Grumman used the model of a wing for the X-29 and associated FEA (Figure 2-6) in combination with a fuselage model to determine the loads in the structure and the dynamic and aeroelastic behavior of the wing required to preclude divergence and flutter. Aerodynamically induced structural divergence was avoided by designing the carbon-epoxy covers to provide bending-twisting coupling to the wing, taking full advantage of the anisotropic properties of the composite material. This model was iteratively appraised by structural analysis, weight optimization, and divergence analysis computer programs to determine the geometry and orientation of the carbon-epoxy tape for each of the 148 plies in the upper wing skin and the 158 plies in the lower wing skin. The same model and computer programs were then used for selection of the materials and the sizing of the cap areas and web thicknesses for the other wing components. As shown in Figure 2-6, the wing covers are carbon-epoxy. The other materials used in the wing component are steel, 6A1-4V titanium, 2024 aluminum, an woven glass-epoxy (Hadcock, 1985).

Three-dimensional models of forgings or machined parts are used for more detailed analysis and sizing of components, such as complex wing-to-fuselage attachment fittings and control surface hinges. These models predict the boundary loads and constraints from the overall FEA. The information from these programs can be electronically transferred to CAD/CAM systems to generate the drawings of the detail parts and assemblies for manufacturing engineering.

In all these programs, material properties and external geometry are generally input data. Structural optimization is done iteratively. Structural geometry, which depends on material properties, panel thicknesses, and stiffener sizes,

Figure 2-6 A model of the wing of the Grumman X-29 and associated FEA. Source: Northrop Grumman.