Morphing Wing Research Articles

A Multiaxial Fracture of Ecoflex Skin with Different Shore Hardness for Morphing Wing Application

The use of elastomer-based skins in morphing wings has become increasingly popular due to their remarkable stretchability and mechanical properties. However, the possibility of the skin fracturing during multiaxial stretching remains a significant design challenge. The propagation of cracks originating from flaws or notches in the skin can lead to the specimen breaking into two parts. This paper presents an experimental study aimed at comprehensively evaluating crack propagation direction, stretchability, and fracture toughness of silicone-based elastomeric skin (Ecoflex) for morphing wing applications, using varying Shore hardness values (10, 30, and 50). The findings show that the lower Shore hardness value of 10 exhibits a unique Sideways crack propagation characteristic, which is ideal for morphing skins due to its high stretchability, low actuation load, and high fracture toughness. The study also reveals that the Ecoflex 10 is suitable for use in span morphing, with a fracture toughness of approximately 1.1 kJ/m2 for all thicknesses at a slower strain rate of 0.4 mm/min. Overall, this work highlights the superior properties of Ecoflex 10 and its potential use as a morphing skin material, offering a groundbreaking solution to the challenges faced in this field.

Functional Hazard Assessment of a Modular Re-Configurable Morphing Wing Using Taguchi and Finite Element Methodologies

Growing concerns over the CO2 footprint due the exponential demand of the aviation industry, along with the requirements for high aerodynamic performance, cost saving, and manoeuvrability during different phases of a flight, pave the path towards adaptable wing design. Morphing wing design encompasses most, if not all, of the flight condition variations, and can respond interactively. However, functional failure of the morphing wing might bring devastating impacts on the passengers, crew, and/or aircraft. In the present work, the dynamic characteristics of a re-configurable modular morphing wing developed in-house by a research group at the Toronto Metropolitan University, are investigated from the perspective of a functional hazard assessment (FHA). This modular morphing wing, developed based on the idea of a parallel robot, consists of a number of structural elements connected to each other and to the wing ribs through eye-bolt joints. Timoshenko’s bending beam theory, in conjunction with the finite element method (FEM), is exploited to model the structural members. Possible hazards, assumed here to be the structural failure of the beam components, have been identified and their failure conditions are assessed. Numerical simulations have been presented to show the impact of various combinations of the identified hazards on the vibration signature of the morphing wing in unmorphed and morphed configurations. Identification of changes in the wing’s vibration signature is a vital component in the fail-safe structural and aeroelastic design of an aircraft. The present study is geared towards the structural response of the system in the absence of any aerodynamic loads.

Structured Expert Judgment Elicitation in Conceptual Aircraft Design

Disruptive technologies and novel aircraft generations represent a potential approach to address the ambitious emission reduction goals in aviation. However, the introduction of innovative concepts is a time-consuming process, which might not necessarily yield an optimal design for a given flight mission and within the defined time frame. In order to address the need for a structured and more exhaustive search for novel concept generations, the Advanced Morphological Approach (AMA) and its further enhancement was introduced earlier. It implies the decomposition of design problems into functional attributes and appropriate technological alternatives. Subsequently, these are evaluated and combined into solutions, which are then projected onto a solution space. The current paper focuses on the technology evaluation step by deriving and integrating structured expert judgment elicitation (SEJE) techniques into conceptual aircraft design with the AMA. For this purpose, the first aim of the work is to justify the developed method by giving an overview and discussing the most prominent SEJE methods and their applications in aerospace. Then, the derived SEJE concept is described and applied in the form of an expert workshop on the use case of wing morphing architecture. As a result, a solution space of wing morphing architecture configurations is generated and analyzed. The workshop conduction and the expert feedback serve as valuable findings for both the further AMA enhancement and similar research.

Design and Experiment of a Seamless Morphing Trailing Edge

Morphing trailing edge wing as an important morphing wing technology has gained wide attention because of its advantages, such as gust mitigation, improved aerodynamic efficiency, and reduced radar reflective area. However, the key problems such as low load carrying capacity and insufficient smooth deformation profile are still not solved in a balanced manner. The purpose of this paper is to design a seamless morphing trailing edge structure that has good load bearing capacity and can realize a chord-wise camber variation with a smooth contour subjected to the required aerodynamic load. In this paper, an innovative seamless trailing edge structure is proposed, and the critical dimensions and parameters are designed through a parametric study based on the 2D and 3D finite element models of the trailing edge structure. A physical prototype was designed and fabricated for deformation and load-bearing experiments. The finite element simulation and experimental results show that the morphing trailing edge can carry a 0.015 MPa aerodynamic load and realize the ±15° smooth camber change. The present study demonstrates the effectiveness and potential of the proposed morphing trailing edge concept for the real application on aircrafts.

