STUDY OF CUSTOMISED COMPOSITE LAMINATED SHIMS USED IN COMBAT AIRCRAFT STRUCTURAL ASSEMBLY

Modern Combat Aircraft uses Advance carbon Pre-impregnated composite material for primary structures like Fuselage and wing for weight reduction, high fatigue strength and better design performance including stealth feature. Composite materials pose manufacturing challenges to achieve outer aerodynamic and inner contour within design values. Composite parts are made by layer build-up method with ply orientation of 0, 45 & 90 deg as shown in Figure 3. They show more variation and waviness at non-tool surface after autoclave curing and de-molding process. At the assembly stage, gaps can go up to 1.5-1.8 mm between sub-structure and skin. An aircraft primary structural assembly of wing and Fuselage require customized shims to fill gaps between structural components in the airframe that arise due to manufacturing process variation adding up across large contoured and complex structures. These shims, whether liquid or solid, are necessary to eliminate gaps to maintain design performance including fuel tight joints, and efficient fasteners joints. Various shims used in the aircraft industry are mentioned in Table 2, which are required to meet aircraft outer contour for aerodynamic efficiency and also to prevent fuel leak from the wing and fuselage assembly and to have fastener joint efficiency. Currently, gap shimming is a time-consuming process, these amounts to significant delays in production, with much of the time being spent in the shim preparation and installation which is of a considerable percentage of the critical path of the aircraft assembly. In this paper, the objective is to bring out a scientific study of shimming process based on process learning and prior data on variation and elicit gap distributions from historical data on prototypes aircraft built. The study has focused on Data from Coordinate Measurement Machine stage and shim prediction after CMM stage. The paper brings out the study on the existing shimming process and presents an existing model and further improvement for an efficient liquid and solid shimming system. The shimming process improvement will ensure better assembly quality and joint efficiency. The typical part joining process shown Figure 7 in the aircraft industry, assembly is made through drilling, followed by fastening. The typical tolerances for part tooling hole location in aircraft assembly are + 0.2 to -0.2 mm. The data have been collected for gaps measured by Feeler gauge between one of the composite spar of Combat aircraft wing and skin which is shown in Figure 8. It is known from Fiureg 9, that gap values are different on top and bottom flange due to Tool design and manufacturing process.