Details of insect wing design and deformation enhance aerodynamic function and flight efficiency

Morphological diversification has led to inter-specific variation in elastic wing deformation during flight in scarab beetles

Insect wing shapes and the internal wing-vein arrangement are remarkably diverse. Although the wings lack intrinsic musculature to adjust shape actively, they elastically deform due to aerodynamic and inertial loads during flapping. In turn, the deformations alter the shape of the wing profile affecting the aerodynamic force. To determine how changes in wing-vein arrangement affect elastic wing deformation during free flight, we compared elastic wing deformations between free-flying rose chafers (Protaetia cuprea) and dung beetles (Scarabaeus puncticollis), complementing the comparison with wing static bending measurements. The broader relevance of the results to scarab beetle divergence was examined in a geometric morphometric (GM) analysis of wing-vein arrangement in 20 species differing in phylogeny and ecology. Despite rose chafers and dung beetles demonstrating similar flapping kinematics and wing size, the rose chafer wings undergo greater elastic deformation during flapping. GM analyses corrected for phylogenetic relatedness revealed that the two beetles represent extremes in wing morphology among the scarab subfamilies. Most of the differences occur at the distal leading edge and the proximal trailing edge of the wing, diversifying the flexibility of these regions, thereby changing the pattern of elastic wing deformation during flapping. Changes to local wing compliance seem to be associated with the diversification of scarab beetles to different food sources, perhaps as an adaptation to meet the demands of diverse flight styles.

Structural characteristics analysis of the hind wings in a bamboo weevil (Cyrtotrachelus buqueti)

The finite element method is a powerful tool for evaluating the experimental results. It can help to study the flight mechanism of insects and the structural characteristics of flying wings. Therefore, the research object based on the hind wings of Cyrtotrachelus buqueti (C. buqueti) was completed here. A finite element model with a length of 45 mm in the spanwise direction and a 16 mm width in the chordwise direction were established. We used a three-dimensional (3D) scanner to scan a real hind wing to obtain point cloud images. The physical model of the hind wing was carried out by using both the software Imageware and Unigraphics NX. To quantify the quality of the finite element model of the hind wing, the material properties of the wing membranes and veins were conducted by the tensile testing machine. The structural static properties of the hind wing, including static characteristics analysis and natural vibration modal analysis, were analysed by ANSYS; the stress and deflection under uniformly distributed load, bending moment, and torque were, respectively, shown. It was found that the model only had a small deformation, which shows that the hind wings of C. buqueti have excellent structural properties.

Three-dimensional wing structure attenuates aerodynamic efficiency in flapping fly wings

The aerial performance of flying insects ultimately depends on how flapping wings interact with the surrounding air. It has previously been suggested that the wing's three-dimensional camber and corrugation help to stiffen the wing against aerodynamic and inertial loading during flapping motion. Their contribution to aerodynamic force production, however, is under debate. Here, we investigated the potential benefit of three-dimensional wing shape in three different-sized species of flies using models of micro-computed tomography-scanned natural wings and models in which we removed either the wing's camber, corrugation, or both properties. Forces and aerodynamic power requirements during root flapping were derived from three-dimensional computational fluid dynamics modelling. Our data show that three-dimensional camber has no benefit for lift production and attenuates Rankine-Froude flight efficiency by up to approximately 12% compared to a flat wing. Moreover, we did not find evidence for lift-enhancing trapped vortices in corrugation valleys at Reynolds numbers between 137 and 1623. We found, however, that in all tested insect species, aerodynamic pressure distribution during flapping is closely aligned to the wing's venation pattern. Altogether, our study strongly supports the assumption that the wing's three-dimensional structure provides mechanical support against external forces rather than improving lift or saving energetic costs associated with active wing flapping.

The impact of dragonfly wing deformations on aerodynamic performance during forward flight

Bulk wing kinematics and wing deformations of free-flying dragonflies of the species Pachydiplax longipennis were measured in a controlled environment. Both upright and inverted straight flights were recorded and analyzed. The inverted dragonflies exhibited similar bulk kinematics to the upright specimens in the global frame, but wing deformations were generally consistent in the body-relative frame. The deformations primarily comprised camber during the body-relative downstroke and twist during the body-relative upstroke. Based on these data, models were developed to incorporate the measured kinematics and deformations into computational fluid dynamics simulations. Both isolated and tandem wings were simulated (rigid and deforming in each case), allowing the effects of deformations and wing-wing interactions to be examined independently. During the upstroke the addition of deformation reduced flow separation on the outboard sections of the wing, whereas the impact of the deformation during the downstroke was found to be dependent on the wing kinematics. The simulations of tandem wings indicated that they produce more force than isolated wings, but the wing deformations reduced the impact of this wing-wing interaction. The changes in average lift and thrust induced by the wing deformations were relatively minor and dependent on the flight orientation, but the aerodynamic efficiency of the deforming wings was significantly higher than that of the rigid wings for all examined cases, including the inverted flights for which the deformations were in the opposite (global) sense to the upright flights.

Pectoral fin kinematics and motor patterns are shaped by fin ray mechanosensation during steady swimming in Scarus quoyi

For many fish species, rhythmic movement of the pectoral fins, or forelimbs, drives locomotion. In terrestrial vertebrates, normal limb-based rhythmic gaits require ongoing modulation with limb mechanosensors. Given the complexity of the fluid environment and dexterity of fish swimming through it, we hypothesize that mechanosensory modulation is also critical to normal fin-based swimming. Here, we examined the role of sensory feedback from the pectoral fin rays and membrane on the neuromuscular control and kinematics of pectoral fin-based locomotion. Pectoral fin kinematics and electromyograms of the six major fin muscles of the parrotfish, Scarus quoyi, a high-performance pectoral fin swimmer, were recorded during steady swimming before and after bilateral transection of the sensory nerves extending into the rays and surrounding membrane. Alternating activity of antagonistic muscles was observed and drove the fin in a figure-of-eight fin stroke trajectory before and after nerve transection. After bilateral transections, pectoral fin rhythmicity remained the same or increased. Differences in fin kinematics with the loss of sensory feedback also included fin kinematics with a significantly more inclined stroke plane angle, an increased angular velocity and fin beat frequency, and a transition to the body-caudal fin gait at lower speeds. After transection, muscles were active over a larger proportion of the fin stroke, with overlapping activation of antagonistic muscles rarely observed in the trials of intact fish. The increased overlap of antagonistic muscle activity might stiffen the fin system in order to enhance control and stability in the absence of sensory feedback from the fin rays. These results indicate that fin ray sensation is not necessary to generate the underlying rhythm of fin movement, but contributes to the specification of pectoral fin motor pattern and movement during rhythmic swimming.

