Wing Hinge Research Articles

Insect‐Inspired Drones: Adjusting the Flapping Kinetics of Insect‐Inspired Wings Improves Aerodynamic Performance

Insects flap their wings through a highly specialized musculoskeletal system that allows the wings to rotate about three degrees of freedom. Consequently, the wingtip trajectory is adjustable in 3D, and accompanied with appropriate wing feathering (wing pitch). Remarkably, the complex flapping motion is achieved by thoracic muscles acting on the wing hinge. The wings themselves do not possess muscles but adjust their shape and orientation by elastically deforming due to the loads applied on them during flapping. Previous attempts to develop insect‐inspired flapping drones have mostly focused on simplified linear flapping mechanisms, which do not utilize the interaction between the wing flexibility and flapping kinematics to its full potential. Here, the aim is to improve flapping drones’ performance by introducing mechanisms that mimic insects’ flight. The first is an elastic beam mechanism, allowing the wing root to swing during flapping, and the second is a passive wing pitch mechanism that allows the wing to rotate at stroke reversals. The two mechanisms are tested using high‐fidelity insect‐inspired 3D‐printed wings and show a sixfold improvement of aerodynamic performance compared to linear flapping kinetics of the same flexible wings. This underscores the necessity of bioinspired flapping mechanisms in future flapping drones.

Aerodynamics and stability of hawkmoth forward flight with flexible wing hinge

Aerodynamics and stability of hawkmoth forward flight with flexible wing hinge

Machine learning reveals the control mechanics of an insect wing hinge.

Insects constitute the most species-rich radiation of metazoa, a success that is due to the evolution of active flight. Unlike pterosaurs, birds and bats, the wings of insects did not evolve from legs1, but are novel structures that are attached to the body via a biomechanically complex hinge that transforms tiny, high-frequency oscillations of specialized power muscles into the sweeping back-and-forth motion of the wings2. The hinge consists of a system of tiny, hardened structures called sclerites that are interconnected to one another via flexible joints and regulated by the activity of specialized control muscles. Here we imaged the activity of these muscles in a fly using a genetically encoded calcium indicator, while simultaneously tracking the three-dimensional motion of the wings with high-speed cameras. Using machine learning, we created a convolutional neural network3 that accurately predicts wing motion from the activity of the steering muscles, and an encoder-decoder4 that predicts the role of the individual sclerites on wing motion. By replaying patterns of wing motion on a dynamically scaled robotic fly, we quantified the effects of steering muscle activity on aerodynamic forces. A physics-based simulation incorporating our hinge model generates flight manoeuvres that are remarkably similar to those of free-flying flies. This integrative, multi-disciplinary approach reveals the mechanical control logic of the insect wing hinge, arguably among the most sophisticated and evolutionarily important skeletal structures in the natural world.

Coupled Aerodynamics–Structure Analysis and Wind Tunnel Experiments on Passive Hinge Oscillation of Wing-Tip-Chained Airplanes

This study examines the wing hinge oscillations in an aircraft concept that employs multiple wings, or small aircraft, chained at the wing tips through freely rotatable hinges with minimal structural damping and no mechanical position-locking system. This creates a single pseudo long-span aircraft that resembles a flying chain oriented perpendicular to the flight direction. Numerical calculations were conducted using the vortex lattice method and modified equations for a multi-link rigid body pendulum. The calculations demonstrated good agreement with small-scale wind tunnel experiments, where the motion of the chained wings was tracked through color tracking, and the forces were measured using six-axis force sensors. The total CL/CD increased for the chained wings, even in the presence of hinge joint oscillations. Furthermore, numerical simulations assuming an unmanned airplane size corroborated the theoretical attainment of passive stability with high chained numbers (≥9 wings), without any structural damping and relying solely on aerodynamic forces. Guidelines for appropriate hinge axis angle δ and angle-of-attack regions for different chained wing numbers to maximize passive oscillation stability were obtained. The results showed that wing-tip-chained airplanes could successfully provide substantially large wing spans while retaining flexibility, light weight and CL/CD, without requiring active hinge rotation control.

