Biomechanics and physiology of gait selection in flying birds

Two wing-beat gaits, distinguished by the presence or absence of lift production during the upstroke, are currently used to describe avian flight. Vortex-visualization studies indicate that lift is produced only during the downstroke in the vortex-ring gait and that lift is produced continuously in the continuous-vortex gait. Tip-reversal and feathered upstrokes represent different forms of vortex-ring gait distinguished by wing kinematics. Useful aerodynamic forces may be produced during tip-reversal upstroke in slow flight and during a feathered upstroke in fast flight, but it is probable that downstroke forces are much greater in magnitude. Uncertainty about the function of these types of upstroke may be resolved when more data are available on wake structure in different flight speeds and modes. Inferring from wing kinematics and available data on wake structure, birds with long wings or wings of high aspect ratio use a vortex-ring gait with tip-reversal upstroke at slow speeds, a vortex-ring gait with a feathered upstroke at intermediate speeds, and a continuous-vortex gait at fast speeds. Birds with short wings or wings of low aspect ratio use a vortex-ring gait with a feathered upstroke at all speeds. Regardless of wing shape, species tend to use a vortex-ring gait for acceleration and a continuous-vortex gait for deceleration. Some correlations may exist between gait selection and the function of the muscular and respiratory system. However, overall variation in wing kinematics, muscle activity, and respiratory activity is continuous rather than categorical. To further our understanding of gait selection in flying birds, it is important to test whether upstroke function varies in a similar manner. Transitions between lifting and nonlifting upstrokes may be more subtle and gradual than implied by a binomial scheme of classification.     

Wing kinematics during natural leaping in the mantids Mantis religiosa and Iris oratoria

High-speed flash photography was used to analyse wing movements of Mantis religiosa and Iris oratoria at the moment of take-off during natural leaping. Wing kinematics are compared with those of the similarly designed locust wing. Iris oratoria showed strong coupling between leg extensor and wing depressor muscle activity immediately prior to take-off, with a possible enhancement of jump momentum. A 'clap and peel' was observed in the hind wings of both species during the first downstroke. Supination in the mantid forewing is accomplished by a backward rotation of the whole of the main wing plate about the claval furrow. Both fore- and hind wings show pronounced ventral flexure at the lower point of stroke reversal. Camber was developed in the hind wing during the upstroke as well as the downstroke. Possible roles of the claval furrow and transverse flexion in protecting the forewing base against torsional forces generated at stroke reversal are discussed.     

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.     

Non-Jumping Take off Performance in Beetle Flight （Rhinoceros Beetle Trypoxylus dichotomus）

In recent decades, the take-off mechanisms of flying animals have received much attention in insect flight initiation. Most of previous works have focused on the jumping mechanism, which is the most common take-off mechanism found in flying animals. Here, we presented that the rhinoceros beetle, Trypoxylus dichotomus, takes offwithout jumping. In this study, we used 3-Dimensional （3D） high-speed video techniques to quantitatively analyze the wings and body kinematics during the initiation periods of flight. The details of the flapping angle, angle of attack of the wings and the roll, pitch and yaw angles of the body were investigated to understand the mechanism of take-off in T. dichotomus. The beetle took off gradually with a small velocity and small acceleration. The body kinematic analyses showed that the beetle exhibited stable take-off. To generate high lift force, the beetle modulated its hind wing to control the angle of attack; the angle of attack was large during the upstroke and small during the downstroke. The legs of beetle did not contract and strongly release like other insects. The hind wing could be con- sidered as a main source of lift for heavy beetle.     

The Role of Elytra in Beetle Flight: I. Generation of Quasi-Static Aerodynamic Forces

We conducted a comprehensive study to investigate the aerodynamic characteristics and force generation of the elytra of abeetle,Allomyrina dichotoma.Our analysis included wind tunnel experiments and three-dimensional computational fluiddynamics simulations using ANSYS-CFX software.Our first approach was a quasi-static study that considered the effect ofinduced flapping flow due to the flapping motion of the fore-wings (elytra) at a frequency of around 30 Hz to 40 Hz.The dihedralangle was varied to represent flapping motion during the upstroke and downstroke.We found that an elytron producespositive lift at 0° geometric angle of attack,negative lift during the upstroke,and always produces drag during both the upstrokeand downstroke.We also found that the lift coefficient of an elytron does not drop even at a very high geometric angle of attack.For a beetle with a body weight of 5 g,based on the quasi-static method,the fore-wings (elytra) can produce lift of less than 1%of its body weight.     