Wasp Waist and Flight: Convergent Evolution in Wasps Reveals a Link between Wings and Body Shapes

Insect flight is made possible by different morphological structures: wings produce the lift, the thorax drives the wings' movements, and the abdomen serves as a secondary control device. As such, the covariation of these structures could reflect functional constraints related to flight performances. This study examines evolutionary convergences in wasp body shapes to provide the first evidence for morphological integration among insect wings, thorax, and abdomen. The shapes of the forewings and hind wings, thorax, and petiole (connecting abdomen and thorax) of 22 Vespidae species were analyzed using computerized tomography and geometric morphometrics. Results show a clear relationship between petiole and wings or thorax shapes but not between wings and thorax. Wasps with elongated bodies have pointed wings, both features thought to improve flight maneuverability. In contrast, stouter species have rounded wings, which may allow for higher flight speeds. These integration patterns suggest that multiple selective regimes on flight performance, some of them biased toward maneuverability or maximal speed, drove the morphological diversity in Vespidae. The results also suggest that wing shapes evolved under constraints related to the body type they have to lift. The abdomen morphology is thus another factor to take into account to understand the flight performance of insects.

Threading the Spindle: A Geometric Study of Chiral Liquid Crystal Polymer Microparticles

Polymeric particles are strong candidates for designing artificial materials capable of emulating the complex twisting-based functionality observed in biological systems. In this Letter, we provide the first detailed investigation of the swelling behavior of bipolar polymer liquid crystalline microparticles. Deswelling from the spherical bipolar configuration causes the microparticles to contract anisotropically and twist in the process, resulting in a twisted spindle-shaped structure. We propose a model to describe the observed spiral patterns and twisting behavior.

Structure, properties and functions of the forewing-hindwing coupling of honeybees

Worker honeybees (Apis mellifera) are morphologically four-winged, but are functionally dipterous insects. During flight, their fore- and hindwings are coupled by means of the forewing posterior rolled margin (PRM) and hindwing hamuli. Morphological analysis shows that the PRM can be connected to the hamuli, so that the fore- and hindwing are firmly hinged, and can rotate with respect to each other. In the present study, using a combination of scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM), we investigate the micromorphology and material composition of the coupling structures on both fore- and hindwings. High-speed filming is utilized to determine the angle variation between the fore- and hindwings in tethered flight. Using sets of two-dimensional (2D) computation fluid dynamic analyses, we further aim to understand the influence of the angle variation on the aerodynamic performance of the coupled wings. The results of the morphological investigations show that both PRM and hamuli are made up of a strongly sclerotized cuticle. The sclerotized hinge-like connection of the coupling structure allows a large angle variation between the wings (135°-235°), so that a change is made from an obtuse angle during the pronation and downstroke to a reflex angle during the supination and upstroke. Our computational results show that in comparison to a model with a rigid coupling hinge, the angle variation of a model having a flexible hinge results in both increased lift and drag with a higher rate of drag increase. This study deepens our understanding of the wing-coupling mechanism and functioning of coupled insect wings.

Extremely large sweep amplitude enables high wing loading in giant hovering insects

Beetle Allomyrina dichotoma is one of the largest insects that performs many remarkable modes of locomotion, particularly hovering flight capability. In order to stay airborne, its flexible hindwings are flapped so as to work as a thrust generator. However, the wing loading of the beetle is relatively large (38.94  ±  3.73 N m^-2) compared to those of other insects and hummingbirds, indicating that it is challenging for it to achieve flight. Here, we measured the hindwing morphology and kinematics of the beetle in order to discover its flight performance. Unlike many other insects, the beetle flaps its hindwings with an extremely large sweep amplitude of about 191.33  ±  6.12 deg at high flapping frequencies ranging from 36 to 41 Hz (mean wing tip speed  ≈  13.45  ±  0.58 m s^-1). These capabilities enable the beetle to produce enough lift force to stay airborne with its bulky body (4-10 g). In order to investigate how the sweep amplitude affects the beetle's flight efficiency, we utilized the unsteady blade element model to estimate the power requirements of the same vertical force production for various sweep amplitudes. The results indicate that the sweep amplitude as high as 190 deg is more beneficial for power requirements than the smaller amplitudes, which require higher frequencies resulting in higher inertial powers to produce the same vertical force. Thus, for this large beetle, high sweep amplitude may be a biological strategy for staying airborne. In addition, we thoroughly discussed the effects of input constraints on the outcome by investigating power loadings for variable sweep amplitudes at a constant vertical force, mean wing tip speed, and flapping frequency. Effect of wing surface area was also investigated and discussed to provide useful information for the development of an insect-inspired flapping-wing robot.

Wing flexibility reduces the energetic requirements of insect flight

Flapping insect wings deform under aerodynamic as well as inertial-elastic forces. This deformation is thought to improve power economy and reduce the energetic costs of flight. However, many flapping wing models employ rigid body simplifications or demand excessive computational power, and are consequently unable to identify the influence of flexibility on flight energetics. Here, we derive a reduced-order model capable of estimating the driving torques and corresponding power of flapping, flexible insect wings. We validate this model by actuating a tobacco hornworm hawkmoth Manduca sexta (L.) forewing with a custom single-degree-of-freedom mechanical flapper. Our model predicts measured torques and instantaneous power with reasonable accuracy. Moreover, the flexible wing model predicts experimental trends that rigid body models cannot, which suggests compliance should not be neglected when considering flight dynamics at this scale. Next, we use our model to investigate flight energetics with realistic flapping kinematics. We find that when the natural frequency of the wing is roughly three times that of the flapping frequency, flexibility can reduce energy expenditures by almost 25% compared to a rigid wing if negative work is stored as potential energy and subsequently released to do positive work. The wing itself can store about 30% of the 1200 [Formula: see text]J of total energy required over a wingbeat. Peak potential energy storage occurs immediately before stroke reversal. We estimate that for a moth weighing 1.5-2.5 g, the peak instantaneous power required for flight is 75-125 W kg^-1. However, these peak values are likely lower in natural insect flight, where the wing is able to exchange strain energy with the compliant thorax. Our findings highlight the importance of flexibility in flapping wing micro aerial vehicle design and suggest tuned flexibility can greatly improve vehicle efficiency.

Canonical description of wing kinematics and dynamics for a straight flying insectivorous bat (Hipposideros pratti)

Bats, with highly articulated wings, are some of the most agile flyers in nature. A novel three-dimensional geometric decomposition framework is developed to reduce the complex kinematics of a bat wing into physical movements used to describe flapping flight: namely flapping, stroke plane deviation and pitching, together with cambering and flexion. The decomposition is combined with aerodynamic simulations to investigate the cumulative effect of each motion on force production, and their primary contribution to the unsteady vortex dynamics. For the nearly straight and level flight of Hipposideros pratti, results show that the flapping motion by itself induced a moderate drag and lift. Stroke plane deviation increased lift, and nullified the drag. With the inclusion of the pitching motion into the kinematics, lift production further increased by a factor of more than 2.5, and exhibited a positive net thrust by virtue of the favorable wing orientation during the upstroke. The primary contribution of cambering, which included a maximum chord line displacement of ≈40% standard mean chord, was the stabilization of the leading edge vortex during the downstroke. This increased mean lift by about 35% at the expense of net thrust. Flexion was perhaps the most complex motion with maximum displacements of 75% standard mean chord. This was instrumental in reducing the negative lift during the upstroke by preventing the formation of strong leading edge vortices. The aerodynamic effective angle of attack emerged as a heuristic parameter to describe lift and net thrust production across movements.