Power synchronisations determine the hovering flight efficiency of passively pitching flapping wings

The power exchange between fluid and structure plays a significant role in the force production and flight efficiency of flapping wings in insects and artificial flyers. This work numerically investigates the performance of flapping wings by using a high-fidelity fluid–structure interaction solver. Simulations are conducted by varying the hinge flexibility (measured by the Cauchy number, $Ch$ , i.e. the ratio between aerodynamic and torsional elastic forces) and the wing shape (quantified by the radius of the first moment of area, $\bar {r}_1$ ). Results show that the lift production is optimal at $0.05 < Ch \leq 0.2$ and larger $\bar {r}_1$ where the minimum angle of attack is around $45^\circ$ at midstroke. The power economy is maximised for wings with lower $\bar {r}_1$ near $Ch=0.2$ . Power analysis indicates that the optimal performance measured by the power economy is obtained for those cases where two important power synchronisations occur: anti-synchronisation of the pitching elastic power and the pitching aerodynamic and inertial powers and nearly in-phase synchronisation of the flapping aerodynamic power and the total input power of the system. While analysis of the kinematics for the different wing shapes and hinge stiffnesses reveals that the optimal performance occurs when the timing of pitch and stroke reversals are matched, thus supporting the effective transfer of input power from flapping to passive pitching and into the fluid. These results suggest that careful optimisation between wing shapes and hinge properties can allow insects and robots to exploit the passive dynamics to improve flight performance.

Effect of Wing Stiffness and Folding Wingtip Release Threshold on Gust Loads

This study presents an aeroelastic wind-tunnel experiment to identify the influence of the wing stiffness and hinge release threshold on the gust load alleviation performance of a folding wingtip design. Five models with different stiffness and tailoring properties are tested, and the wing root bending moment at different conditions is compared to the response with the locked-hinge condition to assess the impact on the gust load alleviation capabilities of the folding wingtip. The results show that the structural properties do not have an important impact on the peak load alleviation but the hinge release threshold and timing do. Releasing with the correct timing can reduce significantly the peak loads. However, the dynamics of the system are affected by this release; the flutter speed is decreased, and, although the performance can improve, load oscillations increase, which can be considered detrimental for reasons such as fatigue or passenger comfort.

Experimental studies suggest differences in the distribution of thorax elasticity between insects with synchronous and asynchronous musculature.

Insects have developed diverse flight actuation mechanisms, including synchronous and asynchronous musculature. Indirect actuation, used by insects with both synchronous and asynchronous musculature, transforms thorax exoskeletal deformation into wing rotation. Though thorax deformation is often attributed exclusively to muscle tension, the inertial and aerodynamic forces generated by the flapping wings may also contribute. In this study, a tethered flight experiment was used to simultaneously measure thorax deformation and the inertial/aerodynamic forces acting on the thorax generated by the flapping wing. Compared to insects with synchronous musculature, insects with asynchronous muscle deformed their thorax 60% less relative to their thorax diameter and their wings generated 2.8 times greater forces relative to their body weight. In a second experiment, dorsalventral thorax stiffness was measured across species. Accounting for weight and size, the asynchronous thorax was on average 3.8 times stiffer than the synchronous thorax in the dorsalventral direction. Differences in thorax stiffness and forces acting at the wing hinge led us to hypothesize about differing roles of series and parallel elasticity in the thoraxes of insects with synchronous and asynchronous musculature. Specifically, wing hinge elasticity may contribute more to wing motion in insects with asynchronous musculature than in those with synchronous musculature.

Model-Based Tracking of Fruit Flies in Free Flight.

Insect flight is a complex interdisciplinary phenomenon. Understanding its multiple aspects, such as flight control, sensory integration, physiology and genetics, often requires the analysis of large amounts of free flight kinematic data. Yet, one of the main bottlenecks in this field is automatically and accurately extracting such data from multi-view videos. Here, we present a model-based method for the pose estimation of free-flying fruit flies from multi-view high-speed videos. To obtain a faithful representation of the fly with minimum free parameters, our method uses a 3D model that includes two new aspects of wing deformation: A non-fixed wing hinge and a twisting wing surface. The method is demonstrated for free and perturbed flight. Our method does not use prior assumptions on the kinematics apart from the continuity of the wing pitch angle. Hence, this method can be readily adjusted for other insect species.

Resilin is needed for wing posture in Drosophila suzukii.

