Abstract
Airliners spend the majority of their flight time in the cruise phase. The induced drag is one of the largest contributors to drag in the cruise phase. The induced drag can be reduced by increasing the wing’s aspect ratio with a longer wingspan, reducing fuel consumption. The airport gate codes specify a range for the wingspan of aircraft that may use each gate type, thereby restricting the types of aircraft that can fly between two airports. Moreover, a long wingspan increases the peak bending moment at the wing root, imposing a structural weight penalty. It further increases the aerodynamic roll damping of the aircraft, reducing its ability to meet the roll manoeuvre requirements for certification. Various solutions are proposed in the literature to circumvent these constraints and achieve the fuel efficiency benefits of high aspect ratio wings.Aircraft with foldable wingtips (e.g., Boeing 777X) extend their wingtips to planar position before take- off to benefit from the long wingspan. The wingtip is folded after landing to reduce the wingspan, enabling the use of airport gate codes with a smaller wingspan range. The Semi-Aeroelastic Hinge (SAH) concept introduced a wingtip attached to the inboard wing with a free/flexible hinge oriented at an outward angle (referred to as flare angle) to the aircraft’s longitudinal axis. The free/flexible hinge reduces the bending moment transferred from the wingtip to the inboard wing and alleviates the aerodynamic roll damping. The flare angle creates a coupling where the local angle of attack at the wingtip decreases with the upward rotation of the wingtip. Aeroelastic studies on the SAH concept show gust load alleviation, which is improved with a high flare angle, low torsional stiffness, and light wingtip. As the SAH wingtip will be in free response for a non-negligible portion of the flight time to alleviate gust load and roll damping, a fairing is required to improve airflow around the joint and seal its components from debris. A morphing fairing is preferred to provide a continuous and smooth surface enclosing joint with its actuator and clutch components.
The thesis presents a numerical analysis of a morphing fairing for folding wingtip joints. The fairing is made of Geometrically Anisotropic Thermoplastic Rubber (GATOR) skin with an accordion-type core sandwiched between two elastomeric facesheets. The GATOR skin is supported by a hinged rib, which is co-located with the SAH hinge but rotates independently of the folding angle. Additional ribs connected only to the fairing are added to maintain its cross-section shape at their locations. The fairing is flexible in direction across the hinge to enable rotation of the wingtip with minimal torsional stiffness. It has near zero Poisson’s ratio to minimise the cross-section distortion due to the wingtip folding. It is also stiff in the out-of-plane direction to carry the pressure loads. The GATOR skin offers these properties and is manufactured via multi-material 3D printing.
The initial study identifies the level of fidelity required to model the GATOR panel fairing using a simplified geometry representing a fairing slice along the span at the thickest chordwise location of the aerofoil. It proposes a multi-scale modelling approach where the fairing is modelled as a shell surface with the equivalent shell properties of the GATOR panel. The study compares analytical and FE-based methods of homogenising GATOR panel stiffness to equivalent shell stiffness. The study further considers the effects of core-facesheet interaction, transverse shear stiffness and geometric nonlinearity in the fairing deformation. It is proposed that FE-based homogenisation for the GATOR panel be used to capture the effects of core-facesheet interaction while ignoring the effects of transverse shear and geometric nonlinearity in the fairing deformation to keep the computation cost affordable. The proposed approach offers good agreement between the multi-scale and full-scale models of the fairing slice for the baseline design case up to a moderate folding angle.
Secondly, a parametric study of the GATOR fairing over the folding wingtip joint is presented using the multi-scale modelling approach. The study aims to reduce the torsional stiffness and the distortion of the fairing, measured as reaction torque and reduction in fairing thickness, respectively, as the wingtip folds. The study identified the feasibility of reducing the trade-off between the objectives by using pairs of variables that have a strong effect on different objectives. It showed thinner facesheets reduced torque with little effect on distortion, and thicker cores reduced distortion with little effect on torque. It also showed that floating ribs reduce distortion at the cost of torque, but the torque increase can be offset by increasing the fairing span. Pre-straining the fairing along the span further reduced distortion by delaying the buckling of the top skin as the wingtip folds.
Thirdly, the benefits of spatially varying the geometry of the fairing are studied using a beam-based model of the core (without the facesheet). The beam-based model explicitly represents the cells of the core with varying orientation, size and shape over the fairing geometry. The spatially varying geometry is derived by using the principal strain directions derived from a deformed shell-based fairing with isotropic properties. The principal strain lines are traced on the surface to create a lattice representing an accordion-type core with spatially varying geometry. This spatially varying core reduced the distortion of the fairing significantly relative to the baseline case with a uniform core. However, the effects of the spatially varying core on torque were dependent on its cell density relative to the baseline core.
Overall, the thesis presents a systematic study of the modelling techniques, the design space offered by the GATOR panel and the benefits of spatially varying geometry on a folding wingtip joint’s fairing. The thesis made novel contributions in developing a framework for the analysis of flexible skins on wing geometry using a multi-scale modelling technique to reduce computation costs. It also made novel contributions in identifying the benefits of spatially varying core geometry to reduce the distortion of the morphing fairing. The thesis further identifies the limitations of the modelling approaches, their implications for the study, and recommendations for future work to address these limitations.
| Date of Award | 13 May 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Ben K S Woods (Supervisor), Branislav Titurus (Supervisor) & Mark Schenk (Supervisor) |