Structural Sizing of Wing Spar in Fixed-Wing Unmanned Aerial Vehicle Concept Using Tsai-Hill Failure Criterion

Abstract

The structural design for a Fixed-Wing Unmanned Aerial Vehicle (UAV) begins with preliminary aeronautical calculations driven by the mission profile. This profile defines key requirements such as endurance, cruise altitude, and payload capacity, which in turn guide the estimation of gross weight, wing loading, and power-to-weight ratio. These parameters determine the wing planform area, span, and aspect ratio, optimizing aerodynamic efficiency. Based on these definitions, wing load calculations are performed, accounting for distributed loads from lift and any concentrated loads from internal masses. Bending moment, shear force, and torsional moment diagrams along the span are obtained for the most critical load case. The results contribute to the subsequent structural design phase. With the maximum loads identified, the sizing of the stringers (longitudinal stiffeners installed along the wing skin). For this purpose, the reinforced shell model was adopted, composed of a core and flanges in polymer matrix composite material and fiber reinforcement, with assumptions of symmetry and linear loading. The stringers are sized to withstand buckling and axial compression, considering the maximum bending moment and the section centroid position. The cross-section of the profiles was selected based on the relationship between structural weight and stiffness. This process ensures that the structure withstands in-flight loads with minimum weight, applying the Tsai-Hill failure criterion. Using a numerical computing code to assess multiple geometries, the optimal configuration achieved reduced structural weight while maintaining an adequate minimum safety margin.