Fins as Mechanosensors for Movement and Touch-Related Behaviors

Mechanosensation is a universal feature of animals that is essential for behavior, allowing detection of animals' own body movement and position as well as physical characteristics of the environment. The extraordinary morphological and behavioral diversity that exists across fish species provide rich opportunities for comparative mechanosensory studies in fins. The fins of fishes have been found to function as proprioceptors, by providing feedback on fin ray position and movement, and as tactile sensors, by encoding pressures applied to the fin surface. Across fish species, and among fins, the afferent response is remarkably consistent, suggesting that the ability of fin rays and membrane to sense deformation is a fundamental feature of fish fins. While fin mechanosensation has been known in select, often highly specialized, species for decades, only in the last decade have we explored mechanosensation in typical propulsive fins and considered its role in behavior, particularly locomotion. In this paper, we synthesize the current understanding of the anatomy and physiology of fin mechanosensation, looking toward key directions for research. We argue that a mechanosensory perspective informs studies of fin-based propulsion and other fin-driven behaviors and should be considered in the interpretation of fin morphology and behavior. In addition, we compare the mechanosensory system innervating the fins of fishes to the systems innervating the limbs of mammals and wings of insects in order to identify shared mechanosensory strategies and how different organisms have evolved to meet similar functional challenges. Finally, we discuss how understanding the biological organization and function of fin sensors can inform the design of control systems for engineered fins and fin-driven robotics.

Functional morphology and structural characteristics of the hind wings of the bamboo weevil Cyrtotrachelus buqueti (Coleoptera, Curculionidae)

Research data of the microstructure and surface morphology of insect wings have been used to help design micro air vehicles (MAV) and coating materials. The present study aimed to examine the microstructure and morphology of the hind wings of Cyrtotrachelus buqueti using inverted fluorescence microscopy (IFM), scanning electron microscopy (SEM), and a mechanical testing system. IFM was used to investigate the distribution of resilin in the hind wing, and SEM was performed to assess the functional characteristics and cross-sectional microstructure of the wings. Moreover, mechanical properties regarding the intersecting location of folding lines and the bending zone (BZ) were examined. Resilin, a rubber-like protein, was found in several mobile joints and in veins walls that are connected to the wing membranes. Taken together, structural data, unfolding motions, and results of tensile testing suggest two conclusions on resilin in the hind wing of C. buqueti: firstly, the resilin distribution is likely associated with specific folding mechanisms of the hind wings, and secondly, resilin occurs at positions where additional elasticity is needed, such as in the bending zone, in order to prevent structural damage during repeated folding and unfolding of the hind wings. The functional significance of resilin joints may shed light on the evolutionary relationship between morphological and structural hind wing properties.

Adaptive evolution of butterfly wing shape: from morphology to behaviour

Butterflies display extreme variation in wing shape associated with tremendous ecological diversity. Disentangling the role of neutral versus adaptive processes in wing shape diversification remains a challenge for evolutionary biologists. Ascertaining how natural selection influences wing shape evolution requires both functional studies linking morphology to flight performance, and ecological investigations linking performance in the wild with fitness. However, direct links between morphological variation and fitness have rarely been established. The functional morphology of butterfly flight has been investigated but selective forces acting on flight behaviour and associated wing shape have received less attention. Here, we attempt to estimate the ecological relevance of morpho-functional links established through biomechanical studies in order to understand the evolution of butterfly wing morphology. We survey the evidence for natural and sexual selection driving wing shape evolution in butterflies, and discuss how our functional knowledge may allow identification of the selective forces involved, at both the macro- and micro-evolutionary scales. Our review shows that although correlations between wing shape variation and ecological factors have been established at the macro-evolutionary level, the underlying selective pressures often remain unclear. We identify the need to investigate flight behaviour in relevant ecological contexts to detect variation in fitness-related traits. Identifying the selective regime then should guide experimental studies towards the relevant estimates of flight performance. Habitat, predators and sex-specific behaviours are likely to be major selective forces acting on wing shape evolution in butterflies. Some striking cases of morphological divergence driven by contrasting ecology involve both wing and body morphology, indicating that their interactions should be included in future studies investigating co-evolution between morphology and flight behaviour.

Local deformation and stiffness distribution in fly wings

Mechanical properties of insect wings are essential for insect flight aerodynamics. During wing flapping, wings may undergo tremendous deformations, depending on the wings’ spatial stiffness distribution. We here show an experimental evaluation of wing stiffness in three species of flies using a micro-force probe and an imaging method for wing surface reconstruction. Vertical deflection in response to point loads at 11 characteristic points on the wing surface reveals that average spring stiffness of bending lines between wing hinge and point loads varies ∼77-fold in small fruit flies and up to ∼28-fold in large blowflies. The latter result suggests that local wing deformation depends to a considerable degree on how inertial and aerodynamic forces are distributed on the wing surface during wing flapping. Stiffness increases with an increasing body mass, amounting to ∼0.6 Nm⁻¹ in fruit flies, ∼0.7 Nm⁻¹ in house flies and ∼2.6 Nm⁻¹ in blowflies for bending lines, running from the wing base to areas near the center of aerodynamic pressure. Wings of house flies have a ∼1.4-fold anisotropy in mean stiffness for ventral versus dorsal loading, while anisotropy is absent in fruit flies and blowflies. We present two numerical methods for calculation of local surface deformation based on surface symmetry and wing curvature. These data demonstrate spatial deformation patterns under load and highlight how veins subdivide wings into functional areas. Our results on wings of living animals differ from previous experiments on detached, desiccated wings and help to construct more realistic mechanical models for testing the aerodynamic consequences of specific wing deformations.

Summary: We determined mechanical properties of wings under load in three fly species. Fly wings broadly deform similar to homogenous beams. Veins and joints shape wing mechanics and govern wing deformation during flapping flight.

On flapping flight mechanisms and their applications to wind and marine energy harvesting

In this paper, we present a short review on some of significative results on insect flapping flight. In particular, we focus on the time varying shape mechanisms observed during the flapping cycle that are used by insects to enhance the production of aerodynamic force. We then discuss a few examples on how these mechanisms are adapted to energy harvesters in engineered applications.

