The control of flight force by a flapping wing: lift and drag production.
01 Aug 2001-The Journal of Experimental Biology (Company of Biologists)-Vol. 204, Iss: 15, pp 2607-2626
TL;DR: A dynamically scaled mechanical model of the fruit fly Drosophila melanogaster is used to study how changes in wing kinematics influence the production of unsteady aerodynamic forces in insect flight, finding no evidence that stroke deviation can augment lift, but it nevertheless may be used to modulate forces on the two wings.
Abstract: We used a dynamically scaled mechanical model of the fruit fly Drosophila melanogaster to study how changes in wing kinematics influence the production of unsteady aerodynamic forces in insect flight. We examined 191 separate sets of kinematic patterns that differed with respect to stroke amplitude, angle of attack, flip timing, flip duration and the shape and magnitude of stroke deviation. Instantaneous aerodynamic forces were measured using a two-dimensional force sensor mounted at the base of the wing. The influence of unsteady rotational effects was assessed by comparing the time course of measured forces with that of corresponding translational quasi-steady estimates. For each pattern, we also calculated mean stroke-averaged values of the force coefficients and an estimate of profile power. The results of this analysis may be divided into four main points.
(i) For a short, symmetrical wing flip, mean lift was optimized by a stroke amplitude of 180° and an angle of attack of 50°. At all stroke amplitudes, mean drag increased monotonically with increasing angle of attack. Translational quasi-steady predictions better matched the measured values at high stroke amplitude than at low stroke amplitude. This discrepancy was due to the increasing importance of rotational mechanisms in kinematic patterns with low stroke amplitude.
(ii) For a 180° stroke amplitude and a 45° angle of attack, lift was maximized by short-duration flips occurring just slightly in advance of stroke reversal. Symmetrical rotations produced similarly high performance. Wing rotation that occurred after stroke reversal, however, produced very low mean lift.
(iii) The production of aerodynamic forces was sensitive to changes in the magnitude of the wing’s deviation from the mean stroke plane (stroke deviation) as well as to the actual shape of the wing tip trajectory. However, in all examples, stroke deviation lowered aerodynamic performance relative to the no deviation case. This attenuation was due, in part, to a trade-off between lift and a radially directed component of total aerodynamic force. Thus, while we found no evidence that stroke deviation can augment lift, it nevertheless may be used to modulate forces on the two wings. Thus, insects might use such changes in wing kinematics during steering maneuvers to generate appropriate force moments.
(iv) While quasi-steady estimates failed to capture the time course of measured lift for nearly all kinematic patterns, they did predict with reasonable accuracy stroke-averaged values for the mean lift coefficient. However, quasi-steady estimates grossly underestimated the magnitude of the mean drag coefficient under all conditions. This discrepancy was due to the contribution of rotational effects that steady-state estimates do not capture. This result suggests that many prior estimates of mechanical power based on wing kinematics may have been grossly underestimated.
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TL;DR: The basic physical principles underlying flapping flight in insects, results of recent experiments concerning the aerodynamics of insect flight, as well as the different approaches used to model these phenomena are reviewed.
Abstract: The flight of insects has fascinated physicists and biologists for more than a century. Yet, until recently, researchers were unable to rigorously quantify the complex wing motions of flapping insects or measure the forces and flows around their wings. However, recent developments in high-speed videography and tools for computational and mechanical modeling have allowed researchers to make rapid progress in advancing our understanding of insect flight. These mechanical and computational fluid dynamic models, combined with modern flow visualization techniques, have revealed that the fluid dynamic phenomena underlying flapping flight are different from those of non-flapping, 2-D wings on which most previous models were based. In particular, even at high angles of attack, a prominent leading edge vortex remains stably attached on the insect wing and does not shed into an unsteady wake, as would be expected from non-flapping 2-D wings. Its presence greatly enhances the forces generated by the wing, thus enabling insects to hover or maneuver. In addition, flight forces are further enhanced by other mechanisms acting during changes in angle of attack, especially at stroke reversal, the mutual interaction of the two wings at dorsal stroke reversal or wing-wake interactions following stroke reversal. This progress has enabled the development of simple analytical and empirical models that allow us to calculate the instantaneous forces on flapping insect wings more accurately than was previously possible. It also promises to foster new and exciting multi-disciplinary collaborations between physicists who seek to explain the phenomenology, biologists who seek to understand its relevance to insect physiology and evolution, and engineers who are inspired to build micro-robotic insects using these principles. This review covers the basic physical principles underlying flapping flight in insects, results of recent experiments concerning the aerodynamics of insect flight, as well as the different approaches used to model these phenomena.
1,182 citations
TL;DR: In this article, a review of the recent progress in flapping wing aerodynamics and aeroelasticity is presented, where it is realized that a variation of the Reynolds number (wing sizing, flapping frequency, etc.) leads to a change in the leading edge vortex (LEV) and spanwise flow structures, which impacts the aerodynamic force generation.
