Aerodynamics of a bio-inspired flexible flapping-wing micro air vehicle

MAVs (micro air vehicles) with a maximal dimension of 15 cm and nominal flight speeds of around 10 m s?1, operate in a Reynolds number regime of 10? or lower, in which most natural flyers including insects, bats and birds fly. Furthermore, due to their light weight and low flight speed, the MAVs' flight characteristics are substantially affected by environmental factors such as wind gust. Like natural flyers, the wing structures of MAVs are often flexible and tend to deform during flight. Consequently, the aero/fluid and structural dynamics of these flyers are closely linked to each other, making the entire flight vehicle difficult to analyze. We have recently developed a hummingbird-inspired, flapping flexible wing MAV with a weight of 2.4-3.0 g and a wingspan of 10-12 cm. In this study, we carry out an integrated study of the flexible wing aerodynamics of this flapping MAV by combining an in-house computational fluid dynamic (CFD) method and wind tunnel experiments. A CFD model that has a realistic wing planform and can mimic realistic flexible wing kinematics is established, which provides a quantitative prediction of unsteady aerodynamics of the four-winged MAV in terms of vortex and wake structures and their relationship with aerodynamic force generation. Wind tunnel experiments further confirm the effectiveness of the clap and fling mechanism employed in this bio-inspired MAV as well as the importance of the wing flexibility in designing small flapping-wing MAVs.     

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 10⁴–10⁵ 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’.     

Aerial locomotion in flies and robots: kinematic control and aerodynamics of oscillating wings

Flight in flies results from a feedback cascade in which the animal converts mechanical power produced by the flight musculature into aerodynamic forces. A major goal of flight research is to understand the functional significance of the various components in this cascade ranging from the generation of the neural code, the control of muscle mechanical power output, wing kinematics and unsteady aerodynamic mechanisms. Here, I attempted to draw a broad outline on fluid dynamic mechanisms found in flapping insect wings such as leading edge vorticity, rotational circulation and wake capture momentum transfer, as well as on the constraints of flight force control by the neuromuscular system of the fruit fly Drosophila. This system-level perspective on muscle control and aerodynamic mechanisms is thought to be a fundamental bridge in any attempt to link the function and performance of the various flight components with their particular role for wing motion and aerodynamic control in the behaving animal. Eventually, this research might facilitate the development of man-made biomimetic autonomous micro air vehicles using flapping wing motion for propulsion that are currently under construction by engineers.     

Pitching Moment Generation in an Insect-Mimicking Flapping-Wing System

Unlike birds, insects lack control surfaces at the tail and hence most insects modify their wing kinematics to produce control forces or moments while flapping their wings. Change of the flapping angle range is one of the ways to modify wing kinematics, resulting in relocation of the mean Aerodynamic force Center （mean AC） and finally creating control moments. In an attempt to mimic this feature, we developed a flapping-wing system that generates a desired pitching moment during flap- ping-wing motion. The system comprises a flapping mechanism that creates a large and symmetric flapping motion in a pair of wings, a flapping angle change mechanism that modifies the flapping angle range, artificial wings, and a power source. From the measured wing kinematics, we have found that the flapping-wing system can properly modify the flapping angle ranges. The measured pitching moments show that the flapping-wing system generates a pitching moment in a desired direction by shifting the flapping angle range. We also demonstrated that the system can in practice change the longitudinal attitude by generating a nonzero pitching moment.     

Design and Demonstration of Insect Mimicking Foldable Artificial Wing Using Four-Bar Linkage Systems

In this work, we develop an artificial foldable wing that mimics the hind wing of a beetle （Allomyrina dichotoma）. In real flight, the beetle unfolds forewings and hind wings, and maintains the unfolded configuration unless it is exhausted. The artificial wing has to be able to maintain a fully unfolded configuration while flapping at a desirable flapping frequency. The artificial foldable hind wing developed in this work is based on two four-bar linkages which adapt the behaviors of the beetle＇s hind wing. The four-bar-linkages are designed to mimic rotational motion of the wing base and the vein folding/unfolding motion of the beetle＇s hind wing. The behavior of the artificial wings, which are installed in a flapping-wing system, is observed using a high-speed camera. The observation shows that the wing could maintain a fully unfolded configuration during flapping motion. A series of thrust measurements are also conducted to estimate the force generated by the flapping-wing system with foldable artificial wings. Although the artificial foldable wings give added burden to the flapping-wing system because of its weight, the thrust measurement results show that the flapping-wing system could still generate reasonable thrust.     