Insect and insect-inspired aerodynamics: unsteadiness, structural mechanics and flight control

Flying insects impress by their versatility and have been a recurrent source of inspiration for engineering devices. A large body of literature has focused on various aspects of insect flight, with an essential part dedicated to the dynamics of flapping wings and their intrinsically unsteady aerodynamic mechanisms. Insect wings flex during flight and a better understanding of structural mechanics and aeroelasticity is emerging. Most recently, insights from solid and fluid mechanics have been integrated with physiological measurements from visual and mechanosensors in the context of flight control in steady airs and through turbulent conditions. We review the key recent advances concerning flight in unsteady environments and how the multi-body mechanics of the insect structure-wings and body-are at the core of the flight control question. The issues herein should be considered when applying bio-informed design principles to robotic flapping wings.

A Numerical Study of the Sound and Force Production of Flexible Insect Wings

We numerically solved the acoustic and flow field around cicada wing models with parametrically varied flexibility using the hydrodynamic/acoustic splitting method. We observed a gradual change of sound directivity with flexibility. We found that flexible wings generally produce lower sound due to reduced aerodynamic forces, which were further found to scale with the dynamic pressure force defined as the integration of dynamic pressure over the wing area. Unlike conventional scaling where the incoming flow velocity is used as the reference to calculate the force coefficients, here only the normal component of the relative velocity of the wing to the flow was used to calculate the dynamic pressure, putting kinematic factors into the dynamic pressure force and leaving the more fundamental physics to the force coefficients. A high correlation was found between the aerodynamic forces and the dynamic pressure. The scaling is also supported by previously reported data of revolving wing experiments.

Physics of unsteady thrust and flow generation by a flexible surface flapping in the absence of a free stream

Inspired by the flexible wings and fins of flying and swimming animals, we investigate the flow induced by the interaction between a flapping flexible surface and the surrounding fluid for the limiting case of Strouhal number S t → ∞ (zero free-stream speed). The model selected for this purpose is a two-dimensional sinusoidally pitching rigid symmetric foil to which is attached at the trailing edge a thin chordwise flexible surface (along the chord line). The flow so generated is a coherent jet aligned along the foil centreline, containing a reverse Bénard–Kármán vortex street and delivering a corresponding unidirectional thrust. We analyse the flow and thrust generation process. The measured velocity field suggests that the flow and thrust generation mainly occurs during the phases when the trailing edge is near the centreline. Flexibility of the surface is important in accelerating the near-wake flow and in transferring momentum and energy to the fluid. We present a detailed account of when and where the momentum and energy are added to the fluid. This study shows that the deformations of the flexible surface are responsible for generating a favourable pressure gradient along the jet direction, and for the observed unsteady actuator disc-type action.

Wing inertia as a cause of aerodynamically uneconomical flight with high angles of attack in hovering insects

Flying insects can maintain maneuverability in the air by flapping their wings, and, to save energy, the wings should operate following optimal kinematics. However, unlike conventional rotary wings, insects operate their wings at aerodynamically uneconomical and high angles of attack (AoA). Although insects have continuously received attention from biologists and aerodynamicists, the high AoA operation in insect flight has not been clearly explained. Here, we used a theoretical blade-element model to examine the impact of wing inertia on the power requirement and flapping AoA, based on 3D free-hovering flight wing kinematics of a horned beetle, Allomyrina dichotoma The relative simplicity of the model allowed us to search for the best AoA distributed along the wingspan, which generate the highest vertical force per unit power. We show that, although elastic elements may be involved in flight muscles to store and save energy, the insect still has to use substantial power to accelerate its wings, because inertial energy stores should be used to overcome aerodynamic drag before being stored elastically. At the same flapping speed, a wing operating at a higher AoA requires lower inertial torque, and therefore lower inertial power output, at stroke reversals than a wing operating at an aerodynamically optimal low AoA. An interactive aerodynamic-inertial effect thereby enables the wing to flap at sufficiently high AoA, which causes an aerodynamically uneconomical flight in an effort to minimize the net flight energy.

Design and Mechanical Analysis of Bionic Foldable Beetle Wings

In order to improve the flight performance of collapsible aircrafts, a novel mechanism of bionic foldable wings of beetle is designed based on the four-plate mechanism theory. The folding and unfolding movements of the bionic foldable wings are driven by motor and torsion hinges. Based on the D-H method, a kinematic model of wings is established to analyze the dihedral angle of adjacent plates. The folding ratio of an area in different plate creasing angles has been derived and calculated. Utilizing the kinematic and static models produced, as well as considering the folding ratio and output motor torque, the optimal physical parameters of folding wings are obtained. Dynamic models of rigid and flexible wings were established using ADAMS, and a motion simulation was performed. The relationship between dihedral angle and torque during the folding process of both rigid and flexible wings was obtained. The results provide a better understanding of the folding mechanism through the formulation of rigid-flexible wing analysis, as well as demonstrating a novel design of insect-mimicking artificial wings for small aerial vehicles.

The effects of wing twist in slow-speed flapping flight of birds: trading brute force against efficiency

In aircraft propellers that are used to propel aircraft forward at some speed, propeller blade twist is important to make the individual propeller 'wings' operate at a relatively constant effective angle of attack over the full span. Wing twist is sometimes also assumed to be essential in flapping flight, especially in bird flight. For small insects, it has however been shown that wing twist has little effect on the forces generated by a flapping wing. The unimportance of twist was attributed to the prominent role of unsteady aerodynamic mechanisms. These were recently also shown to be important in bird flight. It has therefore become necessary to verify the role of wing twist in the flapping flight of birds. The aim of the study is to compare the efficiency and the aerodynamic forces of twisted and non-twisted wings that mimic the slow-speed flapping flight of birds. The analyses were performed by using physical models with different amounts of spanwise twist (0°, 10°, 40°). The flow was mapped in three-dimensions using digital particle image velocimetry. The spanwise circulation, the induced drag, the lift-to-drag ratio and the span efficiency were determined. Twist and Strouhal number (St) both determine the local effective angles of attack of the flapping wing. Wings with low average effective angles of attack (resulting from high twist and/or low St) are more efficient, but generate significantly lower aerodynamic forces. High average effective angles of attack result in lower efficiency and high aerodynamic forces. Efficiency and the magnitude of aerodynamic forces are competing parameters. Wing twist is beneficial only in the cases where efficiency is most important-e.g. in cruising flight. Take-off, landing and maneuvering, however, require large and robust aerodynamic forces to be generated. The additional force comes at the cost of efficiency, but it enables birds to perform extreme manoeuvres, increasing their overall fitness.

A balance between aerodynamic and olfactory performance during flight in Drosophila

The ability to track odor plumes to their source (food, mate, etc.) is key to the survival of many insects. During this odor-guided navigation, flapping wings could actively draw odorants to the antennae to enhance olfactory sensitivity, but it is unclear if improving olfactory function comes at a cost to aerodynamic performance. Here, we computationally quantify the odor plume features around a fruit fly in forward flight and confirm that the antenna is well positioned to receive a significant increase of odor mass flux (peak 1.8 times), induced by wing flapping, vertically from below the body but not horizontally. This anisotropic odor spatial sampling may have important implications for behavior and the algorithm during plume tracking. Further analysis also suggests that, because both aerodynamic and olfactory functions are indispensable during odor-guided navigation, the wing shape and size may be a balance between the two functions.

