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Phasing of dragonfly wings can improve aerodynamic efficiency by removing swirl.

Usherwood JR, Lehmann FO - J R Soc Interface (2008)

Bottom Line: Despite efforts at understanding the implications of flapping flight with two pairs of wings, previous studies have generally painted a rather disappointing picture: interaction between fore and hind wings reduces the lift compared with two pairs of wings operating in isolation.Here, we demonstrate with a mechanical model dragonfly that, despite presenting no advantage in terms of lift, flying with two pairs of wings can be highly effective at improving aerodynamic efficiency.With the appropriate fore-hind wing phasing, aerodynamic power requirements can be reduced up to 22 per cent compared with a single pair of wings, indicating one advantage of four-winged flying that may apply to both dragonflies and, in the future, biomimetic micro air vehicles.

View Article: PubMed Central - PubMed

Affiliation: Structure and Motion Lab, The Royal Veterinary College, North Mymms, Hatfield, Herts, UK.

ABSTRACT
Dragonflies are dramatic, successful aerial predators, notable for their flight agility and endurance. Further, they are highly capable of low-speed, hovering and even backwards flight. While insects have repeatedly modified or reduced one pair of wings, or mechanically coupled their fore and hind wings, dragonflies and damselflies have maintained their distinctive, independently controllable, four-winged form for over 300Myr. Despite efforts at understanding the implications of flapping flight with two pairs of wings, previous studies have generally painted a rather disappointing picture: interaction between fore and hind wings reduces the lift compared with two pairs of wings operating in isolation. Here, we demonstrate with a mechanical model dragonfly that, despite presenting no advantage in terms of lift, flying with two pairs of wings can be highly effective at improving aerodynamic efficiency. This is achieved by recovering energy from the wake wasted as swirl in a manner analogous to coaxial contra-rotating helicopter rotors. With the appropriate fore-hind wing phasing, aerodynamic power requirements can be reduced up to 22 per cent compared with a single pair of wings, indicating one advantage of four-winged flying that may apply to both dragonflies and, in the future, biomimetic micro air vehicles.

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The mechanical dragonfly and results derived from force sensors at the wing bases. (a) The wingtip paths reported for a hovering dragonfly (Wakeling & Ellington 1997a) describe approximately horizontal stroke planes, with vertically stacked wings. (b,c) The mechanical model of a dragonfly's right wings was flapped at controlled fore–hind phases. Wing blade elements and gaze during flow visualization are indicated by the symbol and black lines plotted on the upper wing surfaces, respectively, in (c). The black triangle represents the wing's leading edge. Mean values derived from force sensors at the wing bases, of lift (d), the ratio of mean lift, , to mean drag, , (e), and aerodynamic efficiency expressed as ‘figures of merit’ (f) plotted as a function of fore–hind wing phase shift. Black solid lines show performances of isolated (i) fore wing, (ii) hind wing, (iii) cumulative effect of isolated fore and hind wings, and sine fit to combined-wing data as a function of phase.
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fig1: The mechanical dragonfly and results derived from force sensors at the wing bases. (a) The wingtip paths reported for a hovering dragonfly (Wakeling & Ellington 1997a) describe approximately horizontal stroke planes, with vertically stacked wings. (b,c) The mechanical model of a dragonfly's right wings was flapped at controlled fore–hind phases. Wing blade elements and gaze during flow visualization are indicated by the symbol and black lines plotted on the upper wing surfaces, respectively, in (c). The black triangle represents the wing's leading edge. Mean values derived from force sensors at the wing bases, of lift (d), the ratio of mean lift, , to mean drag, , (e), and aerodynamic efficiency expressed as ‘figures of merit’ (f) plotted as a function of fore–hind wing phase shift. Black solid lines show performances of isolated (i) fore wing, (ii) hind wing, (iii) cumulative effect of isolated fore and hind wings, and sine fit to combined-wing data as a function of phase.

