(Online version in colour. Grey shading denotes FW DS. III. Consistent with the phase difference between the wing pairs, the peak forces produced by the HW led the FW. In the polar plot, black vectors clustered around 90° indicate the body longitudinal axis. Table 1.Morphological parameters for the dragonfly in this study. Taking into account the body motion, we found that αgeom was significantly reduced. The peak vertical and horizontal forces during the flight are about 9 and 5.5 times the body weight, respectively. Hence, the LEV circulation should be much smaller than that measured in the DS. (a) FW DS t/T = 0.35, (b) FW US t/T = 0.82, (c) HW DS t/T = 0.25, (d) HW US t/T = 0.70. The flow features visualized by the λ2-criterion during the second flapping stroke. We report the AoAs at four spanwise locations approximately 0.25, 0.5, 0.75 and 0.9R, where R is the distance from the wing root to tip (figure 4). I went out to go see them and when I looked up there were six large mature dragonflies flying over the house right where yogi my dog was lying at that time. This paper focuses on the effects of structure, mechanical properties, and morphology of dragonfly wings on their flyability, followed by the implications in fabrication and modeling. We captured dragonflies (Erythemis simplicicollis) from the wild and transported them to the laboratory for motion capture. Insects are the only group of invertebrates that have evolved wings and flight. Copyright © 2011 Académie des sciences. Morphological parameters for the dragonfly in this study. Validations of the flow solver are in the works of Wan et al. Dragonfly's, due to their inherent speed do not have an apparent self defense mechanism, their main predators are far too large to defend against (birds, frogs, etc.) The forces and muscle-mass-specific power consumption are displayed in figure 5. (a) Reconstructed dragonfly (ii) overlapped on a real image (i). TEV, trailing edge vortex; TV, tip vortex. We came back out a little later and a black and white dragonfly showed up and was flying around us. A.T.B.-O. The geometric AoA (αgeom) excludes the body velocity. Flow features at maximum force production during second stroke for each wing pair. TEV, trailing edge vortex; TV, tip vortex. These changes influence both (i) the production and (ii) orientation and reorientation of aerodynamic forces, consequently determining the type of free flight manoeuvre that is performed. )Download figureOpen in new tabDownload powerPoint, Figure 4. The current research is aimed towards the development of dragonfly inspired nanocomposite flapping wing for micro air vehicles (MAVs). We also tracked the velocity of the leading edge at the spanwise locations where we calculated the angles of attack (see electronic supplementary material). We observed some interaction between the wings during backward flight (figure 7d). The reason for LEV absence during the US was attributed to very low angles of attack as the wing slices through the air, hence, no flow separation. At these intermediate angles of attack, insect wings usually carry a stable LEV [1,51]. Also, the backward velocity of the body in the upright position enhances the wings' net velocity in the US. Grey shading denotes the FW DS. (a) FW DS t/T = 0.35, (b) FW US t/T = 0.82, (c) HW DS t/T = 0.25, (d) HW US t/T = 0.70. A least-squares reference plane (LSRP) is generated based on the nodes on the reconstructed wing surface to quantify wing twist (see ). Our χ corroborated previous observation in dragonfly backward flight (100°) . The mean stroke plane angle relative to the horizon (βh) is 46.8 ± 5.5° for the FW and hindwings (HW). When a wing flaps at a high AoA, the flow separates at the leading edge and reattaches before the trailing edge, forming a vortex which stays stably attached to wing due to the balance of centripetal and Coriolis accelerations . A–D represent snapshots where WWI occurred as labelled in figure 12. The apparatus includes a fuselage; at least one pair of blade-wings; and an actuator for actuating the blade-wings by flapping the blade-wings in dissonance or resonance frequencies. Figure 6. The wing is designed by taking inspiration from the hind wing of dragonfly (Anax Parthenope Julius).Carbon nanotubes (CNTs)/polypropylene nanocomposite and low-density polyethylene are used as the wing materials. (b) Spanwise vorticity on FW during the (i) DS (dorsal surface shaded in grey) and (ii) US in the third stroke (ventral surface shaded in blue). ), We plotted the iso-surface of the λ2-criterion at two different values (|λ2| = 10, 15) to visualize the flow structures (see electronic supplementary material for CFD simulation video). In addition to the rigid wing kinematics, the wing twist is reported in figure 4. The prototype of the mechanism, built at a scale of four times the size of a dragonfly having a wingspan of 150 mm, is able to create motions in the wing of flapping and feathering, and can vary the stroke plane. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. Similarly, a tilt of the stroke plane has been reported to precede changes in the flight direction of insects . )Download figureOpen in new tabDownload powerPointFigure 11. The average body angle during the entire flight duration was approximately 90°. In these flight modes, the DS is conventionally regarded as vertical force producing and the US, thrust (horizontal force) producing [11,31,50]. Unlike most other insects, such as flies, wasps, and cicadas, that have either reduced hindwings or functionally combined forewings and hindwings as a single pair, dragonflies have maintained two pairs of wings throughout their evolution . Corresponding to these large forces was the presence of a strong leading edge vortex (LEV) at the onset of US which remained attached up until wing reversal. The magnitudes of peak vertical force generated by the FW (all USs) and HW (first DS) are similar (approx. The twist angle is the relative angle of the deformed wing chord line and the LSRP. The bottom row (d–f) represents snapshots during HW US at t/T = 0.52, 0.70 and 0.87, respectively. ), it is known that a wing with an LEV imparts greater momentum to the fluid, leading to the production of larger forces than under steady-state conditions [26–29]. The solid lines and dashed lines indicate the ALL case and where the wings are isolated, respectively. However, χ was significantly larger than those of hummingbirds (50–75°) which use a horizontal stroke plane and waterlily beetles (50–70°), which use an inclined stroke plane [13,38]. Structural Analysis of a Dragonfly Wing S.R. (Online version in colour.). (Online version in colour.). The insect left the platform smoothly while increasingly leaning backward. The LEV was also present in both half strokes with the US LEV being stronger. Relative to the large number of works on its flight aerodynamics, few researchers have focused on the insect wing structure and its mechanical properties. Male-specific color change of dragonflies has been considered as an ecologically important trait for reproductive success. WWI. )Download figureOpen in new tabDownload powerPoint, Figure 6. The peak circulation (figure 9c) occurs in the same region where maximum force is generated for each wing pair (figure 5). Vortex development in backward flight. )Download figureOpen in new tabDownload powerPoint, Figure 3. Dragonfly wings possess great stability and high load-bearing capacity during flapping flight, glide, and hover. The phasing of the FW and HW may help reduce oscillations in the body posture during flight . The insects initiated flight voluntarily, and their motion was recorded by three orthogonally arranged high-speed cameras. The US circulation, shown in dashed lines, is higher than the DS circulation, consistent with greater flight force generated in the US. This table reports the contribution of each half stroke to the total aerodynamic force during a flapping cycle in different flight modes of insects. The steep body angle is in contrast with forward and hovering flight during which the dragonfly keeps its body slightly inclined from the horizontal (approx. The deformed wing is shown in dark grey, and the least deformed wing is shown in light grey with a red outline. Although a steep body posture during backward flight has been thought to generate higher drag due to a higher projected area, Sapir & Dudley  showed that drag forces only differed by 3.6% between backward and forward flight in hummingbirds. Figure 12. They can hover, cruise up to 54km/h, turn 180° in three wing beats, fly sideways, glide, and even fly backwards (Alexander, 1984; Appleton, 1974; Whitehouse, 1941). For display, the meshes coarsened four times. The aerodynamic power is defined as , where is the stress tensor, the velocity of the fluid adjacent to the wing surface, and ds are the unit normal direction and the area of each element, respectively. Ornithopter with two sets of flapping wings based on a Dragonfly, developed by Erich von Holst (1943). The region of interaction is shown in dashed lines with an arrow indicating the direction of vorticity transfer (a (i)). The domain size was totalling 14 million grids. The research objectives are then presented along with the research contributions. We solved the incompressible Navier–Stokes equation (equation (2.1)) using a finite difference method with second-order accuracy in space and a second-order fractional step method for time stepping. The tail angle is the angle between the thorax and the tail. This influx is absent in the HW only case, leading to the formation of a weaker LEV and consequently, a weaker jet below the wing (figure 11b). During the DS, an LEV and TV are observed, and the vorticity in the LEV feeds into a tip vortex (TV). By rotating the body relative to the ground, the insect changes the global orientation of the aerodynamic force