Wing movements in the bush-cricket Tettigonia viridissima and the mantis Ameles spallanziana during natural leaping

The movements of the wings during natural jumps made by Tettigonia viridissima and Ameles spallanziana were analysed by means of high-speed flash photography. Additional data were obtained from the bush-cricket Oecanthus pellucens . In all cases the wings were usually extended before the hind tarsi had left the ground. In most jumps the first downstroke of the wings was completed before take-off and the wings probably contributed directly to initial propulsion. All species showed a 'peel' variation of the 'clap and fling' mechanism in the hind wing downstroke. There was evidence of strong ventral flexure in the forewing at the start of the upstroke in Tettigonia . The implications of the use of the wings in the energetics of jumping are discussed.     

Flow Visualization of Rhinoceros Beetle (Trypoxylus dichotomus) in Free Flight

Aerodynamic characteristics of the beetle,Trypoxylus dichotomus,which has a pair of elytra (forewings) and flexible hind wings,are investigated.Visualization experiments were conducted for various flight conditions of a beetle,Trypoxylus dichotomus:free,tethered,hovering,forward and climbing flights.Leading edge,trailing edge and tip vortices on both wings were observed clearly.The leading edge vortex was stable and remained on the top surface of the elytron for a wide interval during the downstroke of free forward flight.Hence,the elytron may have a considerable role in lift force generation of the beetle.In addition,we reveal a suction phenomenon between the gaps of the hind wing and the elytron in upstroke that may improve the positive lift force on the hind wing.We also found the reverse clap-fling mechanism of the T.dichotomus beetle in hovering flight.The hind wings touch together at the beginning of the upstroke.The vortex generation,shedding and interaction give a better understanding of the detailed aerodynamic mechanism of beetle flight.     

Kinematics of take-off and climbing flight in butterflies

High speed flash photography (flash duration 0.1 ms) was used to analyse wing movements in over 30 species of butterfly. With few exceptions, the insects showed a clap and peel mechanism of lift production at the start of the downstroke. Early in the upstroke the wings showed pronounced ventral flexure which, combined with inertial lag in the posterior parts of both wing pairs and delayed supination in the hind wing, led to the formation of a funnel-like space between the wings. These movements, and the resultant airflow patterns, appear to be an axi-symmetric equivalent of the 'near' clap and peel (here referred to as the funnel). Hind wing movements throughout the stroke are hinged upon the claval furrow. The expanded anal lobes of the hind wing lying medially to the claval furrow help to provide an air-tight seal around the abdomen between the upper and lower wing surfaces, which increases the efficiency of the peel and funnel mechanisms. The role of the intercalary flexion lines in controlling changes in wing surface corrugation during the cycle is also investigated.     

Leading edge vortex in a slow-flying passerine

Most hovering animals, such as insects and hummingbirds, enhance lift by producing leading edge vortices (LEVs) and by using both the downstroke and upstroke for lift production. By contrast, most hovering passerine birds primarily use the downstroke to generate lift. To compensate for the nearly inactive upstroke, weight support during the downstroke needs to be relatively higher in passerines when compared with, e.g. hummingbirds. Here we show, by capturing the airflow around the wing of a freely flying pied flycatcher, that passerines may use LEVs during the downstroke to increase lift. The LEV contributes up to 49 per cent to weight support, which is three times higher than in hummingbirds, suggesting that avian hoverers compensate for the nearly inactive upstroke by generating stronger LEVs. Contrary to other animals, the LEV strength in the flycatcher is lowest near the wing tip, instead of highest. This is correlated with a spanwise reduction of the wing's angle-of-attack, partly owing to upward bending of primary feathers. We suggest that this helps to delay bursting and shedding of the particularly strong LEV in passerines.     

