Mechanics of gliding in birds with special reference to the influence of the ground effect

A model of the mechanics of gliding without loss of altitude (horizontal gliding) is developed. The model can be employed to assess the influence of the principal drag components (induced, profile and parasite drag), choice of initial and final glide velocities and height above the ground on glide distance. For birds gliding near to the ground the ground effect acts to decrease the induced drag and increase the lift to drag ratio of the wings. Minimum drag speed is reduced for birds gliding near to the ground. The model is applied to the gliding flight of the black skimmer (Rhyncops nigra). Glide distances for given initial and final velocities are significantly increased in the influence of the ground effect over out of ground effect values.     

The effect of size on the optimal shapes of gliding insects and seeds

Conventional aerodynamic arguments suggest that possession of high aspect ratio wings will always improve the flight performance of glides. Drag and power will be minimized at intermediate flight speeds. It is shown, however, that as the aspect ratio increases, these minimum drag speeds are reduced, and will fall below the stall speed of the glider. This will happen at lower aspect ratios in small gliders, which operate at higher profile drag coefficients. Increasing the aspect ratio further will improve performance less than this analysis suggests.
A detailed analysis is developed to calculate the optimum shape of small gliders. Profile drag increases with aspect ratio, owing to the fall in the Reynolds number, while induced drag falls with increasing aspect ratio. Minimum drag will be encountered and hence the glide angle will be minimized at intermediate values of aspect ratio. Best glide angles are achieved at low speeds (high lift coefficients) and the optimum aspect ratio increases with the mass of the glider.
Small natural gliders possess large, low aspect ratio wings. The aspect ratios are generally somewhat below those which would produce the best glide angle at stall speed, but should give a reasonable performance over a range of speeds.     

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.     

Flight Characteristics of Two Plume Moths, Alucita pentadacty/a L. and Orneodes hexadactyla L. (Microlepidoptera)

Norberg, R. Å. (Department of Zoology, University of Gothenburg, Göteborg, Sweden.) Flight characteristics of two plume moths, Alucita pentadactyla L. and Orneodes hexadactyla L. (Microlepidoptera). Zool. Scripta 1 (6): 241–246,1972.–Multiple exposure photographs of up to 100 exposures/sec were taken on two plume moth species in free, unrestrained flight, in order to determine approximate lift/drag ratios and other functional characteristics of their wings, which are of a remarkable structure for insects of this size. In Alucita the forewing is cleft in two fringed lobes, the hind-wing in three, while in Orneodes both forewing and hindwing are deeply cleft in six very narrow, fringed lobes. Wing stroke frequencies are ca. 33 Hz in A. pentadactyla and ca. 40 Hz in O. hexadactyla. During both the downstroke and the upstroke the fringed wing lobes lie edge against edge, thus forming a continuous wing surface. The upstroke seems to contribute no useful forces in A. pentadactyla, possibly some propulsive force in O. hexadactyla. The wings are strongly supinated in the upstroke to minimize drag. From relative wind diagrams, lift/drag ratios of 1.1 and 1.4 (minimum values) can be read for A. pentadactyla and O. hexadactyla, respectively. It is thus clear that these species do not make more use of drag forces than of lift forces. However, in A. pentadactyla the drag force in the downstroke may be almost as large as the lift force. Since drag certainly is small in the upstroke, the drag force probably contributes significantly to useful forces for flight in A. pentadactyla. These plume moths operate at Reynolds numbers of ca. 700. Reynolds numbers are calculated for very small insects. It is obvious that the wings of the smallest insects must be operating at Reynolds numbers of about 1. The fringed wings of small insects are briefly discussed.     