877 citations
TL;DR: A standard quasi-steady model of insect flight is modified to include rotational forces, translational forces and the added mass inertia, and the revised model predicts the time course of force generation for several different patterns of flapping kinematics more accurately than a model based solely on translational force coefficients.
Abstract: We used a dynamically scaled model insect to measure the rotational forces produced by a flapping insect wing. A steadily translating wing was rotated at a range of constant angular velocities, and the resulting aerodynamic forces were measured using a sensor attached to the base of the wing. These instantaneous forces were compared with quasi-steady estimates based on translational force coefficients. Because translational and rotational velocities were constant, the wing inertia was negligible, and any difference between measured forces and estimates based on translational force coefficients could be attributed to the aerodynamic effects of wing rotation. By factoring out the geometry and kinematics of the wings from the rotational forces, we determined rotational force coefficients for a range of angular velocities and different axes of rotation. The measured coefficients were compared with a mathematical model developed for two-dimensional motions in inviscid fluids, which we adapted to the three-dimensional case using blade element theory. As predicted by theory, the rotational coefficient varied linearly with the position of the rotational axis for all angular velocities measured. The coefficient also, however, varied with angular velocity, in contrast to theoretical predictions. Using the measured rotational coefficients, we modified a standard quasi-steady model of insect flight to include rotational forces, translational forces and the added mass inertia. The revised model predicts the time course of force generation for several different patterns of flapping kinematics more accurately than a model based solely on translational force coefficients. By subtracting the improved quasi-steady estimates from the measured forces, we isolated the aerodynamic forces due to wake capture.
746 citations
Book•
01 Oct 2007TL;DR: In this paper, the authors introduce fixed, rigid, flexible, and flapping wing aerodynamic models for fixed and flexible wing aerodynamics, and propose a flexible wing model for flapping aerodynamics.
Abstract: 1. Introduction 2. Fixed, rigid wing aerodynamics 3. Flexible wing aerodynamics 4. Flapping wing aerodynamics.
580 citations
TL;DR: In this article, the authors define the terms rotor disk area, sectional drag coefficient, and zero-lift drag coefficient for rotor disk areas, where the sectional coefficient is defined as the ratio of the area of the rotor disk to the length of the chord length.
Abstract: Nomenclature Ar = rotor disk area CD = sectional drag coefficient CD0 = zero-lift drag coefficient Clα = lift-curve slope CP = power coefficient CPi = induced power coefficient CP0 = profile power coefficient CT = thrust coefficient c = chord length D = drag force D.L . = disk loading L = lift force m = mass P.L . = power loading SF = separated flow T = rotor thrust V = local wind velocity perceived by flap W = weight W f = final weight Wo = gross takeoff weight α = blade section angle of attack η = efficiency μ = dynamic viscosity ρ = air density σ = rotor solidity = flapping amplitude (peak to peak)
559 citations
References
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TL;DR: In this paper, the authors show that the enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture.
Abstract: The enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture. Delayed stall functions during the translational portions of the stroke, when the wings sweep through the air with a large angle of attack. In contrast, rotational circulation and wake capture generate aerodynamic forces during stroke reversals, when the wings rapidly rotate and change direction. In addition to contributing to the lift required to keep an insect aloft, these two rotational mechanisms provide a potent means by which the animal can modulate the direction and magnitude of flight forces during steering maneuvers. A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.
2,246 citations
08 Mar 2001
TL;DR: A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.
Abstract: The enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture. Delayed stall functions during the translational portions of the stroke, when the wings sweep through the air with a large angle of attack. In contrast, rotational circulation and wake capture generate aerodynamic forces during stroke reversals, when the wings rapidly rotate and change direction. In addition to contributing to the lift required to keep an insect aloft, these two rotational mechanisms provide a potent means by which the animal can modulate the direction and magnitude of flight forces during steering maneuvers. A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.
2,133 citations
"The control of flight force by a fl..." refers background or methods in this paper
...These include the clap and fling (Spedding and Maxworthy, 1986), dynamic stall (Dickinson and Götz, 1993; Ellington et al., 1996), rotational lift (Bennett, 1970; Dickinson et al., 1999) and wake capture (Dickinson, 1994; Dickinson et al., 1999)....
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...Aerodynamic models have proved valuable in the study of insect flight, particularly in the identification and analysis of unsteady aerodynamics (Bennett, 1977; Maxworthy, 1979; Spedding and Maxworthy, 1986; Dickinson and Götz, 1993; Ellington et al., 1996; Dickinson et al., 1999)....
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...The equations that best fit measured translational force coefficients as functions of angle of attack, α, for the model wing are (Dickinson et al., 1999) These equations are used to generate quasi-steady translational estimates for comparison with measured values....