Resilin is a protein matrix in movable regions of the cuticle conferring resistance to fatigue. The main component of Resilin is Pro‐Rresilin that polymerises via covalent di‐ and tri‐tyrosine bounds (DT). Loss of Pro‐Resilin is nonlethal and causes a held‐down wing phenotype (hdw) in the fruit fly Drosophila melanogaster. To test whether this mild phenotype is recurrent in other insect species, we analysed resilin in the spotted‐wing fruit fly Drosophila suzukii. As quantified by DT autofluorescence by microscopy, DT intensities in the trochanter and the wing hinge are higher in D. suzukii than in D. melanogaster, while in the proboscis the DT signal is stronger in D. melanogaster compared to D. suzukii. To study the function of Pro‐Resilin in D. suzukii, we generated a mutation in the proresilin gene applying the Crispr/Cas9 technique. D. suzukii pro‐resilin mutant flies are flight‐less and show a hdw phenotype resembling respective D. melanogaster mutants. DT signal intensity at the wing hinge is reduced but not eliminated in D. suzukii hdw flies. Either residual Pro‐Resilin accounts for the remaining DT signal or, as proposed for the hdw phenotype in D. melanogaster, other DT forming proteins might be present in Resilin matrices. Interestingly, DT signal intensity reduction rates in D. suzukii and D. melanogaster are somehow different. Taken together, in general, the function of Pro‐Resilin seems to be conserved in the Drosophila genus; small differences in DT quantity, however, allow us to hypothesise that Resilin matrices might be modulated during evolution probably to accommodate the species‐specific lifestyle.

Differential gene expression analysis identified determinants of cell fate plasticity during radiation-induced regeneration in Drosophila.

Ionizing radiation (IR) is used to treat half of all cancer patients because of its ability to kill cells. IR, however, can induce stem cell-like properties in non-stem cancer cells, potentiating tumor regrowth and reduced therapeutic success. We identified previously a subpopulation of cells in Drosophila larval wing discs that exhibit IR-induced stem cell-like properties. These cells reside in the future wing hinge, are resistant to IR-induced apoptosis, and are capable of translocating, changing fate, and participating in regenerating the pouch that suffers more IR-induced apoptosis. We used here a combination of lineage tracing, FACS-sorting of cells that change fate, genome-wide RNAseq, and functional testing of 42 genes, to identify two key changes that are required cell-autonomously for IR-induced hinge-to-pouch fate change: (1) repression of hinge determinants Wg (Drosophila Wnt1) and conserved zinc-finger transcription factor Zfh2 and (2) upregulation of three ribosome biogenesis factors. Additional data indicate a role for Myc, a transcriptional activator of ribosome biogenesis genes, in the process. These results provide a molecular understanding of IR-induced cell fate plasticity that may be leveraged to improve radiation therapy.

The damping and structural properties of dragonfly and damselfly wings during dynamic movement

For flying insects, stability is essential to maintain the orientation and direction of motion in flight. Flight instability is caused by a variety of factors, such as intended abrupt flight manoeuvres and unwanted environmental disturbances. Although wings play a key role in insect flight stability, little is known about their oscillatory behaviour. Here we present the first systematic study of insect wing damping. We show that different wing regions have almost identical damping properties. The mean damping ratio of fresh wings is noticeably higher than that previously thought. Flight muscles and hemolymph have almost no ‘direct’ influence on the wing damping. In contrast, the involvement of the wing hinge can significantly increase damping. We also show that although desiccation reduces the wing damping ratio, rehydration leads to full recovery of damping properties after desiccation. Hence, we expect hemolymph to influence the wing damping indirectly, by continuously hydrating the wing system.

Yaw Control of a Hovering Flapping-Wing Aerial Vehicle With a Passive Wing Hinge

Flapping-wing insect-scale robots (<;$500 mg) rely on small changes in drive signals supplied to actuators to generate angular torques. Previous results on vehicles with passive wing hinges have demonstrated roll, pitch, and position control, but they have not yet been able to control their yaw (heading) angle while hovering. To actuate yaw, the speed of the downstroke can be changed relative to the upstroke by adding a second harmonic signal at double the fundamental flapping frequency. Previous work has shown that pitching dynamics of a passive spring-like wing hinge reduces the aerodynamic drag available to produce yaw torque. We introduce three innovations that increase yaw actuation torque: 1) a new two-actuator robot fly design that increases the moment arm, 2) wider actuators that increase the operating frequency, and 3) a phase shift to the second harmonic signal. We validated these results through simulation and experiment. Further, we present the first demonstration of yaw angle control on a passive-hinge vehicle in a controlled flight. Our new robot fly design, UW Robofly-Expanded, weighs 160 mg (two toothpicks) and requires only two piezo actuators to steer itself.