The effect of forewing depressor activity on wing movement during locust flight

Locusts are passively yawed in the laminar air current of a wind tunnel (Fig. 1). In order to study the influence of depressor muscles of the forewing on its movement, electromyography is combined with true 3-dimensional inductive forewing movement recording. In quick response to the yaw stimulus, many kinematic parameters (e.g. shape of the wing tip path, amplitudes of wingstroke, ratios of downstroke to upstroke duration, time interval between beginning of downstroke and time of maximum pronation etc.) vary differently in both forewings (Figs. 3–5). Pronation changes in correlation to yawing reciprocally on both forewings with comparable differences of pronation angles (Fig. 5a). Maximum pronation is decreased on that side, to which the animal is-passively-yawed, whereas the slope of the wing tip paths remains almost constant. Therefore, decreasing pronation most probably indicates increasing thrust. The animal appears to perform a disturbance avoidance behaviour. Although the burst length of muscle firing is almost constant here, the onset of 8 depressor muscles (1 st basalar and subalar muscles of all 4 wings) varies in correlation to the stimulus (Figs. 6–8). The changing time intervals between the 1 st basalar muscle M97 and subalar muscle M99 are responsible for the alterations of forewing downstroke. Quantitative analysis of combined motor and movement pattern (Fig. 9) shows the following: (i) the maximum pronation and time interval between the onset of 1 st basalar muscle M97 as well as subalar muscle M99 and the beginning of downstroke are positively correlated (Figs. 10 and 12a and b). (ii) Maximum pronation is greatest, when muscles M97 and M99 act simultaneously (Fig. 12c). Thus, both muscles work synergistically, concerning pronation. Muscle M99 is of less importance than muscle M97. On failing activity of the depressor muscle M97, downstroke is greatly reduced. Some depressor as well as elevator muscles are switched on and off separately on each side (Fig. 11).     

A Quasi-Steady Lifting Line Theory for Insect-Like Hovering Flight

A novel lifting line formulation is presented for the quasi-steady aerodynamic evaluation of insect-like wings in hovering flight. The approach allows accurate estimation of aerodynamic forces from geometry and kinematic information alone and provides for the first time quantitative information on the relative contribution of induced and profile drag associated with lift production for insect-like wings in hover. The main adaptation to the existing lifting line theory is the use of an equivalent angle of attack, which enables capture of the steady non-linear aerodynamics at high angles of attack. A simple methodology to include non-ideal induced effects due to wake periodicity and effective actuator disc area within the lifting line theory is included in the model. Low Reynolds number effects as well as the edge velocity correction required to account for different wing planform shapes are incorporated through appropriate modification of the wing section lift curve slope. The model has been successfully validated against measurements from revolving wing experiments and high order computational fluid dynamics simulations. Model predicted mean lift to weight ratio results have an average error of 4% compared to values from computational fluid dynamics for eight different insect cases. Application of an unmodified linear lifting line approach leads on average to a 60% overestimation in the mean lift force required for weight support, with most of the discrepancy due to use of linear aerodynamics. It is shown that on average for the eight insects considered, the induced drag contributes 22% of the total drag based on the mean cycle values and 29% of the total drag based on the mid half-stroke values.     

Hovering and intermittent flight in birds

Two styles of bird locomotion, hovering and intermittent flight, have great potential to inform future development of autonomous flying vehicles. Hummingbirds are the smallest flying vertebrates, and they are the only birds that can sustain hovering. Their ability to hover is due to their small size, high wingbeat frequency, relatively large margin of mass-specific power available for flight and a suite of anatomical features that include proportionally massive major flight muscles (pectoralis and supracoracoideus) and wing anatomy that enables them to leave their wings extended yet turned over (supinated) during upstroke so that they can generate lift to support their weight. Hummingbirds generate three times more lift during downstroke compared with upstroke, with the disparity due to wing twist during upstroke. Much like insects, hummingbirds exploit unsteady mechanisms during hovering including delayed stall during wing translation that is manifest as a leading-edge vortex (LEV) on the wing and rotational circulation at the end of each half stroke. Intermittent flight is common in small- and medium-sized birds and consists of pauses during which the wings are flexed (bound) or extended (glide). Flap-bounding appears to be an energy-saving style when flying relatively fast, with the production of lift by the body and tail critical to this saving. Flap-gliding is thought to be less costly than continuous flapping during flight at most speeds. Some species are known to shift from flap-gliding at slow speeds to flap-bounding at fast speeds, but there is an upper size limit for the ability to bound (~0.3 kg) and small birds with rounded wings do not use intermittent glides.     