A wing-assisted running robot and implications for avian flight evolution

DASH+Wings is a small hexapedal winged robot that uses flapping wings to increase its locomotion capabilities. To examine the effects of flapping wings, multiple experimental controls for the same locomotor platform are provided by wing removal, by the use of inertially similar lateral spars, and by passive rather than actively flapping wings. We used accelerometers and high-speed cameras to measure the performance of this hybrid robot in both horizontal running and while ascending inclines. To examine consequences of wing flapping for aerial performance, we measured lift and drag forces on the robot at constant airspeeds and body orientations in a wind tunnel; we also determined equilibrium glide performance in free flight. The addition of flapping wings increased the maximum horizontal running speed from 0.68 to 1.29 m s?1, and also increased the maximum incline angle of ascent from 5.6° to 16.9°. Free flight measurements show a decrease of 10.3° in equilibrium glide slope between the flapping and gliding robot. In air, flapping improved the mean lift:drag ratio of the robot compared to gliding at all measured body orientations and airspeeds. Low-amplitude wing flapping thus provides advantages in both cursorial and aerial locomotion. We note that current support for the diverse theories of avian flight origins derive from limited fossil evidence, the adult behavior of extant flying birds, and developmental stages of already volant taxa. By contrast, addition of wings to a cursorial robot allows direct evaluation of the consequences of wing flapping for locomotor performance in both running and flying.     

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.     

Thermal impact of migrating birds’ wing color on their flight performance: Possibility of new generation of biologically inspired drones

The thermal impact of the birds’ color on their flight performance are investigated. In most of the large migrating birds, the top of their wings is black. Considering this natural phenomenon in the migrating birds, such as albatross, a thermal analysis of the boundary layer of their wings is performed during the year depending on the solar insulation. It is shown that the temperature difference between the bright and dark colored top wing surface is around 10 °C. The dark color on the top of the wing increases the temperature of the boundary layer over the wing which consequently reduces the skin drag force over the wing. This reduction in the drag force can be considered as one of the effective factors for long endurance of these migrating birds. This research should lead to improved designs of the drones by applying the inspired colors which can help drones increase their endurance.     

The ‘W’ Prawn-Trawl with Emphasised Drag-Force Transfer to Its Centre Line to Reduce Overall System Drag

For prawn trawling systems, drag reduction is a high priority as the trawling process is energy intensive. Large benefits have occurred through the use of multiple-net rigs and thin twine in the netting. An additional positive effect of these successful twine-area reduction strategies is the reduced amount of otter board area required to spread the trawl systems, which leads to further drag reduction. The present work investigated the potential of redirecting the drag-strain within a prawn trawl away from the wings and the otter boards to the centre line of the trawl, where top and bottom tongues have been installed, with an aim to minimise the loading/size of the otter boards required to spread the trawl. In the system containing the new ‘W’ trawl, the drag redirected to the centre-line tongues is transferred forward through a connected sled and towing wires to the trawler. To establish the extent of drag redirection to the centre-line tongues and the relative drag benefits of the new trawl system, conventional and ‘W’ trawls of 3.65 m headline length were tested firstly over a range of spread ratios in the flume tank, and subsequently at optimum spread ratio in the field. The developed ‘W’ trawl effectively directed 64% of netting-drag off the wings and onto the centre tongues, which resulted in drag savings in the field of ∼20% for the associated ‘W’ trawl/otter-board/sled system compared to the traditional trawl/otter-board arrangement in a single trawl or twin rig configuration. Furthermore, based on previously published data, the new trawl when used in a twin rig system is expected to provide approximately 12% drag reduction compared to quad rig. The twin ‘W’ trawl system also has benefits over quad rig in that a reduced number of cod-end/By-catch Reduction Device units need to be installed and attended each tow.     

Biomimetic Drag Reduction Study on Herringbone Riblets of Bird Feather

Birds have gradually formed various excellent structures such as streamlined shape and hollow shaft of feather to improve their flying performance by millions of years of natural selection. As typical property of bird feather, herringbone riblets align along the shaft of each feather, which is caused by perfect link of barbs, especially for the primary and secondary feathers of wings. Such herringbone riblets of feather are assumed to have great impact on drag reduction. In this paper, microstructures of secondary feathers of adult pigeons are investigated by SEM, and their structural parameters are statistically obtained. Based on quantitative analysis of feather structure, novel biomimetic herringbone riblets with narrow smooth edge are proposed to reduce surface drag. In comparison with traditional microgroove riblets and other drag reduction structures, the drag reduction rate of the proposed biomimetic herringbone riblets is experimentally clarified up to 16%, much higher than others. Moreover, the drag reduction mechanism of herringbone riblets are also confirmed and exploited by CFD.     