to perform the desired manoeuvre. The twist was as much as 40°, twice higher than previous measurements on dragonflies . A more detailed study of the 3D reconstruction method is identified elsewhere . The mechanism of WWI which led to increased force production during the second stroke is shown in figures 10 and 11. Flying in reverse: kinematics and aerodynamics of a dragonfly in backward free flight. Compared to hovering , βh in backward flight was about 15° less. In addition to force vectoring, we found that while flying backward, the dragonfly flaps its wings with larger angles of attack in the upstroke (US) when compared with forward flight. Force generation and muscle-specific power consumption. (Online version in colour. (Online version in colour.). The veins and membranes have a complex design within the wing that give rise to whole-wing characteristics which result in dragonflies being supremely versatile, maneuverable fliers. Kinematic parameters of several organisms in flight. From their smoke visualization and analysis, there was no hint of an LEV to enhance lift in the US. Although the magnitude of both US and DS forces change from cycle to cycle, and were produced in a somewhat uniform direction with respect to the longitudinal axis of the body. Because force production is proportional to wing velocity squared, insects adjust wing speed by altering the stroke amplitude and/or frequency [5,11,17]. Figure 1.  noted that the US TV was relatively weak in comparison to the DS's. Hence, unsteady straining and viscous effect need to be eliminated to identify a vortex core properly. Dragonfly, any of a group of roughly 3,000 species of aerial predatory insects most commonly found near freshwater throughout most of the world. αeff and αgeom are the effective and geometric angles of attack. (Online version in colour. A state-of-the-art MAV, the Delfly-II, has also been shown to induce backward flight by increasing its body angle to about 100° from its stable flight configuration . (b) Experimental set-up. (Online version in colour. However, in classical aerodynamics (extended lifting line theory), the three-quarter chord (both for steady and unsteady flow) is the point of choice for calculating the AoA with respect to induced velocities for a wing in curved flow (Pistolesi's theorem) [42,43]. All authors contributed to the final paper. The mass and length measurement uncertainties are ±1 mg and ±1 mm, respectively. The dragonfly's fore and hindwings typically counterstroke, or beat out of phase. This figure shows the mechanism of vorticity transfer from the fore to HW during backward flight. L, body length; R, wing length from root to tip, , mean chord length. Comparing the CD measured from our simulation (Reynolds number based on body length, Reb ∼ 3860) with results for forward flight of dragonflies of similar Reb approximately 2460–7790 in the literature, the results were comparable indicating that an upright body posture did not substantially influence body drag production. The presence of the leading edge vortex (LEV) in insect flight has been associated with enhanced forces on the wing [10,23]. The geometric (dashed lines) and effective angles of attack (solid lines) and twist angles at four spanwise location are reported. Because the dragonfly is accelerating, the advance ratio changes on a half stroke basis and is larger in the second and third flapping strokes. The blood circulation is essential for the maintenance of reasonable water content in wings. In figure 10, the vortical structures are projected on a 2D slice cut at mid-span, similar to figure 9a. (Online version in colour.). Wing kinematics and twist. In the text, the mid-span (0.5R) AoA is reported. (Online version in colour. Figure 5. The vortex structures are visualized by the λ2-criterion , which has been used in previous insect flight studies [44,48]. Owing to their relatively low flapping frequency, the magnitude of body velocity of a dragonfly is comparable to its wing velocity. Table 2.Forces from three different grids set-up. For thrust production, the interaction was detrimental for the FW leading to a 17.5% decrease in force while benefiting the HW by as much as 13.2%. The circulation increases along the span and tapers towards the tip. (a) βh and βb are the stroke plane angles with respect to the horizontal and body longitudinal axis, respectively. Grey shading denotes the FW DS. However, the change in magnitude of the force, as well as production of large aerodynamic forces in US, cannot be explained by force vectoring alone. A classic example is backward flight. The FW could also benefit from interaction due to the distortion of the FW wakes by the HW via the ‘wall effect’ [20,58,59]. A helicopter rotates the force vector by inducing a nose-down motion on the fuselage and tilting the tip-path plane (of the blades) forward to induce forward flight. 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