We verified this finding by calculating the LEV circulation of the wing and found DS-to-US LEV circulation ratios as low as 0.4 and 0.59 for the FW and HW, respectively. Time history of forces (Fv, vertical force; FH, horizontal force; W, weight = 1.275 mN) and muscle-mass-specific power consumption. Insects elicit flight manoeuvres by drastically or subtly changing their wing and body kinematics. (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). A–D represent snapshots where WWI occurred as labelled in figure 12. (Online version in colour.). The twist angle is the relative angle of the deformed wing chord line and the LSRP.  noted that the US TV was relatively weak in comparison to the DS's. The DS-to-US duration ratio changed on a stroke-by-stroke basis from 0.9 (first stroke) to 0.7 (second stroke) to 1 (third stroke) for the FW and from 0.9 (first stroke) to 0.8 (second and third strokes) for the HW. Copyright © 2011 Académie des sciences. Dragonfly wings are highly corrugated, which increases the stiffness and strength of the wing significantly, and results in a lightweight structure with good aerodynamic performance. The coning angle can be set between tests. The mass and length measurement uncertainties are ±1 mg and ±1 mm, respectively. The sum of the FW and HW forces is shown during the second stroke (Fv, vertical force; FH, horizontal force). In addition, we showed that a strong and stable LEV in the US was responsible for greater force production (figure 9 and table 3). Second, the orientation and reorientation of aerodynamic forces is as essential for successful flight as force production and is vital to positioning the insect in its intended flight direction. This figure shows the mechanism of vorticity transfer from the fore to HW during backward flight. (e) Tail angle definition. drafted the initial manuscript. In the polar plot, black vectors clustered around 90° indicate the body longitudinal axis. The biolog oyf dragonflie has s been closely studie bud t few attempts have been made to analyse their flight mechanics. (Online version in colour. The morphological parameters of the selected dragonfly are shown in table 1, and the flight video can be found in the electronic supplementary material. Taking into account the body motion, we found that αgeom was significantly reduced. The LSRP is a planar fitting to the 3D positions of the wing surface points where the sum of the distances of the wing surface points from this plane is minimized. At every time step, a 2D plane normal to the axis of LEV was constructed (figure 9a). The average Euler angles are shown. A vorticity threshold was set to capture the vortex. In contrast with forward flight, during which dragonflies generates little force in US , the magnitude of the half-stroke-averaged force generated in US during backward flight is two to four times the body weight. The dragonflies are coloured based on FW (blue) and HW (black) timing. We compared three simulation cases: (i) with all four wings (ALL; shown in figures 8 and 9), (ii) the FW only (FO), and (iii) HW only (HO), to elucidate WWI during flight (table 4). In previous works, the LEV circulation was significantly larger in DS compared to US where the LEV may be completely absent [20,66,69–71]. )Download figureOpen in new tabDownload powerPoint, Figure 6. A least-squares reference plane (LSRP) is generated based on the nodes on the reconstructed wing surface to quantify wing twist (see ). All authors interpreted the data. Tables 5 and 6 show a summary of previous research on different flight modes. Figure 10. We observed some interaction between the wings during backward flight (figure 7d). , respectively, for forward flight. Previous studies have indicated that the FW experience in-wash due to the HW and the HW are affected by the downwash from the FW with benefits being dependent on the phase difference between wing pairs [31,54–57]. TEV, trailing edge vortex; TV, tip vortex. Insects also modulate the circulation produced by their wings by controlling the angle of attack (AoA) with wing flexibility and rotation speed playing lesser roles . (a,b) Anecdotally using real footage, how dragonflies may appropriate the force vectoring for forward and backward flight. Alterations in kinematics and aerodynamic features which are different from hovering and forward flight characterize backward flight of dragonflies. Overall, the resultant wing velocities squared were higher during the US than the DS by 20 and 15% for the FW and HW at mid-span. However, in contrast with dragonflies, these insects use a horizontal stroke plane in the flight scenarios listed. (Online version in colour. In these flight modes, the DS is conventionally regarded as vertical force producing and the US, thrust (horizontal force) producing [11,31,50]. To better understand the aerodynamics of backward flight in connection with wing and body kinematics, we studied free flying dragonflies in this flight mode. The domain size was totalling 14 million grids. Solid and dashed arrows show resultant force and its components, respectively. For display, the meshes coarsened four times. Contours represent non-dimensional vorticity. An accurate three-dimensional (3D) surface