Wing shape and migration in shorebirds: a comparative study

Migration is an energetically expensive and hazardous stage of the annual cycle of non‐resident avian species, and requires certain morphological adaptations. Wing shape is one of the morphological traits that is expected to be evolutionarily shaped by migration. Aerodynamic theory predicts that long‐distance migrants should have more pointed wings with distal primaries relatively longer than proximal primaries, an arrangement that minimizes induced drag and wing inertia, but this prediction has mostly been tested in passerine species. We applied the comparative method of phylogenetically independent contrasts to assess convergent evolution between wing shape and migration within shorebirds. We confirmed the assumption that long‐distance migrants have less rounded wings than species migrating shorter distances. Furthermore, wing roundedness negatively correlates with fat load and mean distance of migratory flights, the basic components of migration strategies. After controlling for interspecific differences in body size, we found no support for a link between wing length and migration, indicating that wing shape is a more important predictor of shorebird migratory behaviour than wing length. The results suggest that total migration distance and migratory strategy may simultaneously act on the evolution of wing shape in shorebirds, and possibly in other avian species.     

A Brief Review on Aerodynamic Performance of Wingtip Slots and Research Prospect

Wingtip slots,where the outer primary feathers of birds split and spread vertically,are regarded as an evolved favorable feature that could effectively improve their aerodynamic performance.They have inspired many to perform experiments and simulations as well as to relate their results to aircraft design.This paper aims to provide guidance for the research on the aerodynamic mechanism of wingtip slots.Following a review of previous wingtip slot research,four imperfections are put forward:vacancies in research content,inconsistencies in research conclusions,limitations of early research methods,and shortage of the aerodynamic mechanism analysis.On this basis,further explorations and expansion of the influence factors for steady state are needed;more attention should be poured into the application of flow field integration method to decom-pose drag,and evaluation of variation in induced drag seems a more rational choice.Geometric and kinematic parameters of wingtip slot structure in the unsteady state,as well as the flexibility of wingtips,should be taken into account.As for the aerodynamic mechanism of wingtip slots,the emphasis can be placed on the study of the formation,development,and evolution of wingtip vortices on slotted wings.Besides,some research strategies and feasibility analyses are proposed for each part of the research.     

Hydromechanics and biology

Summary To exemplify relations between biology and hydrodynamics the Reynolds number range and the effects of viscosity and inertia in swimming and flying organisms is discussed. Comparing water beetles and penguins it is shown, that the technical drag coefficient is an adequate means to describe flow adaptation in animals. Compared to technical systems, especially the penguins' drag coefficient is astonishingly low. Furthermore, the question, why comparatively thick bodies in penguins and dolphins show rather low drag is discussed. Distributed boundary layer damping in dolphins and secretion of special high molecular slimes in fishes help to keep flow characteristics laminar. As an example of one easily understood thrust mechanism, the drag inducing pair of rowing legs in water beetles is morphologically and hydrodynamically analysed. Fish swimming is discussed as a locomotion principle using lift components. Thrust generation by the moving tail fin of a fish is analysed in detail. Coming back to the influence if Reynolds number, it is finally shown, how very small, bristle bearing swimming legs and wings of insects make use of viscosity effects for locomotion.     