Experimental Study of the Aerodynamic Interaction between the Forewing and Hindwing of a Beetle-Type Ornithopter

Beetles have attracted attention from researchers due to their unique combination of a passively flapping forewing and an actively flapping hindwing during flight. Because the wing loads of beetles are larger than the wing loads of other insects, the mechanism of beetle flight is potentially useful for modeling a small aircraft with a large weight. In this paper, we present a beetle-type ornithopter in which the wings are geometrically and kinematically modeled after an actual beetle. Furthermore, the forewing is designed to be changeable between no-wing, flapping-wing, or fixed-wing configurations. Micro-electro-mechanical systems (MEMS) differential pressure sensors were attached to both the forewing and the hindwing to evaluate the aerodynamic performance during flight. Whether the forewing is configured as a flapping wing or a fixed wing, it generated constant positive differential pressure during forward flight, whereas the differential pressure on the hindwing varied with the flapping motion during forward flight. The experimental results suggest that beetles utilize the forewing for effective vertical force enhancement.

Influence of hydrodynamic pressure and vein strength on the super-elasticity of honeybee wings

The wings of honeybees (Apis mellifera L.) usually produce bending and torsional deformations during flapping wing movement. These deformations endow honeybees with perfect aerodynamic control to escape predators and exploit scattered resources. However, the mechanisms by which honeybee wings recover from large deformations are unclear. This study demonstrates that honeybee wings are super-elastic that they can recover rapidly from one extreme contorted state to their original position. A comparative experiment is conducted to evaluate the difference in super-elastic recovery between attached and detached wings. Results show that the structural stiffness of wings is affected by the reticulate vein and the haemolymph pressure generated by the blood circulation. Further analysis indicates that the haemolymph pressure can increase the stiffness of honeybee wings, especially that of the subcostal veins. This phenomenon shortens the recovery time of wing deflection behaviour.

Design and evaluation of a deformable wing configuration for economical hovering flight of an insect-like tailless flying robot

Studies on wing kinematics indicate that flapping insect wings operate at higher angles of attack (AoAs) than conventional rotary wings. Thus, effectively flying an insect-like flapping-wing micro air vehicle (FW-MAV) requires appropriate wing design for achieving low power consumption and high force generation. Even though theoretical studies can be performed to identify appropriate geometric AoAs for a wing for achieving efficient hovering flight, designing an actual wing by implementing these angles into a real flying robot is challenging. In this work, we investigated the wing morphology of an insect-like tailless FW-MAV, which was named KUBeetle, for obtaining high vertical force/power ratio or power loading. Several deformable wing configurations with various vein structures were designed, and their characteristics of vertical force generation and power requirement were theoretically and experimentally investigated. The results of the theoretical study based on the unsteady blade element theory (UBET) were validated with reference data to prove the accuracy of power estimation. A good agreement between estimated and measured results indicated that the proposed UBET model can be used to effectively estimate the power requirement and force generation of an FW-MAV. Among the investigated wing configurations operating at flapping frequencies of 23 Hz to 29 Hz, estimated results showed that the wing with a suitable vein placed outboard exhibited an increase of approximately 23.7%  ±  0.5% in vertical force and approximately 10.2%  ±  1.0% in force/power ratio. The estimation was supported by experimental results, which showed that the suggested wing enhanced vertical force by approximately 21.8%  ±  3.6% and force/power ratio by 6.8%  ±  1.6%. In addition, wing kinematics during flapping motion was analyzed to determine the reason for the observed improvement.

Effects of flexibility and aspect ratio on the aerodynamic performance of flapping wings

In the current study, we experimentally investigated the flexibility effects on the aerodynamic performance of flapping wings and the correlation with aspect ratio at angle of attack α  =  45°. The Reynolds number based on the chord length and the wing tip velocity is maintained at Re  =  5.3  ×  10³. Our result for compliant wings with an aspect ratio of 4 shows that wing flexibility can offer improved aerodynamic performance compared to that of a rigid wing. Flexible wings are found to offer higher lift-to-drag ratios; in particular, there is significant reduction in drag with little compromise in lift. The mechanism of the flexibility effects on the aerodynamic performance is addressed by quantifying the aerodynamic lift and drag forces, the transverse displacement on the wings and the flow field around the wings. The regime of the effective stiffness that offers improved aerodynamic performance is quantified in a range of about 0.5-10 and it matches the stiffness of insect wings with similar aspect ratios. Furthermore, we find that the aspect ratio of the wing is the predominant parameter determining the flexibility effects of compliant wings. Compliant wings with an aspect ratio of two do not demonstrate improved performance compared to their rigid counterparts throughout the entire stiffness regime investigated. The correlation between wing flexibility effects and the aspect ratio is supported by the stiffness of real insect wings.

Experimental and Numerical Investigation on Dragonfly Wing and Body Motion during Voluntary Take-off

We present a detailed analysis of the voluntary take-off procedure of a dragonfly. The motions of the body and wings are recorded using two high-speed cameras at Beihang University. The experimental results show that the dragonfly becomes airborne after approximately one wingbeat and then leaves the ground. During this process, the maximum vertical acceleration could reach 20 m/s². Evidence also shows that acceleration is generated only by the aerodynamic force induced by the flapping of wings. The dragonfly voluntary take-off procedure is divided into four phases with distinctive features. The variation in phase difference between the forewing and hindwing and angle of attack in the down-stroke are calculated to explain the different features of the four phases. In terms of the key parameters of flapping, the phase difference increases from approximately 0 to 110 degrees; the angle of attack in down-stroke reaches the maximum at first and then decreases in the following take-off procedure. Due to experimental limitations, 2-D simulations are conducted using the immersed boundary method. The results indicate that the phase difference and the angle of attack are highly correlated with the unsteady fluid field around the dragonfly's wings and body, which determines the generation of aerodynamic forces.

Measurement of shape and deformation of insect wing

To measure the shape and deformation of an insect wing, a scanning setup adopting laser triangulation and image matching was developed. Only one industry camera with two light sources was employed to scan the transparent insect wings. 3D shape and point to point full field deformation of the wings could be obtained even when the wingspan is less than 3 mm. The venation and corrugation could be significantly identified from the results. The deformation of the wing under pin loading could be seen clearly from the results as well. Calibration shows that the shape and deformation measurement accuracies are no lower than 0.01 mm. Laser triangulation and image matching were combined dexterously to adapt wings' complex shape, size, and transparency. It is suitable for insect flight research or flapping wing micro-air vehicle development.

