Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Structural Modeling Using AirCONICS
Starting from the existing wetted surface model already prepared in AirCONICS for aerodynamic analysis, one can begin to build a simple structural model. For most of our aiframes, such as Decode-1, the main wings will be made of foam outboard of longitudinal spars that join the empennage to the main wings. Inboard of this, a 3D printed structure will be adopted, which will directly support the various spars, engine bearer and undercarriage pickup points. In addition, a single rib will commonly be positioned midway along each outer wing where the ailerons start; these carry the inboard ends of the aileron hinges (and sometimes the aileron servos). The wing tips will be 3D printed and also carry the outer ends of the aileron hinges. The elevators and rudders will be dealt with in a similar way with ribs at their ends to support the hinge spars and small 3D printed structural elements to join them to the longitudinal spars, see Figure 14.2.
It can be seen from this that our airframes essentially consist of five types of elements placed inside the wetted surface outer mold line:
Figure 14.2 Breakdown of Decode-1 outer mold line model into individual components for structural modeling.
To proceed, we begin by defining the acceptable working thicknesses for all elements. For aircraft up to 30 kg in size, we typically adopt the following values:
Having made these choices, the next task in partitioning the structure is to decide where the various spar elements should be placed. We generally opt for the principal lifting surface spars to run along the sections at the quarter chord location along with smaller hinge spars at 75-80% of chord when control surfaces are present. These then need to be linked by one or more longitudinal spars or (less commonly) the main hull of the fuselage. We then size the lifting surface ribs and tips that will eventually act as control surface hinge supports as well as helping transfer aerodynamic loads to the spars. Finally, we delineate the extent of the foam elements along the lifting surfaces, fixing where they meet the fuselage and local load bearing structures; all these parameters are defined in the AirCONICS codes we use, see Appendix B for example. Figure 14.3 shows just those parts that will eventually be 3D printed or made from laser-cut ply.
Next the 3D printed parts and foam elements are hollowed out, with the foam parts then being skinned with Mylar or fiber-reinforced plastic. During detailed design, our 3D printed parts are always configured as thin-walled structures locally stiffened against buckling, since this can be accomplished very efficiently during this stage. However, during preliminary design work we often use a thick-walled equivalent structure to represent this approach, and for simple spar analysis just completely solid SLS joining parts. For thick-walled equivalent models, a wall thickness of between 2 and 4 mm is usually reasonable.
Figure 14.3 Decode-1 components that will be produced by 3D printing or made from laser-cut ply.
Thus the basic steps in converting from the aerodynamic outer mold line to a simplified structural representations are as follows:
Once the building blocks have been defined in this way, analysis can begin. We start with simple beam theory assessments before moving on to FEA approaches based on exports from the AirCONICS Rhino model (in the form of a ACIS, STEP, STL, or Parasolid files) and read into whatever FEA system is to be used to carry out the more detailed preliminary structural analysis. In either case, we start with simple UDLs set at the peak load factors already defined in the Vn diagram. We follow this up with distributed loads taken directly from XFLR5 analyses, scaled by load factors from the Vn diagram (we do not try and use loads taken directly from XFLR5 at extreme load factors since the panel solver is not compatible with such conditions). For beam theory calculations, loads are most easily transferred as combinations of point and uniformly distributed forces and torques, while for FEA models it is also possible to work with body forces on the spars or pressure loads on the aerodynamic surfaces.
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