In wing design, aerodynamic and structural are major consideration that the design required to consider the loading condition of the wing and the layout of the wing structure to ensure that the wing structure are able to withstand all possible loading condition with minimum if the weight. Since it is often that don’t able to have best solution for the consideration at the same time. The wing design will be optimized. Before processing the wing design, it is necessary to understand the structure of the wing. In the figure-x , it shown the elements and layout of wing structure of the Pilatus PC9/A which include spars, shear webs, ribs, skin and skin stingers. This section will be process the design of those elements base on the loads calculated in section 4. Note that the calculation is for one wing only as the load same for both wings as assumed.
Figure xx: The wing structure layout of Pilatus PC9/A (reference)
In the process of optimum design of the wing, there are several assumptions are made, The wing root thickness is 12% of the chord.
The contribution of the auxiliary spar is neglected.
The spar can be treated as tension field beam.
The stringer/skin can contribute up to 50% of the bending moment, but the bending moment contribution of spar remains 100% because of safety. The material for the entire wing structure is AL2024-T3.
The compressive strength of the material is equal to tensile strength of the material. In rib sizing section, the cross section of the wing is constant that the wing thickness and chord are wing root thickness and mean geometric chord length to simplify the sizing process.
Wing structure design requirements,
No structural failures at design ultimate limit which is 50% more than design limit load. All loading will be added factor of 1.5 to meeting this requirement. No buckling allow at 110% of design limit load.
In the wing design, the ribs used to support and stiffen the skin to maintain the airfoil. The amount of rib spacing affects the thickness of the skin as it is always wish to avoid buckling of the skin, and therefore the spacing of the ribs is critical as it relate to ribs and skin panels. The optimized rib spacing will give best strength and weight efficiency. To simplify the complexity of rib sizing on standard shape of airfoil, the rectangular wing box is considered that the chord length is mean geometric chord length, cm, and thickness is wing root thickness, D. From (wang), to optimize the rib spacing, the weight of the ribs, Wrib, will be the half of the stringer/skin, Wskin, as the following, Wskin=2Wrib
That given from develop from the following equation which relate the thickness of the ribs and the optimum rib spacing, Lop, Lop=4F2D2tr2Eω13
F is constant depend on stringer type which is 0.95 for Z stringer,
tr is thickness of the ribs
ω is end of the stringer/skin which determine as following, From figure(bending), the end load, ω, of the stringer/skin will be, ω=0.5MxcmD=0.5×2.118×1051.26×0.216=3.890×105 N/m
Also, the achieve weight efficiency, the optimized rib spacing given working stress of the stringer/skin, σskin, equal to the allowable stress of the material, σallow, that shown as following, σskin=ωt*=σallow
t* is equivalent thickness which determine as following t*=1FωLopE
To determine the optimum rib spacing, it is necessary to assume the value of the thickness of the ribs. Therefore, iterating will be used to determine the thickness of the ribs which give working stress that equal to allowable stress. The table of the result of iterating will be given in Appendix X. After iterating, the suitable thickness of the ribs is 0.206mm that gives rib spacing of 110.3mm and equivalent thickness of 0.8066. However, since the thickness of the ribs is less than standard gauge of 0.02in which is 0.5080mm. The thickness of ribs of 0.5080mm is selected as 0.3416mm isn’t able to manufacture. Therefore the de-optimized...
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