The effect of wing flexibility on sound generation of flapping wings

In this study, the unsteady flow and acoustic characteristics of a three-dimensional (3D) flapping wing model of a Tibicen linnei cicada in forward-flight are numerically investigated. A single cicada wing is modelled as a membrane with a prescribed motion reconstructed from high-speed videos of a live insect. The numerical solution takes a hydrodynamic/acoustic splitting approach: the flow field is solved with an incompressible Navier-Stokes flow solver based on an immersed boundary method, and the acoustic field is solved with linearized perturbed compressible equations. The 3D simulation allows for the examination of both the directivity and frequency compositions of the flapping wing sound in a full space. Along with the flexible wing model, a rigid wing model that is extracted from real motion is also simulated to investigate the effects of wing flexibility. The simulation results show that the flapping sound is directional; the dominant frequency varies around the wing. The first and second frequency harmonics show different radiation patterns in the rigid and flexible wing cases, which are demonstrated to be highly associated with wing kinematics and loadings. Furthermore, the rotation and deformation in the flexible wing is found to help lower the sound strength in all directions.

Self-adaptive Bioinspired Hummingbird-wing Stimulated Triboelectric Nanogenerators

Bio-inspired technologies have remarkable potential for energy harvesting from clean and sustainable energy sources. Inspired by the hummingbird-wing structure, we propose a shape-adaptive, lightweight triboelectric nanogenerator (TENG) designed to exploit the unique flutter mechanics of the hummingbird for small-scale wind energy harvesting. The flutter is confined between two surfaces for contact electrification upon oscillation. We investigate the flutter mechanics on multiple contact surfaces with several free-standing and lightweight electrification designs. The flutter driven-TENGs are deposited on simplified wing designs to match the electrical performance with variations in wind speed. The hummingbird TENG (H-TENG) device weighed 10 g, making it one of the lightest TENG harvesters in the literature. With a six TENG network, the hybrid design attained a 1.5 W m⁻² peak electrical output at 7.5 m/s wind speed with an approximately linear increase in charge rate with the increased number of TENG harvesters. We demonstrate the ability of the H-TENG networks to operate Internet of Things (IoT) devices from sustainable and renewable energy sources.

Biomechanics and biomimetics in insect-inspired flight systems

Insect- and bird-size drones-micro air vehicles (MAV) that can perform autonomous flight in natural and man-made environments are now an active and well-integrated research area. MAVs normally operate at a low speed in a Reynolds number regime of 10(4)-10(5) or lower, in which most flying animals of insects, birds and bats fly, and encounter unconventional challenges in generating sufficient aerodynamic forces to stay airborne and in controlling flight autonomy to achieve complex manoeuvres. Flying insects that power and control flight by flapping wings are capable of sophisticated aerodynamic force production and precise, agile manoeuvring, through an integrated system consisting of wings to generate aerodynamic force, muscles to move the wings and a control system to modulate power output from the muscles. In this article, we give a selective review on the state of the art of biomechanics in bioinspired flight systems in terms of flapping and flexible wing aerodynamics, flight dynamics and stability, passive and active mechanisms in stabilization and control, as well as flapping flight in unsteady environments. We further highlight recent advances in biomimetics of flapping-wing MAVs with a specific focus on insect-inspired wing design and fabrication, as well as sensing systems.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Fruit fly scale robots can hover longer with flapping wings than with spinning wings

Hovering flies generate exceptionally high lift, because their wings generate a stable leading edge vortex. Micro flying robots with a similar wing design can generate similar high lift by either flapping or spinning their wings. While it requires less power to spin a wing, the overall efficiency depends also on the actuator system driving the wing. Here, we present the first holistic analysis to calculate how long a fly-inspired micro robot can hover with flapping versus spinning wings across scales. We integrate aerodynamic data with data-driven scaling laws for actuator, electronics and mechanism performance from fruit fly to hummingbird scales. Our analysis finds that spinning wings driven by rotary actuators are superior for robots with wingspans similar to hummingbirds, yet flapping wings driven by oscillatory actuators are superior at fruit fly scale. This crossover is driven by the reduction in performance of rotary compared with oscillatory actuators at smaller scale. Our calculations emphasize that a systems-level analysis is essential for trading-off flapping versus spinning wings for micro flying robots.

Allometry of wing twist and camber in a flower chafer during free flight: How do wing deformations scale with body size?

Intraspecific variation in adult body mass can be particularly high in some insect species, mandating adjustment of the wing's structural properties to support the weight of the larger body mass in air. Insect wings elastically deform during flapping, dynamically changing the twist and camber of the relatively thin and flat aerofoil. We examined how wing deformations during free flight scale with body mass within a species of rose chafers (Coleoptera: Protaetia cuprea) in which individuals varied more than threefold in body mass (0.38-1.29 g). Beetles taking off voluntarily were filmed using three high-speed cameras and the instantaneous deformation of their wings during the flapping cycle was analysed. Flapping frequency decreased in larger beetles but, otherwise, flapping kinematics remained similar in both small and large beetles. Deflection of the wing chord-wise varied along the span, with average deflections at the proximal trailing edge higher by 0.2 and 0.197 wing lengths compared to the distal trailing edge in the downstroke and the upstroke, respectively. These deflections scaled with wing chord to the power of 1.0, implying a constant twist and camber despite the variations in wing and body size. This suggests that the allometric growth in wing size includes adjustment of the flexural stiffness of the wing structure to preserve wing twist and camber during flapping.

Allometry of wing twist and camber in a flower chafer during free flight: How do wing deformations scale with body size?

Intraspecific variation in adult body mass can be particularly high in some insect species, mandating adjustment of the wing's structural properties to support the weight of the larger body mass in air. Insect wings elastically deform during flapping, dynamically changing the twist and camber of the relatively thin and flat aerofoil. We examined how wing deformations during free flight scale with body mass within a species of rose chafers (Coleoptera: Protaetia cuprea) in which individuals varied more than threefold in body mass (0.38-1.29 g). Beetles taking off voluntarily were filmed using three high-speed cameras and the instantaneous deformation of their wings during the flapping cycle was analysed. Flapping frequency decreased in larger beetles but, otherwise, flapping kinematics remained similar in both small and large beetles. Deflection of the wing chord-wise varied along the span, with average deflections at the proximal trailing edge higher by 0.2 and 0.197 wing lengths compared to the distal trailing edge in the downstroke and the upstroke, respectively. These deflections scaled with wing chord to the power of 1.0, implying a constant twist and camber despite the variations in wing and body size. This suggests that the allometric growth in wing size includes adjustment of the flexural stiffness of the wing structure to preserve wing twist and camber during flapping.