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...Measures of the quasi-steady-state translational force coefficients CL,t and CD,t were derived from 180 ° sweeps of wing motion with fixed angles of attack, as described elsewhere (Dickinson et al., 1999)....
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...…forces on each wing, we calculated the corresponding mean force coefficients using an equation derived from blade element theory (Ellington, 1984c; Dickinson et al., 1999): where F – is the magnitude of a specific force component (lift, drag, radial, total) averaged over the stroke, Φ is stroke…...
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TL;DR: In this article, the authors visualized the airflow around the wings of the hawkmoth Manduca sexta and a 'hovering' large mechanical model, and found an intense leading-edge vortex was found on the downstroke, of sufficient strength to explain the high-lift forces.
Abstract: INSECTS cannot fly, according to the conventional laws of aerodynamics: during flapping flight, their wings produce more lift than during steady motion at the same velocities and angles of attack1–5. Measured instantaneous lift forces also show qualitative and quantitative disagreement with the forces predicted by conventional aerodynamic theories6–9. The importance of high-life aerodynamic mechanisms is now widely recognized but, except for the specialized fling mechanism used by some insect species1,10–13, the source of extra lift remains unknown. We have now visualized the airflow around the wings of the hawkmoth Manduca sexta and a 'hovering' large mechanical model—the flapper. An intense leading-edge vortex was found on the down-stroke, of sufficient strength to explain the high-lift forces. The vortex is created by dynamic stall, and not by the rotational lift mechanisms that have been postulated for insect flight14–16. The vortex spirals out towards the wingtip with a spanwise velocity comparable to the flapping velocity. The three-dimensional flow is similar to the conical leading-edge vortex found on delta wings, with the spanwise flow stabilizing the vortex.
1,663 citations
"The control of flight force by a fl..." refers background in this paper
...These include the clap and fling (Spedding and Maxworthy, 1986), dynamic stall (Dickinson and Götz, 1993; Ellington et al., 1996), rotational lift (Bennett, 1970; Dickinson et al., 1999) and wake capture (Dickinson, 1994; Dickinson et al., 1999)....
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...Aerodynamic models have proved valuable in the study of insect flight, particularly in the identification and analysis of unsteady aerodynamics (Bennett, 1977; Maxworthy, 1979; Spedding and Maxworthy, 1986; Dickinson and Götz, 1993; Ellington et al., 1996; Dickinson et al., 1999)....
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...However, because of the critical role of unsteady mechanisms and three-dimensional flow structure in insect flight aerodynamics (Ellington et al., 1996; Dickinson et al., 1999), theoretical or numerical approaches have, as yet, offered only limited insight into the aerodynamics of steering....
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TL;DR: In this paper, a projection analysis technique is described that solves for the orientation of the animal with respect to a cam era-based coordinate system, giving full kinematic details for the longitudinal wing and body axes from single-view films.
Abstract: Insects in free flight were filmed at 5000 frames per second to determine the motion of their wings and bodies. General comments are offered on flight behaviour and manoeuvrability. Changes in the tilt of the stroke plane with respect to the horizontal provides kinematic control of manoeuvres, analogous to the type of control used for helicopters. A projection analysis technique is described that solves for the orientation of the animal with respect to a cam era-based coordinate system, giving full kinematic details for the longitudinal wing and body axes from single-view films. The technique can be applied to all types of flight where the wing motions are bilaterally symmetrical: forward, backward and hovering flight, as well as properly banked turns. An analysis of the errors of the technique is presented, and shows that the reconstructed angles for wing position should be accurate to within 1-2° in general. Although measurement of the angles of attack was not possible, visual estimations are given. Only 11 film sequences show flight velocities and accelerations that are small enough for the flight to be considered as ‘hovering’. Two sequences are presented for a hover-fly using an inclined stroke plane, and nine sequences of hovering with a horizontal stroke plane by another hover-fly, two crane-flies, a drone-fly, a ladybird beetle, a honey bee, and two bumble bees. In general, oscillations in the body position from its mean motion are within measurement error, about 1-2 % of the wing length. The amplitudes of oscillation for the body angle are only a few degrees, but the phase relation of this oscillation to the wingbeat cycle could be determined for a few sequences. The phase indicates that the pitching moments governing the oscillations result from the wing lift at the ends of the wingbeat, and not from the wing drag or inertial forces. The mean pitching moment of the wings, which determines the mean body angle, is controlled by shifting the centre of lift over the cycle by changing the mean positional angle of the flapping wings. Deviations of the wing tip path from the stroke plane are never large, and no consistent pattern could be found for the wing paths of different insects; indeed, variations in the path were even observed for individual insects. The wing motion is not greatly different from simple harmonic motion, but does show a general trend towards higher