Application of 3D-scanning for structural and geometric assessment of aerospace structures

The geometric assessment of physical demonstrators are an integral part of several research projects conducted at the Chair of Aircraft Design at the Technical University of Munich. The projects range from several research UAVs, a sailplane morphing wing to propellers. There are different project objectives like the assessment of manufacturing deviations, design and function validation as well as reverse engineering of aerodynamic surfaces for model adaptation and simulation in the projects. Nevertheless, mutual approaches and solutions have been identified. Therefore joint development efforts are undertaken using 3D-scanning technology for data collection and evaluation. This technology captures the surface of a given object typically as a point cloud with comparably high accuracy. Since a manual evaluation process bears disadvantages in terms of reproducibility, custom post-processing software tools are developed. Global geometry data, like wing platform data, as well as airfoils can be extracted from a surface point cloud to analyze UAV wings or propellers. Airfoils can be derotated, normed and smoothed for aerodynamic analysis with low-fidelity aerodynamic tools, such as XFLR5 or XFOIL. For the analysis of morphing airfoil structures, the scanned geometry is aligned with the desired design airfoil shape so they can be compared. In this paper, analysis methods and several example results are presented.

Aero-structural optimization and actuation analysis of a morphing wing section with embedded selectively stiff bistable elements

Morphing wings provide a potential avenue to improve aerodynamic performance of aircraft operating at multiple design conditions. Nevertheless, morphing wing design is constrained by the mutually exclusive goals of high load-carrying capacity, low weight, and sufficient aerodynamic control authority via conformal shape adaptation. This trade-off can be addressed by exploiting the stiffness selectivity and shape “lock-in” properties enabled by using bistable beam-like elements within compliant structures. In this paper, we present an aero-structural optimization method to realize morphing structures with selective stiffness and shape “lock-in” capability from embedded bistable elements. We leverage an embeddable beam element with an invertible curved arch that provides stiffness selectivity and camber variation to the proposed rib geometry. Optimization objectives and constraints are designed to maximize the structure’s stiffness change and camber morphing “lock-in” effect when operating at two distinct flight conditions. Using the optimization results, we manufacture a wing section demonstrator with selective stiffness and “lock-in” morphing featuring two optimized ribs, a load-carrying skin made of a carbon reinforced laminate, Macro-Fiber Composite (MFC) actuators, and a servo-controlled mechanism for switching the bistable elements’ states. The power and energy requirements of actuating and holding a target deflection are experimentally measured and compared. The results show that the bistable elements can assist in holding a target deflection at a reduced energy cost. Finally, we test the experimental demonstrator in a low-speed wind tunnel demonstrating the load carrying capability and lift variation achieved from switching states.

A Novel Actuation Strategy for an Agile Bioinspired FWAV Performing a Morphing-Coupled Wingbeat Pattern

Flying vertebrates exhibit sophisticated wingbeat kinematics. Their specialized forelimbs allow for the wing morphing motion to couple with the flapping motion during their level flight, Previous flyable bionic platforms have successfully applied bio-inspired wing morphing but cannot yet be propelled by the morphing-coupled wingbeat pattern. Spurred by this, we develop a bio-inspired flapping-wing aerial vehicle (FWAV) entitled RoboFalcon, which is equipped with a novel mechanism to drive the bat-style morphing wings, performs a morphing-coupled wingbeat pattern, and overall manages an appealing flight. The novel mechanism of RoboFalcon allows coupling the morphing and flapping during level flight and decoupling these when maneuvering is required, producing a bilateral asymmetric downstroke affording high rolling agility. The bat-style morphing wing is designed with a tilted mounting angle around the radius at the wrist joint to mimic the wrist supination and pronation effect of flying vertebrates' forelimbs. The agility of RoboFalcon is assessed through several rolling maneuver flight tests, and we demonstrate its well-performing agility capability compared to flying creatures and current flapping-wing platforms. Wind tunnel tests indicate that the roll moment of the asymmetric downstroke is correlated with the flapping frequency, and the wrist mounting angle can be used for tuning the angle of attack and lift-thrust configuration of the equilibrium flight state. We believe that this work yields a well-performing bionic platform and provides a new actuation strategy for the morphing-coupled flapping flight.

