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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:

  • • Carbon-fiber reinforced plastic (CFRP) spars (usually of circular cross-section but sometimes other shapes);
  • • 3D printed fuselage elements (in SLS nylon or FDM ABS);
Breakdown of Decode-1 outer mold line model into individual components for structural modeling

Figure 14.2 Breakdown of Decode-1 outer mold line model into individual components for structural modeling.

  • • 3D printed or laser-cut ply local load transfer and clamping structures;
  • • Low-density foam for aerodynamic surfaces;
  • • Foam cladding in either Mylar film or glass fiber cloth for all the foam parts (in both cases bonded to the foam with appropriate resin-based systems).

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:

  • • Carbon-fiber spars - 0.75-2 mm wall thickness depending on the diameter and selected from readily available stock sizes;
  • • 3D printed fuselage skins, ribs, and clamps - 1.5-8 mm wall thickness (although it is possible to print thinner structures, we find them too fragile to work with);
  • • Laser-cut ply ribs and local load transfer structures - from 3 to 10 mm thickness;
  • • Low-density foam for aerodynamic surfaces - minimum wall thickness of 5 mm, more commonly 10 mm (very fine trailing edges are avoided, either by using a trailing edge radius or by simple truncation);
  • • Foam cladding in Mylar film - 125 pm thick;
  • • Foam cladding in glass-fiber cloth - 20 g/m2 woven cloth, which gives a cladding thickness of 100 pm.

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.[1] For thick-walled equivalent models, a wall thickness of between 2 and 4 mm is usually reasonable.

Decode-1 components that will be produced by 3D printing or made from laser-cut ply

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:

  • • Assume all the moving control surfaces will be solid foam with a Mylar or glass-fiber cloth covering and a full-length hinge pin formed from a small diameter CFRP tube. These items can be dealt with separately and simply need to be partitioned into thin skins and foam cores, which themselves have circular holes cut through for hinge pins - we do not add lightening holes to control surfaces;
  • • Cut the main wings just outboard of the junction with the longitudinal boom(s) and again at the start (two cuts) and finish of the aileron to create four outer wing parts on each side. The outboard-most of these will become the (3D printed or laser-cut ply) tip, and the other three will become two foam blocks joined together by a rib (of 3D printed plastic or laser-cut ply).
  • • Cut the elevator and fins off in a similar manner, with end parts forming printed or laser-cut tips and allowing for ribs inboard where the control surface hinges end.
  • • Cut suitable holes in all parts to accept CFRP spars, boom(s), and hinge pins.
  • • Hollow out all the foam items forming the main lifting surfaces to reduce weight, leaving suitable wall thicknesses including around spar holes. These can then also be partitioned into thin skins and foam cores.
  • • Convert the remaining outer mold surface parts into solid or thick-walled structures. These will be the main 3D printed structural elements of the aircraft already illustrated in Figure 14.3.

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.

  • [1] It is tempting to try and decided on more precise local dimensions of the 3D printed parts by moving to thin-walledmodels with suitable approximations of the likely internal local stiffening; caution should be taken with such analyses,however, since the final details of local stressing will often be controlled by subtleties such as clearances in contactanalysis, local fillets and the positions of any clamping bolts, and so on. It is usually best to delay more precisestructural modeling until after the detail design stage has been entered, or even to rely on past practice or experimentaltesting.
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