Der Vorderflügel großer Heuschrecken als Luftkrafterzeuger

Summary The mode is demonstrated in which the three regions of the forewings of large grasshoppers, i.e. the costal, radial and anal parts (Fig. 2) are folded against each other during up- and downstroke (Fig. 1; measurements made by W. Zarnack). The aerodynamic effects of this wing folding are determined from the measurements of lift carried out on wing models in parallel air stream (Fig. 3) and on rotating wing models (Fig. 4). Accordingly wing bending during downstroke generates distinctly higher lift at small and medium angles of attack than a flat wing having the same dimensions and moving at the same speed. It generates less lift at high angles of attack. Wing bending during upstroke generates higher lift only at>15°. Lift remains greater than that of a flat wing up to the highest angles of attack. These conclusions are supported by measurements of downstroke wind velocity (Fig. 5). Therefore it is possible to change the lift of right and left wings in order to generate moments for flight control around all three axes of the animal's system of co-ordinates without changing the rough kinematics of the beating wings.

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Der Verfasser dankt Dr. H.K. Pfau für die Einstellung der Flügelmodelle auf typische Mittelpositionen nach funktionsmorphologischen Gesichtspunkten (Abb. 2a) und Dr. W. Zarnack für die Vorpublikationserlaubnis der in Abb. 1 zusammengefaßten Meßdaten. Für meßtechnische Mitarbeit dankt er Frl. cand. rer. nat. U. Britz.     

Die Bedeutung der Elytren bei Vertretern desMelolontha-Flugtyps (Coleoptera)

The function of the flapping elytra was investigated in garden shafers (Melolontha melolontha L.) and rhinoceros beetles (Oryctes boas Fabr.).

Flow visualization and unsteady aerodynamics in the flight of the hawkmoth,Manduca sexta

The aerodynamic mechanisms employed durng the flight of the hawkmoth, Manduca sexta, have been investigated through smoke visualization studies with tethered moths. Details of the flow around the wings and of the overall wake structure were recorded as stereophotographs and high-speed video sequences. The changes in flow which accompanied increases in flight speed from 0.4 to 5.7 m s^-1 were analysed. The wake consists of an alternating series of horizontal and vertical vortex rings which are generated by successive down- and upstrokes, respectively. The downstroke produces significantly more lift than the upstroke due to a leading-edge vortex which is stabilized by a radia flow moving out towards the wingtip. The leading-edge vortex grew in size with increasing forward flight velocity. Such a phenomenon is proposed as a likely mechanism for lift enhancement in many insect groups. During supination, vorticity is shed from the leading edge as postulated in the ''flex'' mechanism. This vorticity would enhance upstroke lift if it was recaptured diring subsequent translation, but it is not. Instead, the vorticity is left behind and the upstroke circulation builds up slowly. A small jet provides additional thrust as the trailing edges approach at the end of the upstroke. The stereophotographs also suggest that the bound circulation may not be reversed between half strokes at the fastest flight speeds.     

Evolution of Flight in Insects

Norberg, R. Å. (Department of Zoology, University of Gothenburg, Göteborg, Sweden.) Evolution of flight in insects. Zool. Scripta 1 (6): 247–250, 1972.–Two hypotheses on the origin of flight in insects are discussed. 1. Gliding hypothesis. If wings and flight originated in ca. 1 cm large, or larger, insects, a leaping type seems to be a more probable candidate than a non-leaping one, since the former type has, with certainty, a high frequency of voluntary air excursions, during which any extensions come into play. Furthermore, it may attain the equilibrium gliding speed by jumping, and need not, if arboreal, lose any height on a steep initial fall to gain speed. 2. Floating hypothesis. The hypothesis presented here is a modified version of that put forward by Wigglesworth in 1963. It is suggested that wings may have originated in very small insects as thin dorsolateral, fringed extensions (like the wings of the smallest flying insects) acting as viscous drag producers, enabling the insects to float in the air with a very slow sinking speed and to be dispersed passively over long distances by thermal convection currents. Mov-ability of the wings would have increased practicability on the ground, and selection pressure for this could have brought about preadaptation for active flapping flight. Monophyly versus convergence of insect wings of conventional type (aerofoil function) is discussed briefly.     