Optimal Elastic Wing for Flapping-Wing Robots Through Passive Morphing

Flapping wing robots show promise as platforms for safe and efficient flight in near-human operations, thanks to their ability to agile maneuver or perch at a low Reynolds number. The growing trend in the automatization of these robots has to go hand in hand with an increase in the payload capacity. This work provides a new passive morphing wing prototype to increase the payload of this type of UAV. The prototype is based on a biased elastic joint and the holistic research also includes the modelling, simulation and optimization scheme, thus allowing to adapt the prototype for any flapping wing robot. This model has been validated through flight experiments on the available platform, and it has also been demonstrated that the morphing prototype can increase the lift of the robot under study by up to 16% in real flight and 10% of estimated consumption reduction.

Design and Shape Monitoring of a Morphing Wing Trailing Edge

The morphing wing trailing edge is an attractive aviation structure due to its shape-adaptive ability, which can effectively improve the aerodynamic performance of an aircraft throughout the whole flight. In this paper, a mechanical solution for a variable camber trailing edge (VCTE) based on a multi-block rotating rib is proposed. Parametric optimizations are conducted to achieve the smooth and continuous deformation of the morphing rib. A prototype is designed according to the optimized results. In addition, the deformations of the trailing edge are monitored via an indirect method using a fiber Bragg grating (FBG) sensor beam. Finally, ground tests are performed to investigate the morphing capacity of the VCTE and the shape monitoring ability of the proposed method. Our results indicate that a maximum deflection range from 5° upward to 15° downward can be obtained for the VCTE and the indirect sensing system can satisfactorily monitor the deformation of the trailing edge.

Wind Defiant Morphing Drones

Intense winds are a challenge for vertical take‐off and landing drones with wings. In particular, in the hovering regime, wings are sensitive to wind currents that can be detrimental to their operational and energetic performances. Tail‐sitters are particularly prone to those wind currents because their wings are perpendicular to the incoming wind during hovering. This wind generates a large amount of drag and can displace and destabilize the vehicle, possibly leading to catastrophic failures. Herein, our morphing strategy demonstrates in a custom‐built 1.8 kg tail‐sitter with morphing wings that can actively resist winds and leverage them to increase its aerodynamic efficiency. It is shown that adaptive wing morphing during hovering in adverse wind conditions can reduce normalized energy consumption up to 85%, increase attitude and positional stability, and leverage wind energy to increase its yaw angular rate up to 200% while decreasing motor saturation levels.

Enhanced Range and Endurance Evaluation of a Camber Morphing Wing Aircraft

Flight range, endurance, maneuverability, and agility are the key elements that determine an aircraft's performance. Both conventional and morphing wing aircraft have been well studied and estimated in all aspects of performance. When considering the performance of morphing aircraft, most works address aspects of the aerodynamical performance such as L and D as well as flight envelopes for flight dynamics and control perspectives. However, the actual benefits of adopting morphing technologies in practical aspects such as aircraft operation, mission planning, and sustainability have not been addressed so far. Thus, this paper addresses the practical aspect of the benefits when adopting a camber morphing wing aircraft. Identical geometrical and computational conditions were applied to an already-existing aircraft: the RQ-7a Shadow. The wing structure was switched between a fixed wing and a camber morphing wing to generate conventional and morphing wing geometries. The fixed-wing cases had varying flap deflection angles, and the camber morphing wing cases had varying camber rates from 4% to 8%. Once the CL values of the fixed and morphing wing cases were matched up to two significant figures, the CD and CL/CD were analyzed for these matching cases to calculate the flight endurance, range, and improvement. When NACA 6410 is adopted, a 17% improvement in flight range and endurance average was expected. In the case of NACA 8410, an average 60% improvement was expected.

