Research On Variable Camber Wing Structure Based On Variable Stiffness Morphing Skin And Zero Poisson’s Ratio Cellular Structure

A traditional aircraft configuration is designed around an optimal in-flight cruise operation.At different points of the flight envelope the aircraft does not however perform with the same aerodynamic efficiency.Several different operational segments compose a typical aircraft mission,and aeroplanes are also tasked to accomplish multiple aircraft missions.A morphing aircraft constitutes therefore a suitable platform to accomplish these multi-mission profiles by adapting the aerodynamic and structural shape to extend the aeromechanics flight envelope of the airframe,it is treated as an important direction for future aircraft development.As the main component to produce aerodynamic forces in classical aircraft designs,the development of a morphing wing is critical to realize the characteristics of a morphing aircraft.A morphing wing is a complex system that includes morphing skin,adaptive supporting structure,actuator,and their associated mechanisms.Morphing wings are complex and sophisticated systems,which consist of shapechanging skins,actuators,wing substructures and associated mechanisms.Each component should be designed for a performance trade-off between compliance(i.e.,perform the morphed shape),stiffness(to withstand and transf er the aerodynamic loads),and weight(maximize payloads while minimizing the airframe weight).In view of the problems faced by the morphing skin,adaptive supporting structure,and high efficiency driving method,this work presents three kinds of variabl e stiffness morphing skin based on shape memory polymer,and a novel zero Poisson’s ratio honeycomb adaptive supporting structure and its pneumatic driving method.Finally,a variable camber wing structure is proposed which consists of the variable stiffne ss morphing skin and the zero Poisson’s ratio honeycomb adaptive supporting structure actuated by pneumatic driving method.A morphing skin should not only feature flexible properties to obtain large deformations and minimize the driving forces,but also possess high stiffness to withstand and transfer in an adequate manner the aerodynamic pressure load.To meet the needs of a morphing skin,this work presents three kinds of variable stiffness morphing skin based on shape memory polymer.Firstly,an electro-active shape memory polymer variable stiffness morphing skin is proposed and investigated.The investigations are focused on the microstructure morphology,mechanical properties,electrical conductivity,shape recovery behaviors,heating performance,tunable temperature distribution,and temperature and strain sensing abilities.The results show that the electro-active shape memory polymer variable stiffness morphing skin can be electrical heated rapidly and uniformly and performs variable stiffness property induced by its Joule heating without an external heater,and self-sensing property,low cost and good designability.These good properties make it a good candidate for morphing wing changed in medium-small scale.In consideration of the requirement of large deformation,a corrugated morphing structure is designed and fabricated.Its mechanical properties are investigated by theoretical analysis,finite elements method and experiments.The results from three method show a general good agreement.Corrugated structure possesses extreme mechanical anisotropic properties,being flexible in the transverse to conduct deformation and stiff in the longitudinal direction to withstand aerodynamic load.A typical corrugation features a series of parallel ridges and f urrows that cannot provide a smooth aerodynamic surface.This work presents the design and modelling of a variable stiffness corrugated structure based on the combination shape memory polymer and composite corrugated laminates for potential morphing skin applications.Analytical models of the concept related to the transverse and bending stiffness are derived and validated using finite element method.The result shows the proposed concept exhibits both extreme mechanical anisotropy and variable stiffness.Finally,this work designs and fabricates an electro-active variable stiffness corrugated morphing skin by integrate the advantages of electro-active shape memory polymer composite and corrugated structure,and investigate its mechanical properties and heating performance by experiments.The results show the proposed morphing skin exhibits extreme mechanical anisotropy,large deformation ability,and variable stiffness induced by its Joule heating without an external heater.In order to provide adaptive support structures for the morphing wing,this work present a novel zero Poisson’s ratio honeycomb structure that can achieve deformations along two orthogonal directions and avoid the increase of effective stiffness in the morphing direction by the restraining the Poisson’s effect in the nonmorphing direction,and avoid warpage when bended(the positive Poisson’s ratio honeycomb appears saddle shape,the negative Poisson’s ratio honeycomb appears hyperboloid shape).Analytical models related to elasticities a re derived and validated using the finite element techniques and experiments.Based on the analytical model,the mechanical properties of the honeycomb are investigated using a parametric analysis against the geometry of the unit cell and the gauge thickne ss of honeycomb.The mechanical properties of this honeycomb can be predicted accurately use the analytical models.As for the actuator of morphing wing,an adaptive pressurized honeycomb actuator is proposed.The zero Poisson’s ratio honeycomb can be derived by a pneumatic driving method,and turns into an active zero Poisson’s ratio honeycomb.The actuation can be generated by the deformations of honeycomb cell walls due to pressure-controlling.Analytical models related to driving displacement,driving force and equivalent elastic modulus of active zero Poisson’s ratio honeycomb are derived and validated using the finite element techniques.Based on the analytical model,the mechanical properties of the active honeycomb are investigated using a parametric analysis against the geometry of the unit cell.The results provide theoretical guidance for the design of the active zero Poisson’s ratio honeycomb.To overcome the shortcomings of the traditional variable camber wing which cannot perform smooth and continuous camber variation,a new type of variable camber wing structure is designed and fabricated.The variable bending wing structure is mainly composed of electro-active variable stiffness corrugated morphing skin and active zero Poisson’s ratio honeycomb structure,which can perform smooth and continuous camber variation.Firstly,the variable camber wing is investigated by finite element techniques.The finite element simulation reveals the relationship between the deflection of the trailing edge and pre ssure,and exhibits displacement,strain and stress distributions.To verify the feasibility of morphing mechanism and the accuracy of the finite element simulation,the variable camber morphing wing prototype is fabricated,and its deflection of trailing edge and equivalent strain distribution are studied by experiments.The test results show a good agreement with that of finite element simulation.Finally,the effect of wing camber variation on the aerodynamic performance of the wing is studied.The aerodynamic performance of morphing wing in different configurations is analysed by finite element method.It is proved that camber variation can effectively change wing aerodynamics,and help to improve wing aerodynamics in different flight missions.