reconstruction technique coupled with a high-fidelity computational fluid dynamics (CFD) flow solver  is used to quantify the coordination of the wing and body motion and to identify how flight forces are generated during flight. The US is often ‘aerodynamically inactive’ as a result . Electronic supplementary material is available online at https://dx.doi.org/10.6084/m9.figshare.c.4131254. Scientists have been intrigued by them and have carried out research for biomimetic applications. Flow features at maximum force production during second stroke for each wing pair. The geometric (dashed lines) and effective angles of attack (solid lines) and twist angles at four spanwise location are reported. Effect of WWI during flight (all strokes combined). Wang & Sun , using CFD, verified the absence of the LEV in the US in hovering as well as forward flight of dragonflies. Table 4.Effect of WWI during flight (all strokes combined). 4 mN), while the peak vertical force of the HW is about twice FW in the second and third strokes as the insect ascends (see §3.1.1). Figure 6. )Download figureOpen in new tabDownload powerPoint, Figure 3. )Download figureOpen in new tabDownload powerPoint, Figure 5. Lehmann  reported that an HW leading by 90° could achieve the same mean lift as an isolated wing due to wake capture. flying insects. Relative to the large number of works on its flight aerodynamics, few researchers have focused on the insect wing structure and its mechanical properties. Their flight performance far exceeds other insects. The insect left the platform smoothly while increasingly leaning backward. 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 . 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. At the onset of flight, the dragonfly rested on a platform posing at an initial body angle of approximately 87°. As reversal approaches, the LEV deteriorates and sheds from the trailing edge. The average body angle during the entire flight duration was approximately 90°. The effective AoA (αeff) here is the angle between the chord and the vector sum of the body and wing velocity measured at the leading edge. Both wing pairs generate larger forces in US compared to DS.  and Li & Dong . The spanwise distribution of circulation on the wing surface at the instant of maximum force production in the second and third stroke are reported in figure 9d,e. During the DS, horizontal forces for the FW are attenuated by 5.5%. The high body angles (χ) during dragonfly backward flight parallels similar observations of hummingbird  and insect backward flight  and could be a mechanism of convergent evolution . Compared to hovering , βh in backward flight was about 15° less. This βb is slightly less than the stroke plane angle measured in forward flight (relative to the longitudinal axis), which is about 50–60° [37,49]. (Online version in colour. Force vectoring involves redirecting flight forces globally by rotating the body while the force vector remains relatively fixed to the body. (c,d) Measured flight forces. Force vectoring is a mechanism commonly used by insects and birds to change flight direction. The difference is shaded in green. Previous insect flight studies have measured the AoA at locations between the leading edge and quarter-chord or near the rotation axis of the wing [19,41]. An LEV forms as the wings translate during the DS. Forces from three different grids set-up. The TV is also more pronounced and suggests that the strength of the LEV feeding it may be greater than the DS's. Grey shading denotes the FW DS. Contours represent non-dimensional vorticity. Velocities, accelerations and kinematics of flapping flight, Surface tension dominates insect flight on fluid interfaces, Computational investigation of cicada aerodynamics in forward flight, 3D reconstruction and analysis of wing deformation in free-flying dragonflies, Scaling law and enhancement of lift generation of an insect-size hovering flexible wing, State-space representation of the unsteady aerodynamics of flapping flight, Vortex dynamics and new lift enhancement mechanism of wing–body interaction in insect forward flight, A versatile sharp interface immersed boundary method for incompressible flows with complex boundaries, Wing kinematics measurement and aerodynamics of a dragonfly in turning flight, Three-dimensional flow structures and evolution of the leading-edge vortices on a flapping wing, Study of lift enhancing mechanisms via comparison of two distinct flapping patterns in the dragonfly, Dragonfly flight. Thomas et al. At the beginning of the third US, the insect slowed down and reduced its body and tail angle (figure 3e,f). Like helicopters, flying backward in insects may require a similar strategy where the insect will maintain a pitch-up orientation. The flow visualizations corroborated these findings in figures 7 and 8. (Online version in colour. The muscle mass (Mm) is 49% of the body mass based on previous measurements [52,53]. 