Autonomous material composite morphing wing

Aeronautics research has continually sought to achieve the adaptability and morphing performance of avian wings, but in practice, wings of all scales continue to use the same hinged control-surface embodiment. Recent research into compliant and bio-inspired mechanisms for morphing wings and control surfaces has indicated promising results, though often these are mechanically complex, or limited in the number of degrees-of-freedom (DOF) they can control. Seeking to improve on these limitations, we apply a new paradigm denoted Autonomous Material Composites to the design of avian-scale morphing wings. With this methodology, we reduce the need for complex actuation and mechanisms, and allow for three-dimensional placement of stretchable fiber optic strain gauges (Optical Lace) throughout the metamaterial structure. This structure centers around elastomeric conformal lattices, and by applying functionally-graded warping and thickening to this lattice, we allow for local tailoring of the compliance properties to fit the desired morphing. As a result, the wing achieves high-deformation morphing in three DOF: twist, camber, and extension/compression, with these morphed shapes effectively modifying the aerodynamic performance of the wing, as demonstrated in low-Reynolds wind tunnel testing. Our sensors also successfully demonstrate differentiable trends across all degrees of morphing, enabling the future state estimation and control of this wing.

Development of a morphing UAV for optimal multi-segment mission performance

AbstractThe need for innovative solutions to enable aerial platforms to fly faster, higher, and longer continues to remain a primary focus for airframe designers. This paper outlines work undertaken to apply a morphing wing warping technology onto a generic Unmanned Aerial Vehicle to deliver enhanced flight performance, efficiency and control capabilities. The prototype employs wings of novel construction which provide both near resistance-free compliance in twist as well as adequate structural stiffness to resist applied loads; all while preserving an aerodynamically smooth surface. Used in combination with developed and integrated closed-loop feedback control architecture, a real-time, non-linear, span-wise wing twist adjustment capability required for optimised flight under differing operating conditions and flight requirements, is demonstrated. Experimental results obtained from a wind tunnel test program show up to a 72% increase in lift to drag ratio under certain conditions compared to a fixed baseline providing some confidence that the combination could be used to realise a step change in flight performance.

Application Status and Future Prospect of Aircraft Morphing Wing

Application Status and Future Prospect of Aircraft Morphing Wing

On the role of tail in stability and energetic cost of bird flapping flight

Migratory birds travel over impressively long distances. Consequently, they have to adopt flight regimes being both efficient—in order to spare their metabolic resources—and robust to perturbations. This paper investigates the relationship between both aspects, i.e., energetic performance and stability, in flapping flight of migratory birds. Relying on a poly-articulated wing morphing model and a tail-like surface, several families of steady flight regime have been identified and analysed. These families differ by their wing kinematics and tail opening. A systematic parametric search analysis has been carried out, in order to evaluate power consumption and cost of transport. A framework tailored for assessing limit cycles, namely Floquet theory, is used to numerically study flight stability. Our results show that under certain conditions, an inherent passive stability of steady and level flight can be achieved. In particular, we find that progressively opening the tail leads to passively stable flight regimes. Within these passively stable regimes, the tail can produce either upward or downward lift. However, these configurations entail an increase of cost of transport at high velocities penalizing fast forward flight regimes. Our model-based predictions suggest that long range flights require a furled tail configuration, as confirmed by field observations, and consequently need to rely on alternative mechanisms to stabilize the flight.

Numerical Analysis of Variable Morphing Wing for Improved Aerodynamic Performance of a Predator MQ-1B

In this paper, aerodynamic performance benefits of morphing unmanned aerial vehicle’s wing concepts are investigated. A Predator MQ-1B with variable wing structure was utilized for this study. The concept consists of variable twist (-10°< θ <10°, in steps of ±2.5°) and wing sweep (0°< θ <30°, in steps of +10°) to illustrate morphing wing’s performance benefits. All computations were performed with Athena Vortex Lattice modelling with varying degrees of twist and sweep angle considered. The results obtained from this work show that if morphing wings adapted to the Predator MQ-1B, it will provide significant performance benefits and also offer a great opportunity to reduce fuel consumption.

High actuation capability and smooth-deformation piezo morphing wing based on multi-layer parallel pre-compressed MFC bimorph

