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Jingtang Yang - One of the best experts on this subject based on the ideXlab platform.
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Enhanced thrust and speed revealed in the forward flight of a butterfly with transient body translation.
Physical Review E, 2015Co-Authors: Jingtang YangAbstract:: A butterfly with broad wings, flapping at a small frequency, flies an erratic trajectory at an inconstant speed. A large variation of speed within a cycle is observed in the forward flight of a butterfly. A self-propulsion model to simulate a butterfly is thus created to investigate the transient translation of the body; the results, which are in accordance with experimental data, show that the shape of the variation of the flight speed is similar to a sinusoidal wave with a maximum (J=0.89) at the beginning of the Downstroke, and a decrease to a minimum (J=0.17) during a transition from Downstroke to upstroke; the difference between the extrema of the flight speed is enormous in a flapping cycle. At a high speed, a clapping motion of the butterfly wings decreases the generation of drag. At a small speed, a butterfly is able to capture the induced wakes generated in a Downstroke, and effectively generates a thrust at the beginning of an upstroke. The wing motion of a butterfly skillfully interacts with its speed so as to enable an increased speed with the same motion. Considering a butterfly to fly in a constant inflow leads to either an underestimate of its speed or an overestimate of its generated lift, which yields an inaccurate interpretation of the insect's flight. Our results reveal the effect of transient translation on a butterfly in forward flight, which is especially important for an insect with a small flapping frequency.
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A passerine spreads its tail to facilitate a rapid recovery of its body posture during hovering
Journal of the Royal Society Interface, 2012Co-Authors: Jianyuan Su, Shangchieh Ting, Yuhung Chang, Jingtang YangAbstract:We demonstrate experimentally that a passerine exploits tail spreading to intercept the downward flow induced by its wings to facilitate the recovery of its posture. The periodic spreading of its tail by the White-eye bird exhibits a phase correlation with both wingstroke motion and body oscillation during hovering flight. During a Downstroke, a White-eye's body undergoes a remarkable pitch-down motion, with the tail undergoing an upward swing. This pitch-down motion becomes appropriately suppressed at the end of the Downstroke; the bird's body posture then recovers gradually to its original status. Employing digital particle-image velocimetry, we show that the strong downward flow induced by downstroking the wings serves as an external jet flow impinging upon the tail, providing a depressing force on the tail to counteract the pitch-down motion of the bird's body. Spreading of the tail enhances a rapid recovery of the body posture because increased forces are experienced. The maximum force experienced by a spread tail is approximately 2.6 times that of a non-spread tail.
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aerodynamic trick for visual stabilization during Downstroke in a hovering bird
Physical Review E, 2011Co-Authors: Jianyuan Su, Shangchieh Ting, Yuhung Chang, Jingtang YangAbstract:We provide physical insight into how a small hovering bird attains stabilized vision during Downstroke. A passerine generates a lift force greater than its body weight during Downstroke, leading to a substantial swing of the bird body, but the bird's eyes are nearly stable. Employing digital particle-image velocimetry, we demonstrate that a hovering passerine generates a lift force acting dorsal to the center of mass, concurrently resulting in rotational and translational displacements of the bird's body. The most notable finding is that the rotational and translational displacements at the bird's eyes almost cancel each other; the displacement of the eye is \ensuremath{\sim}8% that of the trailing tip of the tail. This aerodynamic trick enables a bird to attain stabilized vision beneficial for the inspection of the environment.
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Aerodynamic trick for visual stabilization during Downstroke in a hovering bird.
Physical review. E Statistical nonlinear and soft matter physics, 2011Co-Authors: Jianyuan Su, Shangchieh Ting, Yuhung Chang, Jingtang YangAbstract:We provide physical insight into how a small hovering bird attains stabilized vision during Downstroke. A passerine generates a lift force greater than its body weight during Downstroke, leading to a substantial swing of the bird body, but the bird's eyes are nearly stable. Employing digital particle-image velocimetry, we demonstrate that a hovering passerine generates a lift force acting dorsal to the center of mass, concurrently resulting in rotational and translational displacements of the bird's body. The most notable finding is that the rotational and translational displacements at the bird's eyes almost cancel each other; the displacement of the eye is ~8% that of the trailing tip of the tail. This aerodynamic trick enables a bird to attain stabilized vision beneficial for the inspection of the environment.