Effect of outer wing separation on lift and thrust generation in a flapping wing system

We explore the implementation of wing feather separation and lead-lagging motion to a flapping wing. A biomimetic flapping wing system with separated outer wings is designed and demonstrated. The artificial wing feather separation is implemented in the biomimetic wing by dividing the wing into inner and outer wings. The features of flapping, lead-lagging, and outer wing separation of the flapping wing system are captured by a high-speed camera for evaluation. The performance of the flapping wing system with separated outer wings is compared to that of a flapping wing system with closed outer wings in terms of forward force and downward force production. For a low flapping frequency ranging from 2.47 to 3.90 Hz, the proposed biomimetic flapping wing system shows a higher thrust and lift generation capability as demonstrated by a series of experiments. For 1.6 V application (lower frequency operation), the flapping wing system with separated wings could generate about 56% higher forward force and about 61% less downward force compared to that with closed wings, which is enough to demonstrate larger thrust and lift production capability of the separated outer wings. The experiments show that the outer parts of the separated wings are able to deform, resulting in a smaller amount of drag production during the upstroke, while still producing relatively greater lift and thrust during the downstroke.     

Artificial Cambered-Wing for a Beetle-Mimicking Flapper

In this paper, we have attempted to improve the aerodynamic force generation ability of an artificial wing by implementing initial wing camber in the flexible artificial wing. This initial camber is used to create passive wing camber during flapping motion. We modified original artificial wing by removing many minor vein structures in the wing and then placed the initial camber between two major veins. Stiffness measurements for the original artificial wing and the present wing with initial camber were conducted to compare the stiffnesses of the two artificial wings, and the similarities of the two wings are discussed. A flapping test was carried out using a previously-built flapper that can flap at higher than 25 Hz flapping frequency to verify the wing camber effect. Finally, a performance comparison between uncambered- and cambered-wings was also undertaken based on observations using a high-speed camera and force measurements from wired-flight tests and swing tests. The comparison showed that the cambered-wing could produce about 10% higher thrust than the uncambered-wing.     

How Could Beetle＇s Elytra Support Their Own Weight during Forward Flight？

The aerodynamic role of the elytra during a beetle＇s flapping motion is not well-elucidated, although it is well-recognized that the evolution of elytra has been a key in the success of coleopteran insects due to their protective function. An experimental study on wing kinematics reveals that for almost concurrent flapping with the hind wings, the flapping angle of the elytra is 5 times smaller than that of the hind wings. Then, we explore the aerodynamic forces on elytra in free forward flight with and without an effect of elytron-hind wing interaction by three-dimensional numerical simulation. The numerical results show that vertical force generated by the elytra without interaction is not sufficient to support even its own weight. However, the elytron-hind wing interaction improves the vertical force on the elytra up to 80%; thus, the total vertical force could fully support its own weight. The interaction slightly increases the vertical force on the hind wind by 6% as well.     

A two-dimensional aerodynamic model of freely flying insects

Possible free flights of insects by a single flapping motion were studied. It is well-known that insects utilize vortices generated by flapping, by which they generate larger lift than that evaluated by the ordinary aerodynamic theory. However, the effect of the motion of the center of mass (CM) of the insect on its flight has not been clarified. To clarify the effect, numerical simulation was performed for a simple model considering the coupling between the vertical CM motion and the separation vortices generated by flapping wing. As a result, it is shown that the flapping flight has the following interesting features. First, despite a single flapping motion, the model exhibits two types of flapping flight: a steady flight in which the CM velocity oscillates and a wandering flight in which the CM velocity varies irregularly. These two types of flights are selected by the initial conditions even when all the parameters are the same. Second, at a certain critical parameter value, the steady flight loses its stability and undergoes an abrupt transition to the wandering flight. Interestingly, at this critical value, the steady flight can be regarded as hovering. The possible flights are analyzed in terms of bifurcation, and the bifurcation structure is qualitatively explained based on a simple assumption. These results suggest the significance of the effect of CM motion.     