A single macro-fiber composite (MFC) unimorph or bimorph wing can only be applied to micro aerial vehicles (MAV) at speeds lower than 20 m/s owing to its relatively small output work. In this study, a high-actuation MFC pre-compressed bimorph actuator (MFC-PBA) multilayer parallel morphing wing is proposed for application in high-speed miniature unmanned aerial vehicles (mini-UAVs). The morphing wing adopts the proposed ribs with an arc groove and three parallel layers of MFC-PBAs to improve the output work and guarantee smooth morphing wing surface. The large deformation finite element (FE) models of a single MFC-PBA and its multilayer parallel morphing wing are established to evaluate the output deformation and output force. Subsequently, a prototype of the morphing wing is fabricated, and the actuation capabilities of the single MFC-PBAs and morphing wing are tested. The experimental and FE simulation results satisfactorily agree. The results show that at the peak driving voltage, with axial compression forces of 18 and 27 N applied to the side and middle MFC-PBAs, respectively, the deflection angle of the morphing wing reaches ±12°, which is twice that of the morphing wing without compression; and its blocking force is maintained at 1.682 N; thus, its output work has doubled. The multiphysics model of the proposed morphing wing under airflow is established to evaluate the deformation capability. The results show that the present morphing wing achieves a deflection of 5.04° on the side of the incoming flow direction, and a unit lift of 249.1 N/m at an air speed of 40 m/s and angle of attack of 15°. Consequently, the proposed morphing wing exhibits a high deformation capability. We expect the proposed morphing wing will be used for the flight control of mini-UAVs at higher speeds and larger sizes.

Vision-Based Nonlinear Incremental Control for a Morphing Wing With Mechanical Imperfections

Morphing structures have acquired much attention in the aerospace community because they enable an aircraft to actively adapt its shape during flight, leading to fewer emissions and fuel consumption. Researchers have designed, manufactured, and tested a morphing wing named SmartX-Alpha, which can actively alleviate loads while achieving the optimal lift distribution. However, the widely existing mechanical imperfections can degrade the performance of the morphing wing and even lead to instabilities. To tackle these issues, this article proposes a vision-based adaptive control approach to actively compensate for mechanical imperfections. In this approach, an incremental model is constructed online to identify the system dynamics using servo commands and vision measurements, and then, nonlinear dynamic inversion control is applied based on the identified model. This data-driven control approach with visual feedback has been validated by real-world experiments on the SmartX-Alpha. The results demonstrate that the vision-based system combined with the proposed control methodology can actively compensate for mechanical imperfections with minimal adjustments to the actual system design. Compared to a controller that only uses a feedforward input-output mapping, this proposed approach improves the system performance and decreases the tracking errors by more than 62% despite disturbances. The results collectively demonstrate the effectiveness of the proposed control system, which sets a foundation for realizing morphing in next-generation aircraft.

Design of a Distributedly Active Morphing Wing Based on Digital Metamaterials

Morphing wings are a typical application of shape-adaptive structures in aviation, which play an important role in improving the comprehensive performance of an aircraft. However, traditional morphing wings based on purely mechanical, rigid-flexible coupling, or purely flexible structures usually cannot achieve a distributed morphing ability and have limitations in weight, intelligence level, and reliability. In this paper, a distributed morphing lattice structure based on variable geometry digital metamaterials is proposed. The innovative structural concept consists of three types of fundamental cells featuring remarkably different mechanical properties and three other types of derived cells. One type of the derived cells embedded with micro-actuators, named an active cell, can autonomously extend or contract. All these cells can be reversibly assembled in a random sequence to form an active distributed morphing lattice structure with the ability to realize different target aerodynamic contours. In addition, taking a simplified variable thickness wing as a designing case, this paper develops a cell combination optimization methodology on the basis of a heuristic algorithm to determine the optimal combination sequence of the six types of basic cells and the actuator inputs of active cells collaboratively. Final results show that the optimized lattice structure can morph its outer surface into a predefined aerodynamic contour with a maximum deviation of 3 mm.

Morphing wing design using integrated and distributed trailing edge morphing

The presented study investigates the design and development of an autonomous morphing wing concept developed in the scope of the SmartX project, which aims to demonstrate in-flight performance optimisation with active morphing. To progress this goal, a novel distributed morphing concept with six translation induced camber morphing trailing edge modules is proposed in this study. The modules are interconnected using elastomeric skin segments to allow seamless variation of local lift distribution along the wingspan. A fluid-structure interaction optimisation tool is developed to produce an optimised laminate design considering the ply orientation, laminate thickness, laminate properties and actuation loads of the module. Analysis of the kinematic model of the integrated actuator system is performed, and a design is achieved, which meets the required continuous load and fulfils both static and dynamic requirements in terms of bandwidth and peak actuator torque with conventional actuators. The morphing design is validated using digital image correlation measurements of the morphing modules. Characterisation of mechanical losses in the actuator mechanism is performed. Out-of-plane deformations in the bottom skin and added stiffness of the elastomer are identified as the impacting factors of the reduced tip deflection.