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an unconventional mechanism of lift production during the Downstroke in a hovering bird zosterops japonicus
Experiments in Fluids, 2011Co-Authors: Yuhung Chang, Shangchieh Ting, Jingtang Yang, C Y SoongAbstract:An unconventional mechanism of ventral clap is exploited by hovering passerines to produce lift. Quantitative visualization of the wake flow, analysis of kinematics and evaluation of the transient lift force was conducted to dissect the biomechanical role of the ventral clap in the asymmetrical hovering flight of passerines. The ventral clap can first abate and then augment lift production during the Downstroke; the net effect of the ventral clap on lift production is, however, positive because the extent of lift augmentation is greater than the extent of lift abatement. Moreover, the ventral clap is inferred to compensate for the zero lift production of the upstroke because the clapping wings induce a substantial elevation of the lift force at the end of the Downstroke. Overall, our observations shed light on the aerodynamic function of the ventral clap and offer biomechanical insight into how a bird hovers without kinematically mimicking hovering hummingbirds.
Kenneth P Dial - One of the best experts on this subject based on the ideXlab platform.
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Asymmetrical Force Production in the Maneuvering Flight of Pigeons
The Auk, 1998Co-Authors: Douglas R. Warrick, Kenneth P Dial, Andrew A BiewenerAbstract:Downstroke force produced by Rock Doves (Columba livia) as they negotiated an obstacle course was measured using in vivo recordings of delto-pectoral crest strain. Dur- ing this slow (
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Kinematic, aerodynamic and anatomical mechanisms in the slow, maneuvering flight of pigeons
The Journal of Experimental Biology, 1998Co-Authors: Douglas R. Warrick, Kenneth P DialAbstract:A high-speed (200 Hz) infrared video system was used in a three-dimensional analysis of pigeon wing and body kinematics to determine the aerodynamic and anatomical mechanisms they use to produce force asymmetries to effect a turn during slow (3 m s-1) flight. Contrary to our expectations, pigeons used Downstroke velocity asymmetries, rather than angle of attack or surface area asymmetries, to produce the disparities in force needed for directional changes. To produce a bank, a velocity asymmetry is created early in the Downstroke and, in the majority of cases, then reversed at the end of the same Downstroke, thus arresting the rolling angular momentum. When the velocity asymmetry was not reversed at the end of Downstroke, the arresting force asymmetry was produced during upstroke, with velocity asymmetries creating disparate drag forces on the wings. Rather than using subtle aerodynamic variables to produce subtle Downstroke force asymmetries, pigeons constantly adjust their position using a series of large alternating and opposing forces during Downstroke and upstroke. Thus, a pigeon creates a precise 9average9 body position (e.g. bank angle) and flight path by producing a series of rapidly oscillating movements. Although the primary locomotor event (Downstroke) is saltatory, maneuvering during slow flight should be considered as a product of nearly continuous, juxtaposed force generation throughout the wingbeat cycle. Further, viewing upstroke as more than stereotypical, symmetrical wing recovery alters the evolutionary and functional context of investigations into the musculoskeletal mechanisms and the associated neural control involved in this unique kinematic event.
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The effects of the wingbeat cycle on respiration in black-billed magpies (Pica pica)
The Journal of Experimental Biology, 1997Co-Authors: Dona F. Boggs, F. A. Jenkins, Kenneth P DialAbstract:Interclavicular and posterior thoracic airsac pressures, tracheal airflows and pectoralis muscle activity were recorded simultaneously to determine the effect of the wingbeat cycle upon the function of the respiratory system. The effects of the wingbeat cycle on the relative positions of thoraco-abdominal skeletal structures were also assessed using high-speed X-ray cinematography of magpies Pica pica flying in a windtunnel. We found that the furcula bends laterally on the Downstroke and recoils medially on the upstroke, as previously described for starlings, and that the coraco-sternal joint (the most consistently visible point on the sternum for digitization) is displaced dorsally during the Downstroke and ventrally, with respect to the vertebral column, during the upstroke. In magpies, there are generally three wingbeat cycles during a respiratory cycle. When Downstroke occurs during inspiration, its compressive effect reduces the inspiratory subatmospheric airsac pressure by an average of 92 % (0.35 kPa), whereas when upstroke occurs during expiration its expansive effect can reduce the expiratory supra-atmospheric airsac pressure by 63 % (0.23 kPa). Corresponding changes occur in tracheal airflow. Changes in respiratory parameters during short flights with respect to resting values include a doubling of tidal volume and a tripling of respiratory frequency. We conclude that the wingbeat cycle can have a substantial impact on respiratory system function in the flying magpie, and that this represents a mechanical basis for breathing patterns and patterns of phasic coordination between wingbeat and respiratory cycles that may result in minimal interference and maximal assistance from the wingbeat upon the respiratory cycle.