Speck, First Identified in Drosophila melanogaster in 1910, Is Encoded by the Arylalkalamine N-Acetyltransferase (AANAT1) Gene.

The pigmentation mutation speck is a commonly used recombination marker characterized by a darkly pigmented region at the wing hinge. Identified in 1910 by Thomas Hunt Morgan, speck was characterized by Sturtevant as the most “workable” mutant in the rightmost region of the second chromosome and eventually localized to 2-107.0 and 60C1-2. Though the first speck mutation was isolated over 110 years ago, speck is still not associated with any gene. Here, as part of an undergraduate-led research effort, we show that speck is encoded by the Arylalkylamine N-acetyltransferase 1 (AANAT1) gene. Both alleles from the Morgan lab contain a retrotransposon in exon 1 of the RB transcript of the AANAT1 gene. We have also identified a new insertion allele and generated multiple deletion alleles in AANAT1 that all give a strong speck phenotype. In addition, expression of AANAT1 RNAi constructs either ubiquitously or in the dorsal portion of the developing wing generates a similar speck phenotype. We find that speck alleles have additional phenotypes, including ectopic pigmentation in the posterior pupal case, leg joints, cuticular sutures and overall body color. We propose that the acetylated dopamine generated by AANAT1 decreases the dopamine pool available for melanin production. When AANAT1 function is decreased, the excess dopamine enters the melanin pathway to generate the speck phenotype.

Indirect actuation reduces flight power requirements in Manduca sexta via elastic energy exchange.

In many insects, wing movements are generated indirectly via exoskeletal deformations. Measurements of inertial and aerodynamic power suggest that elastic recovery of energy between wingstrokes might reduce power requirements of flight. We tested three questions. (1) Can the thorax itself provide significant energy return? (2) Does a simple damped elastic model describe the bulk mechanical behaviour? (3) Are different regions of the thorax specialized for elastic energy exchange? We measured deformation mechanics of the hawkmoth Manduca sexta thorax by recording the force required to sinusoidally deform the thorax over a wide frequency range. Elastic energy storage in the thorax is sufficient to minimize power requirements. However, we find that a structural (frequency-independent) damping model, not a viscoelastic model, best describes the thorax's mechanical properties. We next performed complementary experiments on a structurally damped homogeneous hemisphere. In contrast to the hemispherical shell, we find that mechanical coupling between different regions of the thorax improves energy exchange performance and that local mechanical properties depend on global strain patterns. Specifically, the scutum region provides energy recovery with low dissipation, while the majority of energy loss occurred in the wing hinge region, highlighting the specificity of thorax regions for flight energetics.

A contralateral wing stabilizes a hovering hawkmoth under a lateral gust

Previous analysis on the lateral stability of hovering insects, which reported a destabilizing roll moment due to a lateral gust, has relied on the results of a single wing without considering a presence of the contralateral wing (wing-wing interaction). Here, we investigated the presence of the contralateral wing on the aerodynamic and flight dynamic characteristics of a hovering hawkmoth under a lateral gust. By employing a dynamically scaled-up mechanical model and a servo-driven towing system installed in a water tank, we found that the presence of the contralateral wing plays a significant role in the lateral static stability. The contralateral wing mitigated an excessive aerodynamic force on the wing at the leeward side, thereby providing a negative roll moment to the body. Digital particle image velocimetry revealed an attenuated vortical system of the leading-edge vortex. An excessive effective angle of attack in the single wing case, which was caused by the root vortex of previous half stroke, was reduced by a downwash of the contralateral wing. The contralateral wing also relocated a neutral point in close proximity to the wing hinge points above the actual center of gravity, providing a practical static margin to a hovering hawkmoth.

