Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Once the preliminary design is complete, detail, production, or embodiment design takes over. This stage will also focus on design verification and formal approval or acceptance of the designs. The verification and acceptance process may well involve prototype manufacture and testing. Such manufacturing needs to fully reflect likely production processes if potentially costly re-engineering during production is to be avoided. Apart from concerns over product performance, issues such as robustness, reliability, safety, and maintainability will be major concerns at this stage. The objective of detailed design is a completely specified product that meets both customer and business needs. At this stage in the process, relatively little design work is routinely automated or parameterized and so the staff effort involved in the design will tend to increase greatly. This is just as true in small UAV programs as large civil aerospace ones - detailing CAD representations with all the information needed for manufacturing remains an intensely manual task.
In all aerospace engineering, detailed design is dominated by the CAD system in use. Moreover, the capabilities of that CAD system can significantly affect the way that design is carried forward. If the system in use is little more than a drafting tool, then drafting becomes the fundamental design process. Increasingly, however, more advanced CAD tools are becoming the norm in aerospace companies. These generally allow parametric descriptions of the features to be produced, which permit more rapid changes to be carried through. They may also allow for information other than simple geometry to be captured alongside the drawings. Such information can address manufacturing processes, costing information, supplier details, and so on. The geometric capabilities of the system may also influence the way that complex surfaces are described: it is very difficult to capture the subtleties of modern aerodynamic surfaces with simplistic spline systems, for example.
In the traditional approach, detailed design revolves around the drafting process, albeit one that uses an electronic description of the product. It is, additionally, quite usual for the CAD system description and the analysis models used in preliminary design to be quite separate from each other. Even in detailed design, these two worlds often continue to run in parallel. Consequently, when any fresh analysis is required, the component geometries must be exported from the CAD system into the analysis codes for study. This conversion process, which commonly makes use of standards such as STEP or IGES, is often very far from straightforward. Moreover, even though such standards are continuously being updated, it is almost inevitable that they will never be capable of reflecting all of the complexity in the most modern CAD systems, since these are continually evolving themselves.
The effort required to convert full geometries into descriptions capable of being analyzed by CFD or FEA codes is often so great that such analyses are carried out less often than might otherwise be desirable. Current developments are increasing the ability of knowledge-based systems to control CAD engines so that any redrafting can be carried out automatically. Nonetheless, the analyst is often faced with the choice of using either an analysis discretization level that is far too fine for preference, or of manually stripping out a great deal of local detail from a CAD model to enable a coarser mesh to be used. The further that the design is into the detailed design process, the greater this problem becomes. In many cases, it leads to a parallel analysis geometry being maintained alongside the CAD geometry. In most small-scale UAV programs, it can mean that full stress analysis is rarely carried out, with reliance instead being placed on prototype testing.
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