Elastomer-based skins for morphing aircraft applications: Effect of biaxial strain rates and prestretch

There is an emerging trend in the morphing aircraft research where two or more morphing degrees of freedom are used on a wing which leads to the concept of polymorphing. The skin of the morphing wing must be flexible in the morphing direction but stiff in other directions to withstand the aerodynamic loads and maintain the airfoil shape. Polymorphing changes the loadings profile (from uniaxial to biaxial) and increases the complexity of designing suitable morphing skins. Furthermore, elastomeric materials used on morphing wings are usually prestretched to prevent wrinkling and to increase their out-of-plane stiffness. This paper focuses on elastomeric morphing skins and it studies the effect of biaxial strain rates and prestretch ratios on important mechanical properties such as stiffness, hysteresis losses (%), and stress relaxations (%) from an experimental perspective. Three polymeric materials are considered: Latex, Oppo, and Ecoflex. This study provides a mechanical comparative understanding of the three polymers used in the morphing wing under biaxial loading (two morphing degrees of freedom)

A Polymorphing Wing Capable of Span Extension and Variable Pitch

This paper presents the development of a novel polymorphing wing capable of Active Span morphing And Passive Pitching (ASAPP) for small UAVs. The span of an ASAPP wing can be actively extended by up to 25% to enhance aerodynamic efficiency, whilst its pitch near the wingtip can be passively adjusted to alleviate gust loads. To integrate these two morphing mechanisms into one single wing design, each side of the wing is split into two segments (e.g., inboard and outboard segments). The inboard segment is used for span extension whilst the outboard segment is used for passive pitch. The inboard segment consists of a main spar that can translate in the spanwise direction. Flexible skin is used to cover the inboard segment and maintain its aerodynamic shape. The skin transfers the aerodynamic loads to the main spar through a number of ribs that can slide on the main spar through linear plain bearings. A linear actuator located within the fuselage is used for span morphing. The inboard and outboard segments are connected by an overlapping spar surrounded by a torsional spring. The overlapping spar is located ahead of the aerodynamic center of the outboard segment to facilitate passive pitch. The aero-structural design, analysis, and sizing of the ASAPP wing are detailed here. The study shows that the ASAPP wing can be superior to the baseline wing (without morphing) in terms of aerodynamic efficiency, especially when the deformation of the flexible skin is minimal. Moreover, the passive pitching near the wingtip can reduce the root loads significantly, minimizing the weight penalty usually associated with morphing

The Transformer aircraft: a multimission unmanned aerial vehicle capable of symmetric and asymmetric span morphing

This paper presents the development and extensive testing of the Transformer aircraft, a multimission UAV capable of symmetric and asymmetric span morphing. The UAV utilises a novel actuation system based on a rack and pinion mechanism to achieve span extensions up to 50%. The Transformer can morph symmetrically to enhance flight performance and asymmetrically to provide roll control. Extensive mechanical testing followed by wind-tunnel testing in the RJ Mitchell Wind-tunnel at the University of Southampton were conducted to ensure structural integrity and assess the behaviour of the UAV. Finally, a series of flight-testing were performed and the flight mechanics aspects associated with both symmetric and asymmetric span morphing were investigated.</p

On the Aeroelasticity of the Active Span and Passive Pitching Polymorphing Wing: Effect of Morphing Rate

This paper studies the effect of morphing rate on the aeroelasticity of a polymorphing wing capable of active span extension and passive twist/pitch. A variable domain size finite element model is developed to capture the dynamic effects due to the presence of a variable span in the Euler–Bernoulli beam model, which introduces a structural damping term in the equations of motion. The effect of various morphing rates on the aeroelastic boundaries of the system, namely, flutter velocity and flutter frequency, is examined for a beam undergoing span extension and retraction, from baseline span to 25% span extension and vice versa, respectively. Three points of interest are analyzed: at the start of the span morphing, at the mid-point of morphing, and just before the morphing process ends. The parametric analysis is carried out to determine the effect of varying critical parameters, such as the elastic axis location of the outboard wing section and adjoining spring torsional rigidity on the aeroelastic boundaries of the polymorphing wing

Multiaxial Deformations of Elastomeric Skins for Morphing Wing Applications: Theoretical Modeling and Experimental Investigations

An elastomeric class of flexible skin-based polymorphing wings changes its configuration to maximize performance at radically different flight conditions. One of the key design challenges for such an aircraft technology is the multiaxial deformation characterization and modeling of nonlinear elastomeric skins of polymorphing wings. In the current study, three elastomeric materials, Latex, Oppo, and Ecoflex, are experimentally characterized and modeled under all possible deformation modes such as uniaxial, pure shear, biaxial, and equibiaxial relevant for flexible skin-based morphing wing applications. Additionally, a novel material model with four material constants is proposed to model the considered elastomers-based morphing wings keeping all the material parameters constant for all the possible deformation modes. The present experimental and theoretical study provides a concise comparative study of the three elastomers used in the morphing wings tested in all possible deformation modes

On the Aeroelasticity of the Active Span and Passive Pitching Polymorphing Wing: A Parametric Study

This paper presents an aeroelastic analysis of a polymorphing wing capable of active span extension and passive pitch variation. The wing is split into two segments: an inboard segment responsible for span extension/retraction and an outboard segment capable of pitch variation. The two segments are connected through an overlapping spar and a torsional spring. A finite element aeroelastic model is developed where the wing structure is discretized into Euler&ndash;Bernoulli beam elements and the aerodynamic loads are calculated using Theodorsen&rsquo;s unsteady model. A comprehensive parametric analysis is carried out with and without span extension to analyze the effect of varying critical design parameters, such as elastic axis position of outboard section and torsional spring rigidity, and conditions for aeroelastic phenomena of flutter and divergence are studied. A gust load analysis is carried out to quantify the capability of the outboard wing passive twist mechanism to alleviate loads. Finally, a nonlinear analysis is carried out by replacing the linear torsional spring with a nonlinear cubic spring to study the effects of cubic hardening and softening on the aeroelasticity of the polymorphing wing

Morphing aircraft: the need for a new design philosophy

This paper proposes a novel framework for classification of morphing technology based on its functionality, operation, and the structural layout. In addition, it highlights the limitations of the conventional design approach to exploit the benefits of the technology using representative examples and results

On the Aeroelasticity of a Cantilever Wing Equipped with the Spanwise Morphing Trailing Edge Concept

This paper studies the aeroelastic behavior of a rectangular, cantilever wing equipped with the spanwise morphing trailing edge (SMTE) concept. The SMTE consists of multiple trailing edge flaps that allow controlling the spanwise camber distribution of a wing. The flaps are attached at the wing’s trailing edge using torsional springs. The Rayleigh–Ritz method is used to develop the equations of motion of the wing-flap system. The use of shape functions allows for representing the wing as an equivalent 2D airfoil with generalized coordinates that are defined at the wingtip. Strip theory, based on Theodorsen’s unsteady aerodynamic model, is used to compute the aerodynamic loads acting on the wing. A representative Padé approximation for Theodorsen’s function is utilized to model the aerodynamic behaviors in a state-space form allowing time-domain simulation and analysis. The model is validated using a rectangular cantilever wing and the data are available in the literature. A comprehensive parametric comparison study is conducted to assess the impact of flap stiffness on the aeroelastic boundary. In addition, the potential of the SMTE to provide load alleviation and flutter suppression is assessed for a wide range of flight conditions, using a discrete (1-cosine) gust. Finally, the implementation and validation of a controller for a wing with SMTE for gust load alleviation are studied and controller parameters are tuned for a specific gust model