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 flow features visualized by the λ2-criterion during the second flapping stroke. The dragonfly's fore and hindwings typically counterstroke, or beat out of phase. The deformed wing is shown in dark grey, and the least deformed wing is shown in light grey with a red outline. Grey shading denotes the DS phase. Slices similar to figure 9a,b are shown here to elucidate WWI. The tail angle is the angle between the thorax and the tail. The mechanical properties of dragonfly wings need to be understood in order to perform simulated models. The loop creates a downward jet which boosts vertical force production. Wing kinematics and twist. The structure and mechanical properties of dragonfly wings and their role on flyability. Finally, wing–wing interaction was found to enhance the aerodynamic performance of the hindwings (HW) during backward flight. This video is unavailable. )Download figureOpen in new tabDownload powerPoint, Figure 1. 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. ), Figure 11. Most of the tilt is accomplished through fuselage rotation because the tilt of the tip-path is limited by the range of motion of the swash plates. (a) Schematic of a dragonfly with 2D slices on the wings with the virtual camera looking through a line passing through the LEV core. Contrary to previous works on dragonfly forward flight [1,30,62], the presence of the LEV was not limited to the FW but was evident on the HW as well . The presence of the FW induces an additional inflow into the LEV which is favourable in this case. Figure 4. The body of a dragonfly looks like a helical structure wrapped with metal. During backward flight, the US must become active because of its weight supporting role. (d) Montage of 3D model of dragonfly used in CFD simulation. A–D represent snapshots where the flow field is evaluated in figure 10. We dotted the dragonflies' wings for tracking purposes and placed the insects in a filming area. ϕ, θ and ψ are the flap, deviation and pitch angles.  also arrived at a similar conclusion with smoke visualizations on dragonflies in tethered and free forward light. 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]. (b) Twist angle (θtwist). (b) Twist angle (θtwist). Here, for simplifying the mechanism both opposite halves of a wing are rigidly fixed to a unit. The upright body posture was used to reorient the stroke plane and the flight force in the global frame; a mechanism known as ‘force vectoring’ which was previously observed in manoeuvres of other flying animals. We also tracked the velocity of the leading edge at the spanwise locations where we calculated the angles of attack (see electronic supplementary material). produce larger forces during the DS due to the higher relative wing velocity and the AoA in comparison to the US [31,32]. Our observations corroborate these reports as we consistently witnessed an upright body posture during the backward flight of dragonflies in our experiment. While body drag is present, we measured it to be 11 times smaller than the horizontal forces being generated by the wings during flight. At the onset of interaction, vorticity emanating from the FW's trailing edge feeds into an already stronger LEV on the HW, boosting its circulation (figure 10a(i)). χ is the body angle. Dragonfly is one of the most maneuverable insects and one of the oldest flying species on earth. In the US, the LEV formed covers the entirety of the wing surface (figures 7e,f and 8b,d). The forces and muscle-mass-specific power consumption are displayed in figure 5. (f) Body kinematics. The bottom row (d–f) represents snapshots during HW US at t/T = 0.52, 0.70 and 0.87, respectively. dragonfly has not yet been achieved though only relatively large size flying dragonfly shaped robot OPEN ACCESS.  reported that a stroke plane tilted backward, and a steep body angle between 50° and 70° from the horizontal induced backward flight in waterlily beetles (Galerucella nymphaeae). During backward flight, the dragonfly maintained an upright body posture of approximately 90° relative to the horizon. (Online version in colour.). Table 3.Quantification of LEV circulation. WWI. The combined effect of the angle of attack and wing net velocity yields large aerodynamic force generation in the US, with the average magnitude of the force reaching values as high as two to three times the body weight. 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 8. Thus, the motion of the body can yield significant effects on the net wing velocity. The peak circulation (figure 9c) occurs in the same region where maximum force is generated for each wing pair (figure 5). Dragonfly wings possess great stability and high load-bearing capacity during flapping flight, glide, and hover. Abstract. Solid and dashed arrows show resultant force and its components, respectively. The wings of dragonflies … Figure 9. The Costa (C), the … Zoom In Zoom Out Reset image size Figure 1. (Online version in colour.). Force vectors in mid-sagittal plane. More precisely, we aim to identify the role that force vectoring plays in the execution of a backward flight manoeuvre. Grey shading denotes the DS phase. Male-specific color change of dragonflies has been considered as an ecologically important trait for reproductive success. Body motion during backward flight. Top row (a–c) represents snapshots during HW DS at t/T = 0.07, 0.19 and 0.34, respectively. 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