Structural Analysis and Testing of a Student-Designed UAV Wing

Uncrewed Aerial Vehicles (UAVs) offer versatile operational capabilities, ranging in size from micro-scale vehicles to long-range, jet-powered autonomous aircraft, and have found applications worldwide in both civilian and military roles. The AIAA Design/Build/Fly (DBF) competition is an opportunity for student teams to design, construct, and fly a UAV in accordance with a specified mission profile. The competition’s fast-paced development timeline and mission diversity requires the aircraft to be constructed in a manner that allows rapid changes to its structure in accordance with aerodynamic and payload refinements made throughout the development cycle, while also producing a sound and reliable final product. For the 2025 DBF competition, the Clarkson University Knighthawks will be constructing a monoplane UAV with a maximum takeoff weight of 15.9 kg (35 lb.), of which up to 70% is allocated for payload weight. The aircraft can accommodate a battery powered propulsion system, radio control system, belly mounted payload release mechanism, and twin wing pylons. The UAV wing is constructed using lightweight extruded polystyrene (XPS) foam with unidirectional carbon fiber reinforced polymer (CFRP) spars. This design provides an easily manufacturable and strong wing, which is well suited for the DBF competition. To determine the strength and stiffness of the wing assembly in competition conditions, finite element (FE) modeling and a series of structural tests will be performed. Sections of the proposed paper will be dedicated to the layout of the wing structure, the FE model, and the testing conducted. The objectives of the final paper will be to present the strength characteristics of the XPS foam wings and correlate analytical with experimental data. The UAV wings are a single cantilever design with a span of 1.81 m and a root chord of 0.559 m tapering to a tip chord of 0.398 m. The wing construction consists of an XPS foam core, cut to an airfoil shape using a hot wire cutter. This core has a tubular CFRP spar bonded within it to carry the bending loads exerted in flight. A joiner member socketed within the spar connects the wings together through the fuselage. To add additional strength in conjunction with a more aerodynamic surface finish, a skin material is to be applied to the outside of the wings. Two different skin materials will be tested to determine an ideal material for wing skins. The materials tested will be a polyester laminating film applied using a thermal bonding process, and carbon fiber woven spread-tow applied using an epoxy resin matrix. These two materials differ greatly in mechanical properties, the carbon fiber can provide additional strength and stiffness to the wing structure, while the laminating film will be a lightweight option that provides an aerodynamic finish and moisture barrier. The experimental test regimen will consist of two tests, both of which are aimed at measuring the strength and stiffness of the wing under bending loads. The first test is a three-point bend test using a prismatic airfoil section with a chord equal to the mean wing chord. This test will be used to determine the contribution of the skin materials to the overall structure and will be used to compare the bending properties of the skin materials. To isolate the strength contribution of the spars, these samples will be constructed without them. The three point bending test will be conducted using an Instron 4505 material testing machine with a custom test apparatus built to hold the samples in a way that applies a uniform load across the chord. The deflection and applied load data will be measured using the frame’s integrated load cell and recorded for analysis. Additionally, strain gauges will be placed on the top and bottom surfaces of each sample to record surface strains. The second test will be a tip loading test of the full-scale wing. This test is designed to replicate the required wing tip loading test during the DBF competition and will serve as a final verification of the wing structure. This test will be conducted using a custom fixture that holds the wing joiners in encastré while applying a load to the wingtip by means of a jackscrew. Wing deflection and surface strain will be recorded at four equally spaced stations along the span of the wing. Vertical deflection and surface strain will be measured at each station using dial indicators and axial strain gauges, respectively. An FE model of the wing and 3-point bend test setup was created using ABAQUS. This model predicts the stresses and strains in the wing during the 3-point bending test. The model was constructed using CAD geometry imported into ABAQUS, with appropriate additional geometry added as needed. The foam core and laminating film were modeled using isotropic elements, while the composite skin was modeled using orthotropic shell elements with laminate properties computed using the ABAQUS composite layup tool. Boundary conditions were applied to simulate an idealization of each experiment condition. The final paper will present findings from both the analytical and experimental tests. Analytical data will be compared against experimental data to assess the accuracy of the FE model in predicting the sample and wing deflections. An error assessment will accompany the results and investigate instrument calibration and other potential error sources. Finally, a conclusion on the overall efficiency of the wing design and recommendations on the wing skin material will be presented. Should this abstract be accepted, a final manuscript will be submitted by February 7, 2025.