches per ply orientation.
Phase 2: Design Fine Tuning (Size Optimization)
Design fine tuning Size optimization identifies the optimal thicknesses of each ply bundle.
Phase 3: Ply Stacking Sequence Optimization
Ply stacking sequence optimization Shuffling optimization obtains an optimal stacking sequence.

The process expands upon three important and advanced optimization techniques; free-size optimization, size optimization and ply stacking sequence optimization. By stringing these three techniques together, OptiStruct offers a unique and comprehensive process for the design and optimization of composite laminates. The process is automated and integrated in Altair Simulation by generating the input data for a subsequent phase automatically from the previous design phase.

Model Definition

The finite element model of the underbelly fairing was generated in HyperMesh. Material properties for carbon-fiber were considered and represented using an orthotropic material (MAT8) for two dimensional elements. The fairing was modeled with four ply orientations (0°, 90°, 45° and -45°) of uniform thickness. The SMEAR option is applied in the PCOMP card to eliminate stack biasing.

Two load cases were defined to represent the operating conditions - an internal uniform pressure loading of 0.02MPa and an external gravity loading of 6.75g. The fairing was considered to be riveted along its edges to the surrounding structure. Two equipment masses, weighing 2Kg and 3Kg each, were mounted to the fairing through RBE3 elements. The fairing has been designed considering two major performance criteria: the first natural frequency is at least 20Hz, and the maximum strain is less than 1000 micro-strain.

Figure 1.