Kinematic compensation for wing loss in flying damselflies

Flying insects can tolerate substantial wing wear before their ability to fly is entirely compromised. In order to keep flying with damaged wings, the entire flight apparatus needs to adjust its action to compensate for the reduced aerodynamic force and to balance the asymmetries in area and shape of the damaged wings. While several studies have shown that damaged wings change their flapping kinematics in response to partial loss of wing area, it is unclear how, in insects with four separate wings, the remaining three wings compensate for the loss of a fourth wing. We used high-speed video of flying blue-tailed damselflies (Ischnura elegans) to identify the wingbeat kinematics of the two wing pairs and compared it to the flapping kinematics after one of the hindwings was artificially removed. The insects remained capable of flying and precise maneuvering using only three wings. To compensate for the reduction in lift, they increased flapping frequency by 18 ± 15.4% on average. To achieve steady straight flight, the remaining intact hindwing reduced its flapping amplitude while the forewings changed their stroke plane angle so that the forewing of the manipulated side flapped at a shallower stroke plane angle. In addition, the angular position of the stroke reversal points became asymmetrical. When the wingbeat amplitude and frequency of the three wings were used as input in a simple aerodynamic model, the estimation of total aerodynamic force was not significantly different (paired t-test, p = 0.73) from the force produced by the four wings during normal flight. Thus, the removal of one wing resulted in adjustments of the motions of the remaining three wings, exemplifying the precision and plasticity of coordination between the operational wings. Such coordination is vital for precise maneuvering during normal flight but it also provides the means to maintain flight when some of the wings are severely damaged.     

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.     

The vortex wake of a 'hovering' model hawkmoth

Visualization experiments with Manduca sexta have revealed the presence of a leading-edge vortex and a highly three-dimensional flow pattern. To further investigate this important discovery, a scaled-up robotic insect was built (the ''flapper'') which could mimic the complex movements of the wings of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing revealed a small but strong leading-edge vortex on the downstroke. This vortex had a high axial flow velocity and was stable, separating from the wing at approximately 75 per cent of the wing length. It connected to a large, tangled tip vortex, extending back to a combining stopping and starting vortex from pronation. At the end of the downstroke, the wake could be approximated as one vortex ring per wing. Based on the size and velocity of the vortex rings, the mean lift force during the downstroke was estimated to be about 1.5 times the body weight of a hawkmoth, confirming that the downstroke is the main provider of lift force.     

A Review of Research on the Mechanical Design of Hoverable Flapping Wing Micro-Air Vehicles

Most insects and hummingbirds can generate lift during both upstroke and downstroke with a nearly horizontal flapping stroke plane,and perform precise hovering flight.Further,most birds can utilize tails and muscles in wings to actively control the flight performance,while insects control their flight with muscles based on wing root along with wing's passive deformation.Based on the above flight principles of birds and insects,Flapping Wing Micro Air Vehicles(FWMAVs)are classified as either bird-inspired or insect-inspired FWMAVs.In this review,the research achievements on mechanisms of insect-inspired,hoverable FWMAVs over the last ten years(2011-2020)are provided.We also provide the definition,func-tion,research status and development prospect of hoverable FWMAVs.Then discuss it from three aspects:bio-inspiration,motor-driving mechanisms and intelligent actuator-driving mechanisms.Following this,research groups involved in insect-inspired,hoverable FWMAV research and their major achievements are summarized and classified in tables.Problems,trends and challenges about the mechanism are compiled and presented.Finally,this paper presents conclusions about research on mechanical structure,and the future is discussed to enable further research interests.