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The functional anatomy of the shoulder in the European starling (Sturnus vulgaris)
Journal of Morphology, 1991Co-Authors: Kenneth P Dial, G E Goslow, Farish A JenkinsAbstract:The excursions of wing elements and the activity of eleven shoulder muscles were studied by cineradiography and electromyography in European starlings (Sturnus vulgaris) flying in a wind tunnel at speeds of 9–20 m s−1. At the beginning of Downstroke the humerus is elevated 80–90° above horizontal, and both elbow and wrist are extended to 90° or less. During Downstroke, protraction of the humerus (55°) remains constant; elbow and wrist are maximally extended (120° and 160°, respectively) as the humerus passes through a horizontal orientation. During the Downstroke-upstroke transition humeral depression ceases (at about 20° below horizontal) and the humerus begins to retract. However, depression of the distal wing continues by rotation of the humerus and adduction of the carpometacarpus. Humeral retraction (to within about 30° of the body axis) is completed early in upstroke, accompanied by flexion of the elbow and carpometacarpus. Thereafter the humerus begins to protract as elevation continues. At mid-upstroke a rapid counterrotation of the humerus reorients the ventral surface of the wing to face laterad; extension of the elbow and carpometacarpus are initiated sequentially. The upstroke-Downstroke transition is characterized by further extension of the elbow and carpometacarpus, and the completion of humeral protraction. Patterns of electromyographic activity primarily coincide with the transitional phases of the wingbeat cycle rather than being confined to Downstroke or upstroke. Thus, the major Downstroke muscles (pectoralis, coracobrachialis caudalis, sternocoracoideus, subscapularis, and humerotriceps) are activated in late upstroke to decelerate, extend, and reaccelerate the wing for the subsequent Downstroke; electromyographic activity ends well before the Downstroke is completed. Similarly, the upstroke muscles (supracoracoideus, deltoideus major) are activated in late Downstroke to decelerate and then reaccelerate the wing into the upstroke; these muscles are deactivated by mid-upstroke. Only two muscles (scapulohumeralis caudalis, scapulotriceps) exhibit electromyographic activity exclusively during the Downstroke. Starlings exhibit a functional partitioning of the two heads of the triceps (the humerotriceps acts with the pectoralis group, and does not overlap with the scapulotriceps). The biphasic pattern of the biceps brachii appears to correspond to this partitioning.
Bret W Tobalske - One of the best experts on this subject based on the ideXlab platform.
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Three-dimensional kinematics of hummingbird flight.
The Journal of experimental biology, 2020Co-Authors: Bret W Tobalske, Douglas R. Warrick, Donald R. Powers, Tyson L Hedrick, Christopher J Clark, Gabriel A Hyder, Andrew A BiewenerAbstract:Hummingbirds are specialized for hovering flight, and substantial research has explored this behavior. Forward flight is also important to hummingbirds, but the manner in which they perform forward flight is not well documented. Previous research suggests that hummingbirds increase flight velocity by simultaneously tilting their body angle and stroke-plane angle of the wings, without varying wingbeat frequency and upstroke: Downstroke span ratio. We hypothesized that other wing kinematics besides stroke-plane angle would vary in hummingbirds. To test this, we used synchronized high-speed (500 Hz) video cameras and measured the three-dimensional wing and body kinematics of rufous hummingbirds (Selasphorus rufus, 3 g, N=5) as they flew at velocities of 0-12 m s(-1) in a wind tunnel. Consistent with earlier research, the angles of the body and the stroke plane changed with velocity, and the effect of velocity on wingbeat frequency was not significant. However, hummingbirds significantly altered other wing kinematics including chord angle, angle of attack, anatomical stroke-plane angle relative to their body, percent of wingbeat in Downstroke, wingbeat amplitude, angular velocity of the wing, wingspan at mid-Downstroke, and span ratio of the wingtips and wrists. This variation in bird-centered kinematics led to significant effects of flight velocity on the angle of attack of the wing and the area and angles of the global stroke planes during Downstroke and upstroke. We provide new evidence that the paths of the wingtips and wrists change gradually but consistently with velocity, as in other bird species that possess pointed wings. Although hummingbirds flex their wings slightly at the wrist during upstroke, their average wingtip-span ratio of 93% revealed that they have kinematically ;rigid' wings compared with other avian species.