accelerations and decelerations at either end of the wingbeat, with constant velocities during the middle of half-strokes. Root mean square and cube root mean cube angular velocities are on average about 4 and 9% lower than simple harmonic motion. Angles of attack are nearly constant during the middle of half-strokes, typically 35° at a position 70 % along the wing length. The wing is twisted along its length, with angles of attack at the wing base some 10-20° greater than at the tip. The wings rotate through about 110° at either end of the wingbeat during 10-20 % of the cycle period. The mean velocity of the wing edges during rotation is similar to the mean flapping velocity of the wing tip and greater than the flapping velocity for more proximal wing regions, which indicates that vortex shedding during rotation is com parable with that during flapping. The wings tend to rotate as a flat plate during the first half of rotation, which ends just before, or at, the end of the half-stroke. The hover-fly using an inclined stroke plane provides a notable exception to this general pattern : pronation is delayed and overlaps the beginning of the downstroke. The wing profile flexes along a more or less localized longitudinal axis during the second half of rotation, generating the ‘flip’ profile postulated by Weis-Fogh for the hover-flies. This profile occurs to some extent for all of the insects, and is not exceptionally pronounced for the hover-fly. By the end of rotation the wings are nearly flat again, although a slight camber can sometimes be seen. Weis-Fogh showed that beneficial aerodynamic interference can result when the left and right wings come into contact during rotation at the end of the wingbeat. His ‘fling’ mechanism creates the circulation required for wing lift on the subsequent half-stroke, and can be seen on my films of the Large Cabbage White butterfly, a plum e moth, and the Mediterranean flour moth. However, their wings ‘peel’ apart like two pieces of paper being separated, rather than fling open rigidly about the trailing edges. A ‘partial fling’ was found for some insects, with the wings touching only along posterior wing areas. A ‘ near fling ’ with the wings separated by a fraction of the chord was also observed for m any insects. There is a continuous spectrum for the separation distance between the wings, in fact, and the separation can vary for a given insect during different manoeuvres. It is suggested that these variants on Weis-Fogh’s fling mechanism also generate circulation for wing lift, although less effectively than a complete fling, and that changes in the separation distance may provide a fine control over the amount of lift produced.
735 citations
"The control of flight force by a fl..." refers background or methods or result in this paper
...…2 ν −1 AR −1 , where Φ is stroke amplitude, n is wingbeat frequency, R is wing length, ν is kinematic viscosity, aspect ratio AR is 4R 2 S −1 and S is the surface area of a wing pair; Ellington, 1984c) and the reduced frequency parameter (body velocity/wing velocity) constant (Spedding, 1993)....
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...It is worth noting that the range of CD – values is much higher than has been previously reported for Drosophila virilis wings under steady-state conditions (Vogel, 1967) or estimated on the basis of Reynolds number (CD≈0.7; Ellington, 1984c)....
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...From the forces on each wing, we calculated the corresponding mean force coefficients using an equation derived from blade element theory (Ellington, 1984c; Dickinson et al., 1999): where F – is the magnitude of a specific force component (lift, drag, radial, total) averaged over the stroke, Φ is…...
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...These patterns were chosen because they roughly approximate patterns described for a variety of insects (Ellington, 1984b; Zanker, 1990a)....
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...The waveform was smoothed to minimize inertial accelerations during stroke reversal and to match more closely published stroke kinematics from a variety of insects (Ellington, 1984b; Zanker, 1990a)....
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TL;DR: In this article, the authors measured the time dependence of aerodynamic forces for a simple yet important motion, rapid acceleration from rest to a constant velocity at a fixed angle of attack, and found that at angles of attack below 13.5°, there was virtually no evidence of a delay in the generation of lift, in contrast to similar studies made at higher Reynolds numbers.
Abstract: The synthesis of a comprehensive theory of force production in insect flight is hindered in part by the lack of precise knowledge of unsteady forces produced by wings. Data are especially sparse in the intermediate Reynolds number regime (10
675 citations
"The control of flight force by a fl..." refers background in this paper
...These include the clap and fling (Spedding and Maxworthy, 1986), dynamic stall (Dickinson and Götz, 1993; Ellington et al., 1996), rotational lift (Bennett, 1970; Dickinson et al., 1999) and wake capture (Dickinson, 1994; Dickinson et al., 1999)....
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...We chose this particular set of parameters because fruit flies actively vary them during flight maneuvers (Götz et al., 1979; Zanker, 1990a; Dickinson et al., 1993; Lehmann and Dickinson, 1998)....
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...Aerodynamic models have proved valuable in the study of insect flight, particularly in the identification and analysis of unsteady aerodynamics (Bennett, 1977; Maxworthy, 1979; Spedding and Maxworthy, 1986; Dickinson and Götz, 1993; Ellington et al., 1996; Dickinson et al., 1999)....
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