Effect of flexural and torsional wing flexibility on lift generation in hoverfly flight

The effect of wing flexibility in hoverflies was investigated using an at-scale mechanical model. Unlike dynamically-scaled models, an at-scale model can include all phenomena related to motion and deformation of the wing during flapping. For this purpose, an at-scale polymer wing mimicking a hoverfly was fabricated using a custom micromolding process. The wing has venation and corrugation profiles which mimic those of a hoverfly wing and the measured flexural stiffness of the artificial wing is comparable to that of the natural wing. To emulate the torsional flexibility at the wing-body joint, a discrete flexure hinge was created. A range of flexure stiffnesses was chosen to match the torsional stiffness of pronation and supination in a hoverfly wing. The polymer wing was compared with a rigid, flat, carbon-fiber wing using a flapping mechanism driven by a piezoelectric actuator. Both wings exhibited passive rotation around the wing hinge; however, these rotations were reduced in the case of the compliant polymer wing due to chordwise deformations during flapping which caused a reduced effective angle of attack. Maximum lift was achieved when the stiffness of the hinge was similar to that of a hoverfly in both wing cases and the magnitude of measured lift is sufficient for hovering; the maximum lift achieved by the single polymer and carbon-fiber wings was 5.9?×?10(2)(?)μN and 6.9?×?10(2)(?)μN, respectively. These results suggest that hoverflies could exploit intrinsic compliances to generate desired motions of the wing and that, for the same flapping motions, a rigid wing could be more suitable for producing large lift.     

Coordination of wingbeat and respiration in the Canada goose. I. Passive wing flapping.

The effects of passive wing flapping on respiratory pattern were examined in decerebrate Canada geese. The birds were suspended dorsally with two spine clamps while the extended wings were continuously moved up and down with a device designed to reproduce actual wing flapping. Passive wing motion entrained respiration over limited ranges by both increasing and decreasing the respiratory period relative to rest. All ratios of wingbeat frequency to respiratory frequency seen during free flight (Soc. Neurosci. Abstr. 15: 391, 1989) were produced during passive wing flapping. In addition, the phase relationship between wingbeat frequency and respiratory frequency, inspiration starting near the peak of wing upstroke, was similar to that seen during free flight and was unaffected by perturbations of the wing-flapping cycle. Removal of all afferent activity from the wings did not affect the ability of continuous passive wing movement to entrain respiration. However, feedback from the wings was required to produce rapid within-breath shifts in the respiratory period in response to single wing flaps. In conclusion, although feedback from the chest wall/lung may be more important in producing entrainment during the stable conditions of passive wing flapping, wing-related feedback may be critically involved in mediating the rapid adjustments in respiratory pattern required to maintain coordination between wing and respiratory movements during free flight.     

The Role of Soft Vein Joints in Dragonfly Flight

Dragonflies are excellent flyers among insects and their flight ability is closely related to the architecture and material properties of their wings.The veins are main structure components of a dragonfly wing,which are found to be connected by resilin with high elasticity at some joints.A three-dimensional (3D) finite element model of dragonfly wing considering the soft vein joints is developed,with some simplifications.Passive deformation under aerodynamic loads and active flapping motion of the wing are both studied.The functions of soft vein joints in dragonfly flight are concluded.In passive deformation,the chordwise flexibility is improved by soft vein joints and the wing is cambered under loads,increasing the action area with air.In active flapping,the wing rigidity in spanwise direction is maintained to achieve the required amplitude.As a result,both the passive deformation and the active control of flapping work well in dragonfly flight.The present study may also inspire the design of biomimetic Flapping Micro Air Vehicles (FMAVs).     

