Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Summary of Preliminary Structural Analysis
To summarize, when building simple models to check the spars, either by hand or using FEA, stress levels should be well below the failure stress of the material to be used. In contrast, modest local overstressing of the nylon can usually be ignored, but not if it spreads significantly through the nylon or leads to overly large stresses being reported in the adjacent CFRP parts. When there is a concern about the design of the SLS parts, a more detailed geometrical model should be built and meshed using local seed control, and perhaps just of the junction under consideration, to see how typical flights loads will impact the design. The foam parts with outer cladding can also be analyzed for basic strength, though these results depend greatly on the thickness of the cladding model used and this is somewhat difficult to accurately assess. Provided local foam stresses do not greatly exceed those for the material in use, the design is probably acceptable as some local crushing of the foam will not be a problem. It is also important to understand that built-up structures that can be disassembled for transportation are intrinsically much more difficult to model than permanently bonded ones, since details of assembly and clearances around joints are extremely hard to model accurately. Aeroelastic checks should be carried out to ensure that divergence, control reversal, and flutter will not be a problem. Note, however, that the only fully satisfactory way of assessing the behavior of real built-up structures is by experimental testing, a subject we discuss in Chapter 16.
Once the CFD and FEA analysis processes are complete and the design has been reassessed, attention can be turned to any knock-on impacts on weight and center of gravity location.
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