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Aerodynamics of tip-reversal upstroke in a revolving pigeon wing.
The Journal of experimental biology, 2011Co-Authors: Kristen E Crandell, Bret W TobalskeAbstract:During slow flight, bird species vary in their upstroke kinematics using either a 'flexed wing' or a distally supinated 'tip-reversal' upstroke. Two hypotheses have been presented concerning the function of the tip-reversal upstroke. The first is that this behavior is aerodynamically inactive and serves to minimize drag. The second is that the tip-reversal upstroke is capable of producing significant aerodynamic forces. Here, we explored the aerodynamic capabilities of the tip-reversal upstroke using a well-established propeller method. Rock dove (Columba livia, N=3) wings were spread and dried in postures characteristic of either mid-upstroke or mid-Downstroke and spun at in vivo Reynolds numbers to simulate forces experienced during slow flight. We compared 3D wing shape for the propeller and in vivo kinematics, and found reasonable kinematic agreement between methods (mean differences 6.4% of wing length). We found that the wing in the upstroke posture is capable of producing substantial aerodynamic forces. At in vivo angles of attack (66 deg at mid-upstroke, 46 deg at mid-Downstroke), the upstroke wings averaged for three birds produced a lift-to-drag ratio of 0.91, and the Downstroke wings produced a lift-to-drag ratio of 3.33. Peak lift-to-drag ratio was 2.5 for upstroke and 6.3 for Downstroke. Our estimates of total force production during each half-stroke suggest that Downstroke produces a force that supports 115% of bodyweight, and during upstroke a forward-directed force (thrust) is produced at 36% of body weight.
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Aerodynamics of tip-reversal upstroke in a revolving pigeon wing
The Journal of Experimental Biology, 2011Co-Authors: Kristen E Crandell, Bret W TobalskeAbstract:During slow flight, bird species vary in their upstroke kinematics using either a ‘flexed wing’ or a distally supinated ‘tip-reversal’ upstroke. Two hypotheses have been presented concerning the function of the tip-reversal upstroke. The first is that this behavior is aerodynamically inactive and serves to minimize drag. The second is that the tip-reversal upstroke is capable of producing significant aerodynamic forces. Here, we explored the aerodynamic capabilities of the tip-reversal upstroke using a well-established propeller method. Rock dove ( Columba livia, N =3) wings were spread and dried in postures characteristic of either mid-upstroke or mid-Downstroke and spun at in vivo Reynolds numbers to simulate forces experienced during slow flight. We compared 3D wing shape for the propeller and in vivo kinematics, and found reasonable kinematic agreement between methods (mean differences 6.4% of wing length). We found that the wing in the upstroke posture is capable of producing substantial aerodynamic forces. At in vivo angles of attack (66 deg at mid-upstroke, 46 deg at mid-Downstroke), the upstroke wings averaged for three birds produced a lift-to-drag ratio of 0.91, and the Downstroke wings produced a lift-to-drag ratio of 3.33. Peak lift-to-drag ratio was 2.5 for upstroke and 6.3 for Downstroke. Our estimates of total force production during each half-stroke suggest that Downstroke produces a force that supports 115% of bodyweight, and during upstroke a forward-directed force (thrust) is produced at 36% of body weight. * c : chord length C D : coefficient of drag C h : mean horizontal force coefficient C L : coefficient of lift C v : mean vertical force coefficient D : drag Fh : horizontal force FR : resultant force Fv : vertical force L : lift Q : torque about the motor r : radial distance along the wing R : wing length S : wing area S 2 : second moment of area S 3 : third moment of area V T : wing translational velocity α : geometric angle of attack e : downwash angle ρ : air density Ω : angular velocity
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The Aerodynamics of Hummingbird Flight
45th AIAA Aerospace Sciences Meeting and Exhibit, 2007Co-Authors: Douglas R. Warrick, Bret W Tobalske, Donald R. Powers, Michael H DickensonAbstract:[Abstract] Hummingbirds fly with their wings almost fully extended during their entire wingbeat. This pattern, associated with having proportionally short humeral bones, long distal wing elements, and assumed to be an adaptation for extended hovering flight, has lead to predictions that the aerodynamic mechanisms exploited by hummingbirds during hovering should be similar to those observed in insects. To test these predictions, we flew rufous hummingbirds (Selasphorus rufus, 3.3 g, n = 6) in a variable–speed wind tunnel (0-12 ms) and measured wake structure and dynamics using digital particle image velocimetry (DPIV). Unlike hovering insects, hummingbirds produced 75% of their weight support during Downstroke and only 25% during upstroke, an asymmetry due to the inversion of their cambered wings during upstroke. Further, we have found no evidence of sustained, attached leading edge vorticity (LEV) during up or Downstroke, as has been seen in similarly-sized insects although a transient LEV is produced during the rapid change in angle of attack at the end of the Downstroke. Finally, although an extended-wing upstroke during forward flight has long been thought to produce lift and negative thrust, we found circulation during Downstroke alone to be sufficient to support body weight, and that some positive thrust was produced during upstroke, as evidenced by a vortex pair shed into the wake of all upstrokes at speeds of 4 – 12 m s.