STUDY OF THE VERTICAL AUTOROTATION OF A SINGLEWINGED SAMARA

1. Autorotation of a single-winged samara is a highly nonlinear phenomenon that represents a delicate equilibrium between gravity, inertia and aerodynamic effects. Therefore, in order to analyse this phenomenon, an accurate detailed model is necessary. Such a model has not been presented in the past. Recently the authors derived a detailed model which is briefly described in the paper. 2. The aerodynamic contributions present the most complicated part of the phenomenon. These contributions are treated using the blade-element/momentuin method, with certain improvements and additions. These improvements are necessary due to inherent differences between samara wings and other rotary wing systems (aircraft propellers, helicopter rotors, etc.). 3. The cross-sectional aerodynamics of the samara is characterized by relatively small Reynolds numbers, high angles of attack and rough surfaces. While these characteristics are different from other rotary wings, they are typical of the wing cross-sections of insects and birds. Therefore the lift and drag coefficients, which are necessary for the analysis, are obtained using available data for insect and bird wings. 4. The results of the theoretical model are compared with experimental results of tlvo kinds. The first kind includes results for a samara of an Acer platanoides that were reported in the literature. In addition, a special experimental model of a samiira was built and tested. This model offers a simple way of varying the configuration in order to study (experimentally) the effect of different geometric parameters on the autorotation. 5. In the light of the uncertainty in the aerodynamic coefficients, it can be concluded that there is quite a good agreement between the theoretical and experimental results. Thus, after LTalidation, the theoretical model is used for a parametric study to find the influence of different parameters on the autorotation. The important results of this study are outlined below. 6. The spanwise flolv component and the tangential component of the induced velocity have a very small influence and thus can be neglected. 7. It is important to include in the analysis the effects of the axial induced velocity, the tip effect, and the drag of the root region. 8. Since chordwise variations of the centre of pressure location, as a function of the angle of attack, were seen in the past (based on over simplified analyses) as the mechanism which is responsible for the samara stability, this effect is also added to the model. While the influence of this effect on the pitch angle is large and small on the sinking rate, it results in an increase in the deviation between the theoretical and experimental results. 9. Autorotation is sensitive to the cross-sectional aerodynamic coefficients. This sensitivity is critical since the available data on these coefficients is, to say the least, unsatisfactory and require significant improvement.     

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.     

Estimation of Unsteady Aerodynamics in the Wake of a Freely Flying European Starling (Sturnus vulgaris)

Wing flapping is one of the most widespread propulsion methods found in nature; however, the current understanding of the aerodynamics in bird wakes is incomplete. The role of the unsteady motion in the flow and its contribution to the aerodynamics is still an open question. In the current study, the wake of a freely flying European starling has been investigated using long-duration high-speed Particle Image Velocimetry (PIV) in the near wake. Kinematic analysis of the wings and body of the bird has been performed using additional high-speed cameras that recorded the bird movement simultaneously with the PIV measurements. The wake evolution of four complete wingbeats has been characterized through reconstruction of the time-resolved data, and the aerodynamics in the wake have been analyzed in terms of the streamwise forces acting on the bird. The profile drag from classical aerodynamics was found to be positive during most of the wingbeat cycle, yet kinematic images show that the bird does not decelerate. It is shown that unsteady aerodynamics are necessary to satisfy the drag/thrust balance by approximating the unsteady drag term. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag during flapping flight.     

Sequential organization of grooming behaviors of the mosquito,Aedes triseriatus

Grooming behavior was studied in adult females of the mosquito Aedes triseriatus(Say). The grooming repertoire consisted of 12 different behaviors (accounting for bilateral symmetry) organized into five sequences. The proboscis and antennae were scraped by the forelegs, whereas the dorsal and ventral surfaces of the wings, and the forelegs, midlegs, hindlegs, and tip of the abdomen were scraped by the hindlegs. Each sequence ended with hindleg groomng. Tibial grooming combs were found on the ventral apices of the fore- and hind-tibiae but not on the mid-tibiae. A multidimensional scaling procedure grouped the grooming behaviors in two ways: (1) by the relative position of the groomed structure along the anteroposterior axis of the insect's body, and (2) by whether the groomed structure has a locomotory or sensory function. This suggests that mosquitoes groom both to clear sensilla of obstructive matter and to clean and smooth scales on legs and wings, possibly to decrease drag during flight.     