Artificial insect wings with biomimetic wing morphology and mechanical properties

The pursuit of a high lift force for insect-scale flapping-wing micro aerial vehicles (FMAVs) requires that their artificial wings possess biomimetic wing features which are close to those of their natural counterpart. In this work, we present both fabrication and testing methods for artificial insect wings with biomimetic wing morphology and mechanical properties. The artificial cicada (Hyalessa maculaticollis) wing is fabricated through a high precision laser cutting technique and a bonding process of multilayer materials. Through controlling the shape of the wing venation, the fabrication method can achieve three-dimensional wing architecture, including cambers or corrugations. Besides the artificial cicada wing, the proposed fabrication method also shows a promising versatility for diverse wing types. Considering the artificial cicada wing's characteristics of small size and light weight, special mechanical testing systems are designed to investigate its mechanical properties. Flexural stiffness, maximum deformation rate and natural frequency are measured and compared with those of its natural counterpart. Test results reveal that the mechanical properties of the artificial cicada wing depend strongly on its vein thickness, which can be used to optimize an artificial cicada wing's mechanical properties in the future. As such, this work provides a new form of artificial insect wings which can be used in the field of insect-scale FMAVs.

Aerodynamics and flow features of a damselfly in takeoff flight

Flight initiation is fundamental for survival, escape from predators and lifting payload from one place to another in biological fliers and can be broadly classified into jumping and non-jumping takeoffs. During jumping takeoffs, the legs generate most of the initial impulse. Whereas the wings generate most of the forces in non-jumping takeoffs, which are usually voluntary, slow, and stable. It is of great interest to understand how these non-jumping takeoffs occur and what strategies insects use to generate large amount of forces required for this highly demanding flight initiation mode. Here, for the first time, we report accurate wing and body kinematics measurements of a damselfly during a non-jumping takeoff. Furthermore, using a high fidelity computational fluid dynamics simulation, we identify the 3D flow features and compute the wing aerodynamics forces to unravel the key mechanisms responsible for generating large flight forces. Our numerical results show that a damselfly generates about three times its body weight during the first half-stroke for liftoff. In generating these forces, the wings flap through a steeply inclined stroke plane with respect to the horizon, slicing through the air at high angles of attack (45°-50°). Consequently, a leading edge vortex (LEV) is formed during both the downstroke and upstroke on all the four wings. The formation of the LEV, however, is inhibited in the subsequent upstrokes following takeoff. Accordingly, we observe a drastic reduction in the magnitude of the aerodynamic force, signifying the importance of LEV in augmenting force production. Our analysis also shows that forewing-hindwing interaction plays a favorable role in enhancing both lift and thrust production during takeoff.

Quantifying the dynamic wing morphing of hovering hummingbird

Animal wings are lightweight and flexible; hence, during flapping flight their shapes change. It has been known that such dynamic wing morphing reduces aerodynamic cost in insects, but the consequences in vertebrate flyers, particularly birds, are not well understood. We have developed a method to reconstruct a three-dimensional wing model of a bird from the wing outline and the feather shafts (rachides). The morphological and kinematic parameters can be obtained using the wing model, and the numerical or mechanical simulations may also be carried out. To test the effectiveness of the method, we recorded the hovering flight of a hummingbird (Amazilia amazilia) using high-speed cameras and reconstructed the right wing. The wing shape varied substantially within a stroke cycle. Specifically, the maximum and minimum wing areas differed by 18%, presumably due to feather sliding; the wing was bent near the wrist joint, towards the upward direction and opposite to the stroke direction; positive upward camber and the 'washout' twist (monotonic decrease in the angle of incidence from the proximal to distal wing) were observed during both half-strokes; the spanwise distribution of the twist was uniform during downstroke, but an abrupt increase near the wrist joint was found during upstroke.

An experimental comparative study of the efficiency of twisted and flat flapping wings during hovering flight

This work presents a parametric study to find a proper wing configuration for achieving economical flight using unsteady blade element theory, which is based on the 3D kinematics of a flapping wing. Power loading was first considered as a performance parameter for the study. The power loadings at each wing section along the wingspan were obtained for various geometric angles of attack (AoAs) by calculating the ratios of the vertical forces generated and the power consumed by that particular wing section. The results revealed that the power loading of a negatively twisted wing could be higher than the power loading that a flat wing can have; the power loading of the negatively twisted wing was approximately 5.9% higher. Given the relatively low average geometric AoA (α _A,root  ≈  44° and α _A,tip  ≈  25°), the vertical force produced by the twisted wing for the highest power loading was approximately 24.4% less than that produced by the twisted wing for the strongest vertical force. Therefore, for a given wing geometry and flapping amplitude, a flapping-wing micro air vehicle required a 13.5% increase in flapping frequency to generate the same strongest cycle-average vertical force while saving about 24.3% power. However, when force³/power² and force²/power ratios were considered as performance indices, the twisted wings for the highest force³/power² (α _A,root  ≈  43° and α _A,tip  ≈  30°) and force²/power (α _A,root  ≈  43° and α _A,tip  ≈  36°) required only 6.5% and 4% increases in flapping frequency and consumed 26.2% and 25.3% less power, respectively. Thus, it is preferable to use a flapping wing operating at a high frequency using the geometric AoAs for the highest power loading, force³/power² ratio, and force²/power ratio over a flapping wing operating at a low frequency using a high geometric AoA with the strongest vertical force. Additionally, by considering both aerodynamic and inertial forces, this study obtained average geometric AoAs in the range of 30° to 40°, which are similar to those of a typical hovering insect's wings. Therefore, the operation of an aerodynamically uneconomical, high AoA in a hovering insect's wings during flight is explainable.

Design and stable flight of a 21 g insect-like tailless flapping wing micro air vehicle with angular rates feedback control

An insect-like tailless flapping wing micro air vehicle (FW-MAV) without feedback control eventually becomes unstable after takeoff. Flying an insect-like tailless FW-MAV is more challenging than flying a bird-like tailed FW-MAV, due to the difference in control principles. This work introduces the design and controlled flight of an insect-like tailless FW-MAV, named KUBeetle. A combination of four-bar linkage and pulley-string mechanisms was used to develop a lightweight flapping mechanism that could achieve a high flapping amplitude of approximately 190°. Clap-and-flings at dorsal and ventral stroke reversals were implemented to enhance vertical force. In the absence of a control surface at the tail, adjustment of the location of the trailing edges at the wing roots to modulate the rotational angle of the wings was used to generate control moments for the attitude control. Measurements by a 6-axis load cell showed that the control mechanism produced reasonable pitch, roll and yaw moments according to the corresponding control inputs. The control mechanism was integrated with three sub-micro servos to realize the pitch, roll and yaw controls. A simple PD feedback controller was implemented for flight stability with an onboard microcontroller and a gyroscope that sensed the pitch, roll and yaw rates. Several flight tests demonstrated that the tailless KUBeetle could successfully perform a vertical climb, then hover and loiter within a 0.3 m ground radius with small variations in pitch and roll body angles.

