Study On Flexible Skin And Supporting Substructure Of Morphing Aircraft

It is generally accepted that the geometry of traditional wings should be designed according to the special mission and usually optimized in the most concerned flight performance. However, in an integrated flight cycle with continuously varied flight parameters, the geometry of wings is not always optimal. Morphing aircrafts can realize the optimal objective through changing its aerodynamic layout according to the outside flight environment. One of the key techniques to realize morphing aircraft is the morphing skin, which should endure enough deformation if significant changes of chord length, span length, sweepback angle and wing area are accomplished by the aircraft wing, as well as possessing enough stiffness to maintain the aerodynamics configuration during the deformation process. Meanwhile, low in-plane stiffness of the morphing skin is desired to reduce the energy consumption of actuators. Moreover, the recently emerged smart materials and their composites open an avenue to the development of morphing aircrafts.Based on the development status of morphing aircrafts and the requirement of morphing wing, flexible morphing skin with in-plane negative Poisson’s ratio (NPR), active morphing skin embedded with pneumatic muscle fibers (PMFs) and variable stiffness tube structures have been designed, manufactured and investigated from a theoretical and experimental point of view. Then, both kind of supporting substructures of anti-tetrachiral honeycomb with NPR and SILICOMB honeycomb with zero Poisson’s ratio (ZPR) have been also manufactured and studied through theoretical, experimental and finite element (FE) methods. Finally, a kind of sandwiched skins with flexible face-sheets and cellular cores has been designed and investigated. The deformability and stress-bearing capability were demonstrated by experimental tests. The sandwiched skin could be used in the in-plane shear deformation of morphing wing.First of all, in consideration of the requirement of morphing wing, three kind of morphing skins with special functions were designed and investigated. Firstly, a novel fiber-reinforced composite flexible skin with in-plane NPR has been fabricated. To describe the elastic behavior of the flexible skin, a three-dimensional theoretical model was developed, and the theory accuracy was validated by the experimental results. Based on the theoretical model, the relation between in-plane NPR and material parameters was analyzed and discussed. The analysis result showed that it is also possible to identify fiber-reinforced composite skins configurations with required in-plane auxeticity and stiffness performance. The analysis presented in this work provides useful guidelines to develop flexible skins with negative Poisson’s ratio. Secondly, a kind of morphing skin embedded with PMFs was proposed from the bionics perspective, and studied by tensile test and output force testing. Based on this research result, the active morphing skin composed of PMFs and silicone material was fabricated and studied by output force testing and stress-bearing capability test. The experimental result showed that the contraction ratio of the morphing skin could be regulated by changing the applied pressure, which could realize the active deformation of the skin. Furthermore, the experimental result also showed that the objective of avoiding or restraining the swelling can be accomplished through regulating the pressure value and changing the transverse stiffness value. Finally, a new kind of variable stiffness skin was proposed, and variable stiffness tube is one of the key components. Based on flexible matrix and shape memory polymer (SMP) respectively, two kind of variable stiffness tubes were fabricated and studied. They could realize the variable stiffness properties through structure design and material itself, respectively. Two different theoretical models were developed respectively to describe the variable stiffness properties of the tubes, and the accuracy for both theoretical models was validated by the experimental results. The validated models were used to discuss and analyze the relation between material parameters and variable stiffness properties of the tubes. Therefore, the investigated tube could serve as potential candidate for morphing skin applications with variable stiffness.Then, in consideration of the requirement of morphing wing, two kinds of honeycomb supporting substructures with different Poisson’s ratios were designed and investigated. On one hand, the anti-tetrachiral honeycomb samples with NPR were manufactured using a Rapid Prototyping (PR) Fusion Deposition Molding (FDM) machine. Theoretical expressions were derived to describe the in-plane and out-of-plane elastic constants. In comparison with the FE and experimental results, the theoretical method is deemed to possess satisfying accuracy. Based on this fact, the in-plane and out-of-plane elastic constants were predicted through FE and theoretical methods. The predicted results showed that anti-tetrachiral honeycomb structures with minimum density but maximum transverse shear modulus (or in-plane elastic modulus) can be identified through careful selection of the geometry parameters. On the other hand, a novel curved SILICOMB cellular structure has been produced using Kirigami techniques. The out-of-plane stiffness properties was investigated using full-scale FE method and cyclic compression tests. The large deformation capability with good shape recovery could be observed in the experimental tests. In addition, the relation between stiffness and geometry parameters of the curved SILICOMB cellular structure was also discussed and analyzed.Finally, in view of the existent problems of sliding skin proposed by NextGen Aeronautics, a new kind of sandwiched skins with flexible face-sheets and cellular cores has been designed and investigated using ANSYS finite element software. The sandwiched skin sample was fabricated with improved cosine-shaped honeycomb supporting substructures and spandex-reinforced flexible facesheet. FE simulation and experimental tests were used to analyze and discuss the in-plane shear deformability and out-of-plane stress-bearing capability. The experimental test results demonstrated the feasibility of design and optimization honeycomb supporting substructures with FE simulation. The research results could provide useful guidelines to design morphing wings with in-plane shear deformation.