A modified blade element theory for estimation of forces generated by a beetle-mimicking flapping wing system

We present an unsteady blade element theory (BET) model to estimate the aerodynamic forces produced by a freely flying beetle and a beetle-mimicking flapping wing system. Added mass and rotational forces are included to accommodate the unsteady force. In addition to the aerodynamic forces needed to accurately estimate the time history of the forces, the inertial forces of the wings are also calculated. All of the force components are considered based on the full three-dimensional (3D) motion of the wing. The result obtained by the present BET model is validated with the data which were presented in a reference paper. The difference between the averages of the estimated forces (lift and drag) and the measured forces in the reference is about 5.7%. The BET model is also used to estimate the force produced by a freely flying beetle and a beetle-mimicking flapping wing system. The wing kinematics used in the BET calculation of a real beetle and the flapping wing system are captured using high-speed cameras. The results show that the average estimated vertical force of the beetle is reasonably close to the weight of the beetle, and the average estimated thrust of the beetle-mimicking flapping wing system is in good agreement with the measured value. Our results show that the unsteady lift and drag coefficients measured by Dickinson et al are still useful for relatively higher Reynolds number cases, and the proposed BET can be a good way to estimate the force produced by a flapping wing system.     

Improvement of Artificial Foldable Wing Models by Mimicking the Unfolding/Folding Mechanism of a Beetle Hind Wing

<正> In an attempt to realize a flapping wing micro-air vehicle with morphing wings, we report on improvements to our previousfoldable artificial hind wing.Multiple hinges, which were implemented to mimic the bending zone of a beetle hind wing, weremade of small composite hinge plates and tiny aluminum rivets.The buck-tails of rivets were flared after the hinge plates wereassembled with the rivets so that the folding/unfolding motions could be completed in less time, and the straight shape of theartificial hind wing could be maintained after fabrication.Folding and unfolding actions were triggered by electrically-activatedShape Memory Alloy (SMA) wires.For wing folding, the actuation characteristics of the SMA wire actuator were modifiedthrough heat treatment.Through a series of flapping tests, we confirmed that the artificial wings did not fold back and arbitrarilyfluctuate during the flapping motion.     

Automated Kinematics Measurement and Aerodynamics of a Bioinspired Flapping Rotary Wing

A physical model for a micro air vehicle with Flapping Rotary Wings (FRW) is investigated by measuring the wing kinematics in trim conditions and computing the corresponding aerodynamic force using computational fluid dynamics.In order to capture the motion image and reconstruct the positions and orientations of the wing,the photogrammetric method is adopted and a method for automated recognition of the marked points is developed.The characteristics of the realistic wing kinematics are presented.The results show that the non-dimensional rotating speed is a linear function of non-dimensional flapping frequency regardless of the initial angles of attack.Moreover,the effects of wing kinematics on aerodynamic force production and the underlying mechanism are analyzed.The results show that the wing passive pitching caused by elastic deformation can significantly enhance lift production.The Strouhal number of the FRW is much higher than that of general flapping wings,indicating the stronger unsteadiness of flows in FRW.     

Analysis of Maneuvering Flight of an Insect

Wing motion of a dragonfly in the maneuvering flight, which was measured by Wang et al. was investigated. Equations of motion for a maneuvering flight of an insect were derived. These equations were applied for analyzing the maneuvering flight. Inertial forces and moments acting on a body and wings were estimated by using these equations and the measured motions of the body and the wings. The results indicated the following characteristics of this flight: ( 1 ) The phase difference in flapping motion between the two fore wings and two hind wings, and the phase difference between the flapping motion and the feathering motion of the four wings are equal to those in a steady forward flight with the maximum efficiency. (2)The camber change and the feathering motion were mainly controlled by muscles at the wing bases.     

Normalized lift: an energy interpretation of the lift coefficient simplifies comparisons of the lifting ability of rotating and flapping surfaces