Das Geheimnisdes Storchenflugs

Lasertracking of white storks: Slotted wing tips reduce induced drag Utilizing a target‐tracking system VECTOR IV based on an infrared laser system, numerous measurements of trajectories of free gliding white storks were performed. The measured data allowed the calculation of forward‐ and sinking speeds over the native velocity scale. Furthermore numerous investigations of morphometric data, as wing span and aspect ratio, feather construction were performed. The weights of 36 adult individuals were gathered by outdoor scales located on a favored landing place. Furthermore isolated primaries werde investigated in a windtunnel. Utilising mathematical methods, we could separate the induced and the non‐induced drag components. Compared to an ideal planar aeroplane wing with elliptical lift distribution, soaring storks with fully opened primary cascade feature 20 % lower induced drag‐values.     

Die Bionik der Schwungfederkaskade

Bionics of the primary cascade – solution of an optimization problem Investigating of the properties of the primary cascade in soaring birds to date resulted in some spectacular insights. It was disclosed that the primary cascade with its staggered feathers acts like a vortex diffusor and, hence, considerably reduces the wing drag. That advance enlarges the range in migration and food sourcing. Newly the reason for the circular respectively spherical positioning of the primary tips could be resolved. This spatial arrangement features an optimum with respect to enlarging the distances of the feather tips and span, respectively, at limited root bending moments at once. This synergistic solution leads to an optimized fracture behavior and favors the lightweight construction of avian wings.     

Courtship dives of Anna's hummingbird offer insights into flight performance limits

Behavioural displays are a common feature of animal courtship. Just as female preferences can generate exaggerated male ornaments, female preferences for dynamic behaviours may cause males to perform courtship displays near intrinsic performance limits. I provide an example of an extreme display, the courtship dive of Anna''s hummingbird (Calypte anna). Diving male Anna''s hummingbirds were filmed with a combination of high-speed and conventional video cameras. After powering the initial stage of the dive by flapping, males folded their wings by their sides, at which point they reached an average maximum velocity of 385 body lengths s⁻¹ (27.3 m s⁻¹). This is the highest known length-specific velocity attained by any vertebrate. This velocity suggests their body drag coefficient is less than 0.3. They then spread their wings to pull up, and experienced centripetal accelerations nearly nine times greater than gravitational acceleration. This acceleration is the highest reported for any vertebrate undergoing a voluntary aerial manoeuvre, except jet fighter pilots. Stereotyped courtship behaviours offer several advantages for the study of extreme locomotor performance, and can be assessed in a natural context.     

Physiological characterization of the cold-shock-induced humoral factor for wing color-pattern changes in butterflies

Butterfly wing color patterns can be modified by the application of temperature shock to pupae immediately after pupation, which has been attributed to a cold-shock-induced humoral factor called cold-shock hormone (CSH). Here, we physiologically characterized CSH and pharmacological action of tungstate, using a nymphalid butterfly Junonia orithya. We first showed that the precise patterns of modification were dependent on the time-point of the cold-shock treatment after pupation, and confirmed that the modification properties induced in a cold-shocked pupa were able to be transferred to another pupa in a parabiosis experiment. Cold-shock application after removal of the head and prothorax together still produced modified wings, excluding major involvement of the brain-retrocerebral neuroendocrine complex. Furthermore, tungstate injection induced modifications even in individuals whose head and prothorax were removed. Importantly, transplantation of tracheae isolated from cold-shocked pupae induced modifications in the recipient wings. We identified a chemical peak in hemolymph of the cold-shocked individuals using HPLC, which corresponded to dopamine, and demonstrated that dopamine and its related biogenic amines have ability to induce small color-pattern changes. Taken together, the present study suggests that CSH is likely to be secreted from trachea-associated endocrine cells upon cold-shock treatment and that tungstate may change color patterns via its direct action on wings.