Identification of genes underlying phenotypic plasticity of wing size via insulin signaling pathway by network-based analysis in Sogatella furcifera

BackgroundPhenotypic plasticity is a common and highly adaptive phenomenon where the same genotype produces different phenotypes in response to environmental cues. Sogatella furcifera, a migratory pest of rice exhibits wing dimorphism, is a model insect for studying phenotypic plasticity of wing size. The Insullin-PI3K-Akt-FOXO signaling pathway plays a crucial role in the manipulation of wing size in the migratory insects. However, the regulatory mechanism via the pathway involved in wing dimorphism are still unexplored.ResultsAccompanied by special alternative splicing, genes involved in muscle contraction and energy metabolism were highly expressed in the wing hinges of macropters, demonstrating their adaptation for energy-demanding long-distance flights. Based on FOXO ChIP-Seq analysis, a total of 1259 putative target genes were observed in the wing hinges, including wing morph development, flight muscle and energy metabolism genes. An integrated gene interaction network was built by combining four heterogeneous datasets, and the IIS-PI3K-Akt-FOXO pathway was clustered in a divided functional module. In total, 45 genes in the module directly interacting with the IIS-PI3K-Akt-FOXO pathway showed differential expression levels between the two wing hinges, thus are regarded as potential Insulin pathway mediated wing dimorphism related genes (IWDRGs). Of the 45 IWDRGs, 5 were selected for verification by gene knockdown experiments, and played significant roles in the insect wing size regulation.ConclusionsWe provided valuable insights on the genetic basis of wing dimorphism, and also demonstrated that network analysis is a powerful approach to identify new genes regulating wing dimorphic development via insulin signaling pathway in the migratory insect.

Local deformation and stiffness distribution in fly wings

ABSTRACTMechanical 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.

Optimal wing hinge position for fast ascent in a model fly

It was thought that the wing hinge position can be tuned to stabilize an uncontrolled fly. However here, our Floquet stability analysis shows that the hinge position has a weak dependence on the flight stability. As long as the hinge position is within the fly’s body length, both hovering and ascending flight are unstable. Instead, there is an optimal hinge position, $h^{\ast }$, at which the ascending speed is maximized. $h^{\ast }$ is approximately half way between the centre of mass and the top of the body. We show that the optimal $h^{\ast }$ is associated with the anti-resonance of the body–wing coupling, and is independent of the stroke amplitude. At $h^{\ast }$, the torque due to wing inertia nearly cancels the torque due to aerodynamic lift, minimizing the body oscillation thus maximizing the upward force. Our analysis using a simplified model of two coupled masses further predicts, $h^{\ast }=(m_{t}/2m_{w})(g/\unicode[STIX]{x1D714}^{2})$. These results suggest that the ascending speed, in addition to energetics and stability, is a trait that insects are likely to optimize.

Slow viscoelastic response of resilin.

The high importance of resilin in invertebrate biomechanics is widely known. It is generally assumed to be an almost perfect elastomer in different tissues. Whereas mechanical properties of resilin were previously determined mainly in tension, here we aimed at studying its mechanical properties in compression. Microindentation of resilin from the wing hinge of Locusta migratoria revealed the clear viscoelastic response of resilin: about a quarter of the mechanical response was assigned to a viscous component in our experiments. Mechanical properties were characterized using a generalized Maxwell model with two characteristic time constants, poroelasticity theory, and alternatively using a 1D model with just one characteristic time constant. Slow viscous responses with 1.7 and 16s characteristic times were observed during indentation. These results demonstrate that the locust flight system is adapted to both fast and slow mechanical processes. The fast highly elastic process is related to the flight function and the slow viscoelastic process may be related to the wing folding.

Embryological evidence substantiates the subcoxal theory on the origin of pleuron in insects

The lateral body plate pleuron is a significant structure in insects that contributes to the development and elaboration of wings and limbs (appendages). Although the pleuron is thought to originate from the proximal-most appendicular segment, the subcoxa, details remain unclear, and the morphological boundary between the dorsal body plate tergum and appendage (BTA) has not been clearly specified. Employing low-vacuum scanning electron microscopy (SEM) and the nano-suit method for SEM, we followed, in detail, the development of the thoracic segments of the two-spotted cricket Gryllus bimaculatus and succeeded in clearly defining the BTA. This study demonstrates the subcoxal origin of the pleuron, suggests the tergal origin of spiracles, and reveals that the wing proper originates exclusively from the tergum, whereas the wing hinge and direct muscles may be appendicular in origin, suggesting the dual origin (i.e., tergal plus appendicular origin) of wings.