Mentions: We observed the effect of hovering with two pairs of wings by measuring the forces and wakes produced by an intermediate Reynolds number robotic hovering model dragonfly, and demonstrate the significance of hovering with a range of fore–hind wing phases. Reynolds numbers, based on the mean wing chord wing tip velocity, were 105 (fore wing) or 125 (hind wing); this is at the low end for small hovering dragonflies (calculated as between 250 and 500; Maybury & Lehmann (2004) from Rüppell (1989)), but is considered well above the transitional Reynolds number (Wang & Russell 2007). Fore–hind wing phases are described here as the proportion (per cent) of the stroke period at which the hind wing leads the fore: +25 per cent indicates that the hind wing leads the fore wing by a quarter cycle; 50 per cent indicates total anti-phase. The robotic model represents the two dynamically scaled right wings of a hovering dragonfly with realistic wing shapes and hinges vertically separated by 1.25 chord lengths, yielding fore wings beating directly above the hind wings (figure 1a–c). Both fore and hind wings followed identical, generalized kinematics sweeping a horizontal stroke plane (see the electronic supplementary material). The aim of the robot kinematics was not to precisely reproduce any single set of measured wing motions, but to provide a moderately realistic test bed with which the significance of wing phasing could be investigated without introducing confounding aerodynamic factors such as the direction of the net force vector. Thus, while a range of stroke plane angles have been described for hovering dragonflies, we selected vertically stacked horizontal stroke planes following the observations of hovering Sympetrum sanguineum (Wakeling & Ellington 1997a). Instantaneous aerodynamic lift, defined as the vertical force that provides weight support, and drag, the force impeding the motion of the wing in the horizontal plane, were measured with force sensors at the wing bases. From these values, we calculated the mean lift force, the ratio of mean lift to mean drag, the power required to overcome drag, and the aerodynamic efficiency. Aerodynamic efficiency is represented by the ‘figure of merit’ (FoM), a special case of ‘propeller efficiency’ used for hovering helicopters (electronic supplementary material). This term describes the ratio of the minimum theoretical power required for hovering to the measured aerodynamic power (Prouty 2005). In effect, the FoM expresses aerodynamic efficiency by comparison with an ideal helicopter.


Phasing of dragonfly wings can improve aerodynamic efficiency by removing swirl.

Usherwood JR, Lehmann FO - J R Soc Interface (2008)

The mechanical dragonfly and results derived from force sensors at the wing bases. (a) The wingtip paths reported for a hovering dragonfly (Wakeling & Ellington 1997a) describe approximately horizontal stroke planes, with vertically stacked wings. (b,c) The mechanical model of a dragonfly's right wings was flapped at controlled fore–hind phases. Wing blade elements and gaze during flow visualization are indicated by the symbol and black lines plotted on the upper wing surfaces, respectively, in (c). The black triangle represents the wing's leading edge. Mean values derived from force sensors at the wing bases, of lift (d), the ratio of mean lift, , to mean drag, , (e), and aerodynamic efficiency expressed as ‘figures of merit’ (f) plotted as a function of fore–hind wing phase shift. Black solid lines show performances of isolated (i) fore wing, (ii) hind wing, (iii) cumulative effect of isolated fore and hind wings, and sine fit to combined-wing data as a function of phase.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC2607445&req=5