For a century, researchers have used the standard lift coefficient C(L) to evaluate the lift, L, generated by fixed wings over an area S against dynamic pressure, ?ρv(2), where v is the effective velocity of the wing. Because the lift coefficient was developed initially for fixed wings in steady flow, its application to other lifting systems requires either simplifying assumptions or complex adjustments as is the case for flapping wings and rotating cylinders.This paper interprets the standard lift coefficient of a fixed wing slightly differently, as the work exerted by the wing on the surrounding flow field (L/ρ·S), compared against the total kinetic energy required for generating said lift, ?v(2). This reinterpreted coefficient, the normalized lift, is derived from the work-energy theorem and compares the lifting capabilities of dissimilar lift systems on a similar energy footing. The normalized lift is the same as the standard lift coefficient for fixed wings, but differs for wings with more complex motions; it also accounts for such complex motions explicitly and without complex modifications or adjustments. We compare the normalized lift with the previously-reported values of lift coefficient for a rotating cylinder in Magnus effect, a bat during hovering and forward flight, and a hovering dipteran.The maximum standard lift coefficient for a fixed wing without flaps in steady flow is around 1.5, yet for a rotating cylinder it may exceed 9.0, a value that implies that a rotating cylinder generates nearly 6 times the maximum lift of a wing. The maximum normalized lift for a rotating cylinder is 1.5. We suggest that the normalized lift can be used to evaluate propellers, rotors, flapping wings of animals and micro air vehicles, and underwater thrust-generating fins in the same way the lift coefficient is currently used to evaluate fixed wings.     

枯叶蛱蝶扑翼飞行的力学分析

模仿昆虫扑翼飞行的飞行器具有重量轻、质量小、噪音低、效率高、隐蔽性好等优点,在军用、民用领域被广泛地关注与应用.枯叶蛱蝶是典型的扑翼昆虫,在连续上升飞行过程中会出现停顿和跃升的现象.为了研究停顿和跃升现象的产生原因,对枯叶蛱蝶的翅型和扑翼行为进行了力学分析.通过测量鳞翅结构参数,记录飞行行为,运用能量守恒与动量守恒原理,考虑生物能的作用,视空气为不可压缩颗粒,建立了数学模型模拟枯叶蛱蝶飞行情况.结果表明,扑翼行为通过改变飞行动力的动量和分力大小来影响枯叶蛱蝶的飞行轨迹,鳞翅形状则通过改变飞行动力的大小来影响枯叶蛱蝶的飞行轨迹,扑翼行为导致停顿和跃升现象的产生.本文为设计扑翼型飞行器提供了力学仿生学基础与生物学模型,为进一步设计出更优化的仿生飞行器提供科学依据.     

The Aerodynamics of Flapping Animal Flight

Our understanding of the aerodynamics of flapping animal flightis largely based on the quasi-steady assumption: the instantaneousaerodynamic forces on a flapping wing are assumed to be identicalwith those which the wing would experience in steady motionat the same instantaneous speed and angle of attack. Researchup to a decade ago showed that the assumption was sufficientto explain the flight of the vast majority of animals, but didnot rule out the possibility that alternative aerodynamic mechanismswere employed instead. Results are presented here for four hoveringanimals for which the quasi-steady explanation fails. Theseanimals apparently use lift mechanisms that rely on vorticesshed during the rotational motion of the wing at either endof the wingbeat. The postulated rotational lift mechanisms shouldalso apply to other hovering animals, even though the quasi-steadyassumption could explain their flight. Measurements of the wingforces produced by locusts cast doubt on the validity of thequasi-steady assumption for fast forward flight as well.     

Better Endurance and Load Capacity: An Improved Design of Manta Ray Robot (RoMan-II)

An improved design of a biomimetic underwater vehicle (RoMan-II) inspired by manta ray is presented in this paper. The design of the prototype and the swimming motion control are discussed. Instead of using rigid multiple degree-of-freedom linkages as fin rays in the first version, six flexible fin rays are adopted to drive two sided fins which generate thrust through flapping motions. Furthermore, in order to save the energy for a long distance cruising, a bio-inspired gliding motion is incorporated onto the motion control of the improved prototype. With a closed-loop buoyancy control system, the vehicle can perform gliding locomotion in water, which reduces the overall energy consumption. The vehicle can also perform pivot turning and backward locomotion without turning its body. It can achieve an average velocity of one body length per second. The vehicle is able to carry various sensors or communication equipments, as the payload capacity is about 4 kg. Initial testing shows that the operation time of the buoyancy body is estimated to about 6 hours for free swimming and 90 hours for a pure gliding. The flapping frequency, flapping amplitude, and the number of waves performed across the fin's chord and wave directions can be independently tuned through the proposed control scheme. In general, the present prototype provides a useful platform to study the ray-like swimming motion in a single or combination mode of flapping, undulation and gliding.