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Aerodynamics of the hovering hummingbird
Nature, 2005Co-Authors: Douglas R. Warrick, Bret W Tobalske, Donald R. PowersAbstract:Since the first high-speed films were taken of hummingbirds nearly seventy years ago, it has been assumed that their mirror-image upstroke and Downstroke shared the burden of weight support roughly equally. This led to the view that in hovering flight hummingbirds have converged — both biomechanically, and aerodynamically — on the flight style adopted by insects. A new study of the aerodynamics of the hovering hummingbird shows that while they share some aerodynamic ‘tricks’ with insects, they remain birds. They have a bird-like aerofoil and produce a Downstroke three times as effective at generating lift as the upstroke. Despite profound musculoskeletal differences, hummingbirds (Trochilidae) are widely thought to employ aerodynamic mechanisms similar to those used by insects. The kinematic symmetry of the hummingbird upstroke and Downstroke1,2,3 has led to the assumption that these halves of the wingbeat cycle contribute equally to weight support during hovering, as exhibited by insects of similar size4. This assumption has been applied, either explicitly or implicitly, in widely used aerodynamic models1,5,6,7 and in a variety of empirical tests8,9. Here we provide measurements of the wake of hovering rufous hummingbirds (Selasphorus rufus) obtained with digital particle image velocimetry that show force asymmetry: hummingbirds produce 75% of their weight support during the Downstroke and only 25% during the upstroke. Some of this asymmetry is probably due to inversion of their cambered wings during upstroke. The wake of hummingbird wings also reveals evidence of leading-edge vortices created during the Downstroke, indicating that they may operate at Reynolds numbers sufficiently low to exploit a key mechanism typical of insect hovering10,11. Hummingbird hovering approaches that of insects, yet remains distinct because of effects resulting from an inherently dissimilar—avian—body plan.
Andrew A Biewener - One of the best experts on this subject based on the ideXlab platform.
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Three-dimensional kinematics of hummingbird flight.
The Journal of experimental biology, 2020Co-Authors: Bret W Tobalske, Douglas R. Warrick, Donald R. Powers, Tyson L Hedrick, Christopher J Clark, Gabriel A Hyder, Andrew A BiewenerAbstract:Hummingbirds are specialized for hovering flight, and substantial research has explored this behavior. Forward flight is also important to hummingbirds, but the manner in which they perform forward flight is not well documented. Previous research suggests that hummingbirds increase flight velocity by simultaneously tilting their body angle and stroke-plane angle of the wings, without varying wingbeat frequency and upstroke: Downstroke span ratio. We hypothesized that other wing kinematics besides stroke-plane angle would vary in hummingbirds. To test this, we used synchronized high-speed (500 Hz) video cameras and measured the three-dimensional wing and body kinematics of rufous hummingbirds (Selasphorus rufus, 3 g, N=5) as they flew at velocities of 0-12 m s(-1) in a wind tunnel. Consistent with earlier research, the angles of the body and the stroke plane changed with velocity, and the effect of velocity on wingbeat frequency was not significant. However, hummingbirds significantly altered other wing kinematics including chord angle, angle of attack, anatomical stroke-plane angle relative to their body, percent of wingbeat in Downstroke, wingbeat amplitude, angular velocity of the wing, wingspan at mid-Downstroke, and span ratio of the wingtips and wrists. This variation in bird-centered kinematics led to significant effects of flight velocity on the angle of attack of the wing and the area and angles of the global stroke planes during Downstroke and upstroke. We provide new evidence that the paths of the wingtips and wrists change gradually but consistently with velocity, as in other bird species that possess pointed wings. Although hummingbirds flex their wings slightly at the wrist during upstroke, their average wingtip-span ratio of 93% revealed that they have kinematically ;rigid' wings compared with other avian species.