fig1: The mechanical dragonfly and results derived from force sensors at the wing bases. (a) The wingtip paths reported for a hovering dragonfly (Wakeling & Ellington 1997a) describe approximately horizontal stroke planes, with vertically stacked wings. (b,c) The mechanical model of a dragonfly's right wings was flapped at controlled fore–hind phases. Wing blade elements and gaze during flow visualization are indicated by the symbol and black lines plotted on the upper wing surfaces, respectively, in (c). The black triangle represents the wing's leading edge. Mean values derived from force sensors at the wing bases, of lift (d), the ratio of mean lift, , to mean drag, , (e), and aerodynamic efficiency expressed as ‘figures of merit’ (f) plotted as a function of fore–hind wing phase shift. Black solid lines show performances of isolated (i) fore wing, (ii) hind wing, (iii) cumulative effect of isolated fore and hind wings, and sine fit to combined-wing data as a function of phase.
Mentions: We observed the effect of hovering with two pairs of wings by measuring the forces and wakes produced by an intermediate Reynolds number robotic hovering model dragonfly, and demonstrate the significance of hovering with a range of fore–hind wing phases. Reynolds numbers, based on the mean wing chord wing tip velocity, were 105 (fore wing) or 125 (hind wing); this is at the low end for small hovering dragonflies (calculated as between 250 and 500; Maybury & Lehmann (2004) from Rüppell (1989)), but is considered well above the transitional Reynolds number (Wang & Russell 2007). Fore–hind wing phases are described here as the proportion (per cent) of the stroke period at which the hind wing leads the fore: +25 per cent indicates that the hind wing leads the fore wing by a quarter cycle; 50 per cent indicates total anti-phase. The robotic model represents the two dynamically scaled right wings of a hovering dragonfly with realistic wing shapes and hinges vertically separated by 1.25 chord lengths, yielding fore wings beating directly above the hind wings (figure 1a–c). Both fore and hind wings followed identical, generalized kinematics sweeping a horizontal stroke plane (see the electronic supplementary material). The aim of the robot kinematics was not to precisely reproduce any single set of measured wing motions, but to provide a moderately realistic test bed with which the significance of wing phasing could be investigated without introducing confounding aerodynamic factors such as the direction of the net force vector. Thus, while a range of stroke plane angles have been described for hovering dragonflies, we selected vertically stacked horizontal stroke planes following the observations of hovering Sympetrum sanguineum (Wakeling & Ellington 1997a). Instantaneous aerodynamic lift, defined as the vertical force that provides weight support, and drag, the force impeding the motion of the wing in the horizontal plane, were measured with force sensors at the wing bases. From these values, we calculated the mean lift force, the ratio of mean lift to mean drag, the power required to overcome drag, and the aerodynamic efficiency. Aerodynamic efficiency is represented by the ‘figure of merit’ (FoM), a special case of ‘propeller efficiency’ used for hovering helicopters (electronic supplementary material). This term describes the ratio of the minimum theoretical power required for hovering to the measured aerodynamic power (Prouty 2005). In effect, the FoM expresses aerodynamic efficiency by comparison with an ideal helicopter.

Bottom Line: Despite efforts at understanding the implications of flapping flight with two pairs of wings, previous studies have generally painted a rather disappointing picture: interaction between fore and hind wings reduces the lift compared with two pairs of wings operating in isolation.Here, we demonstrate with a mechanical model dragonfly that, despite presenting no advantage in terms of lift, flying with two pairs of wings can be highly effective at improving aerodynamic efficiency.With the appropriate fore-hind wing phasing, aerodynamic power requirements can be reduced up to 22 per cent compared with a single pair of wings, indicating one advantage of four-winged flying that may apply to both dragonflies and, in the future, biomimetic micro air vehicles.

View Article: PubMed Central - PubMed

Affiliation: Structure and Motion Lab, The Royal Veterinary College, North Mymms, Hatfield, Herts, UK.

ABSTRACT
Dragonflies are dramatic, successful aerial predators, notable for their flight agility and endurance. Further, they are highly capable of low-speed, hovering and even backwards flight. While insects have repeatedly modified or reduced one pair of wings, or mechanically coupled their fore and hind wings, dragonflies and damselflies have maintained their distinctive, independently controllable, four-winged form for over 300Myr. Despite efforts at understanding the implications of flapping flight with two pairs of wings, previous studies have generally painted a rather disappointing picture: interaction between fore and hind wings reduces the lift compared with two pairs of wings operating in isolation. Here, we demonstrate with a mechanical model dragonfly that, despite presenting no advantage in terms of lift, flying with two pairs of wings can be highly effective at improving aerodynamic efficiency. This is achieved by recovering energy from the wake wasted as swirl in a manner analogous to coaxial contra-rotating helicopter rotors. With the appropriate fore-hind wing phasing, aerodynamic power requirements can be reduced up to 22 per cent compared with a single pair of wings, indicating one advantage of four-winged flying that may apply to both dragonflies and, in the future, biomimetic micro air vehicles.

Show MeSH
Related in: MedlinePlus