Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Finite Element Analysis (FEA)
Although it is quite simple to build a finite element model directly from beam elements and continue the kind of analysis set out in the previous section, having already generated a simplified parametric AirCONICS CAD model that includes the main structural spar and boom elements it is usually more straightforward to carry out FEA by importing this CAD model into an FEA package where a suitable 3D mesh of the structure can be built and loads and boundary conditions applied to it. From a structural perspective, it will be clear that the key locations in such a parametric model will be based around the main spar and tail booms and their junctions with the tail and fin spars and the fuselage elements. Figure 14.5 shows these basic components of the structure, here controlled by just the five sets of spar diameters, the junction locations, and spar lengths. Note that the tail and fin spars take the form of the tail shape adopted (H, V, inverted V, etc.). For the simplest forms of analysis, all that need be considered are these spars and the (typically SLS nylon) junction parts. If being analyzed in detail, the internal dimensions of those SLS parts that directly transfer load between the spars clearly need to be parametrically driven, possibly with a thick-walled structure as already mentioned.
Figure 14.5 Preliminary spar layout for Decode-1. Here the linking parts are taken directly from AirCONICS without being reduced to either thick-walled or thin-walled rib-reinforced structures.
The precise detailing of the junctions would be finalized later on; however, for sizing spars they do not matter greatly and can be replaced by simple fully solid SLS parts to begin with, as in the figure, the aim being merely to provide the supporting boundary conditions. The structure can then be meshed and loaded with the various forces already derived to assess its suitability for the design in question. By using appropriate material properties for the junction parts, more realistic boundary conditions can be imposed than by directly restraining the spars themselves.
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