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wing inertia and whole body acceleration an analysis of instantaneous aerodynamic force production in cockatiels nymphicus hollandicus flying across a range of speeds
The Journal of Experimental Biology, 2004Co-Authors: Tyson L Hedrick, James R. Usherwood, Andrew A BiewenerAbstract:SUMMARY We used a combination of high-speed 3-D kinematics and three-axis accelerometer recordings obtained from cockatiels flying in a low-turbulence wind tunnel to characterize the instantaneous accelerations and, by extension, the net aerodynamic forces produced throughout the wingbeat cycle across a broad range of flight speeds (1–13 m s –1 ). Our goals were to investigate the variation in instantaneous aerodynamic force production during the wingbeat cycle of birds flying across a range of steady speeds, testing two predictions regarding aerodynamic force generation in upstroke and the commonly held assumption that all of the kinetic energy imparted to the wings of a bird in flapping flight is recovered as useful aerodynamic work. We found that cockatiels produce only a limited amount of lift during upstroke (14% of Downstroke lift) at slower flight speeds (1–3 m s –1 ). Upstroke lift at intermediate flight speeds (7–11 m s –1 ) was moderate, averaging 39% of Downstroke lift. Instantaneous aerodynamic forces were greatest near mid-Downstroke. At the end of each half-stroke, during wing turnaround, aerodynamic forces were minimal, but inertial forces created by wing motion were large. However, we found that the inertial power requirements of Downstroke (minimum of 0.29±0.10 W at 7 m s –1 and maximum of 0.56±0.13 W at 1 m s –1 ) were consistent with the assumption that nearly all wing kinetic energy in Downstroke was applied to the production of aerodynamic forces and therefore should not be added separately to the overall power cost of flight. The inertial power requirements of upstroke (minimum of 0.16±0.04 W at 7 m s –1 and maximum of 0.35±0.11 W at 1 m s –1 ) cannot be recovered in a similar manner, but their magnitude was such that the power requirements for the upstroke musculature (minimum of 54±13 W kg –1 at 7 m s –1 and maximum of 122±35 W at 1 m s –1 ) fall within the established range for cockatiel flight muscle (<185 W kg –1 ).
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Asymmetrical Force Production in the Maneuvering Flight of Pigeons
The Auk, 1998Co-Authors: Douglas R. Warrick, Kenneth P Dial, Andrew A BiewenerAbstract:Downstroke force produced by Rock Doves (Columba livia) as they negotiated an obstacle course was measured using in vivo recordings of delto-pectoral crest strain. Dur- ing this slow (
James Richard Usherwood - One of the best experts on this subject based on the ideXlab platform.
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The aerodynamics of avian take-off from direct pressure measurements in Canada geese (Branta canadensis)
Journal of Experimental Biology, 2003Co-Authors: James Richard UsherwoodAbstract:Direct pressure measurements using electronic differential pressure transducers along bird wings provide insight into the aerodynamics of these dynamically varying aerofoils. Acceleration-compensated pressures were measured at five sites distributed proximally to distally from the tertials to the primaries along the wings of Canada geese. During take-off flight, ventral-to-dorsal pressure is maintained at the proximal wing section throughout the wingstroke cycle, whereas pressure sense is reversed at the primaries during upstroke. The distal sites experience double pressure peaks during the Downstroke. These observations suggest that tertials provide weight-support throughout the wingbeat, that the wingtip provides thrust during upstroke and that the kinetic energy of the rapidly flapping wings may be dissipated via retarding aerodynamic forces (resulting in aerodynamic work) at the end of Downstroke.