Bactericidal Effects of Natural Nanotopography of Dragonfly Wing on Escherichia coli

Nanotextured surfaces (NTSs) are critical to organisms as self-adaptation and survival tools. These NTSs have been actively mimicked in the process of developing bactericidal surfaces for diverse biomedical and hygiene applications. To design and fabricate bactericidal topographies effectively for various applications, understanding the bactericidal mechanism of NTS in nature is essential. The current mechanistic explanations on natural bactericidal activity of nanopillars have not utilized recent advances in microscopy to study the natural interaction. This research reveals the natural bactericidal interaction between E. coli and a dragonfly wing's (Orthetrum villosovittatum) NTS using advanced microscopy techniques and proposes a model. Contrary to the existing mechanistic models, this experimental approach demonstrated that the NTS of Orthetrum villosovittatum dragonfly wings has two prominent nanopillar populations and the resolved interface shows membrane damage occurred without direct contact of the bacterial cell membrane with the nanopillars. We propose that the bacterial membrane damage is initiated by a combination of strong adhesion between nanopillars and bacterium EPS layer as well as shear force when immobilized bacterium attempts to move on the NTS. These findings could help guide the design of novel biomimetic nanomaterials by maximizing the synergies between biochemical and mechanical bactericidal effects.

The relationship between pectoral fin ray stiffness and swimming behavior in Labridae: insights into design, performance, and ecology

The functional capabilities of flexible, propulsive, appendages are directly influenced by their mechanical properties. The fins of fishes have undergone extraordinary evolutionary diversification in structure and function, which raises questions of how fin mechanics relate to swimming behavior. In the fish family Labridae, pectoral fin swimming behavior ranges from rowing to flapping. Rowers are more maneuverable than flappers, but flappers generate greater thrust at high speeds and achieve greater mechanical efficiency at all speeds. Interspecific differences in hydrodynamic capability are largely dependent on fin kinematics and deformation, and are expected to correlate with fin stiffness. Here we examine fin ray stiffness in two closely related species that employ divergent swimming behaviors, the flapping Gomphosus varius and the rowing Halichoeres bivittatus. To determine the spatial distribution of flexural stiffness across the fin, we performed three-point bending tests at the center of the proximal, middle, and distal regions of four equally spaced fin rays. Pectoral fin ray flexural stiffness ranged from 0.0001-1.5109 microNewtons•m2, and the proximal regions of G. varius fin rays were nearly an order of magnitude stiffer than those of H. bivittatus. In both species, fin ray flexural stiffness decreased exponentially along the proximo-distal span of fin rays, and flexural stiffness decreased along the fin chord from the leading to trailing edge. Further, the proportion of fin area occupied by fin rays was significantly greater in G. varius than in H. bivittatus, suggesting that the proportion of fin ray to fin area contributes to differences in fin mechanics.

An analytical model and scaling of chordwise flexible flapping wings in forward flight

Aerodynamic performance of biological flight characterized by the fluid structure interaction of a flapping wing and the surrounding fluid is affected by the wing flexibility. One of the main challenges to predict aerodynamic forces is that the wing shape and motion are a priori unknown. In this study, we derive an analytical fluid-structure interaction model for a chordwise flexible flapping two-dimensional airfoil in forward flight. A plunge motion is imposed on the rigid leading-edge (LE) of teardrop shape and the flexible tail dynamically deforms. The resulting unsteady aeroelasticity is modeled with the Euler-Bernoulli-Theodorsen equation under a small deformation assumption. The two-way coupling is realized by considering the trailing-edge deformation relative to the LE as passive pitch, affecting the unsteady aerodynamics. The resulting wing deformation and the aerodynamic performance including lift and thrust agree well with high-fidelity numerical results. Under the dynamic balance, the aeroelastic stiffness decreases, whereas the aeroelastic stiffness increases with the reduced frequency. A novel aeroelastic frequency ratio is derived, which scales with the wing deformation, lift, and thrust. Finally, the dynamic similarity between flapping in water and air is established.

Clap-and-fling mechanism in a hovering insect-like two-winged flapping-wing micro air vehicle

This study used numerical and experimental approaches to investigate the role played by the clap-and-fling mechanism in enhancing force generation in hovering insect-like two-winged flapping-wing micro air vehicle (FW-MAV). The flapping mechanism was designed to symmetrically flap wings at a high flapping amplitude of approximately 192°. The clap-and-fling mechanisms were thereby implemented at both dorsal and ventral stroke reversals. A computational fluid dynamic (CFD) model was constructed based on three-dimensional wing kinematics to estimate the force generation, which was validated by the measured forces using a 6-axis load cell. The computed forces proved that the CFD model provided reasonable estimation with differences less than 8%, when compared with the measured forces. The measurement indicated that the clap and flings at both the stroke reversals augmented the average vertical force by 16.2% when compared with the force without the clap-and-fling effect. In the CFD simulation, the clap and flings enhanced the vertical force by 11.5% and horizontal drag force by 18.4%. The observations indicated that both the fling and the clap contributed to the augmented vertical force by 62.6% and 37.4%, respectively, and to the augmented horizontal drag force by 71.7% and 28.3%, respectively. The flow structures suggested that a strong downwash was expelled from the opening gap between the trailing edges during the fling as well as the clap at each stroke reversal. In addition to the fling phases, the influx of air into the low-pressure region between the wings from the leading edges also significantly contributed to augmentation of the vertical force. The study conducted for high Reynolds numbers also confirmed that the effect of the clap and fling was insignificant when the minimum distance between the two wings exceeded 1.2c (c = wing chord). Thus, the clap and flings were successfully implemented in the FW-MAV, and there was a significant improvement in the vertical force.

Basal Complex and Basal Venation of Odonata Wings: Structural Diversity and Potential Role in the Wing Deformation

Dragonflies and damselflies, belonging to the order Odonata, are known to be excellent fliers with versatile flight capabilities. The ability to fly over a wide range of speeds, high manoeuvrability and great agility are a few characteristics of their flight. The architecture of the wings and their structural elements have been found to play a major role in this regard. However, the precise influence of individual wing components on the flight performance of these insects remains unknown. The design of the wing basis (so called basal complex) and the venation of this part are responsible for particular deformability and specific shape of the wing blade. However, the wing bases are rather different in representatives of different odonate groups. This presumably reflects the dimensions of the wings on one hand, and different flight characteristics on the other hand. In this article, we develop the first three-dimensional (3D) finite element (FE) models of the proximal part of the wings of typical representatives of five dragonflies and damselflies families. Using a combination of the basic material properties of insect cuticle, a linear elastic material model and a nonlinear geometric analysis, we simulate the mechanical behaviour of the wing bases. The results reveal that although both the basal venation and the basal complex influence the structural stiffness of the wings, it is only the latter which significantly affects their deformation patterns. The use of numerical simulations enabled us to address the role of various wing components such as the arculus, discoidal cell and triangle on the camber formation in flight. Our study further provides a detailed representation of the stress concentration in the models. The numerical analysis presented in this study is not only of importance for understanding structure-function relationship of insect wings, but also might help to improve the design of the wings for biomimetic micro-air vehicles (MAVs).