Honeybees, drone flies, damselflies and fruit flies all increase stroke amplitude to generate larger flight forces. Because force production is proportional to wing velocity squared, insects adjust wing speed by altering the stroke amplitude and/or frequency. To better understand the aerodynamics of backward flight in connection with wing and body kinematics, we studied free flying dragonflies in this flight mode.ДЇirst, to fly, insects need to produce forces by controlling both the velocity of and circulation generated by their wings. Now, engineers are interested in incorporating retro-flight capabilities into state-of-the-art MAVs for additional manoeuvrability. Although just qualitatively characterized in the literature, it has been documented that insects use backward flight for predator evasion, prey capture, flight initiation, station keeping and load lifting. However, some flight modes found in nature which may lead to further insights are yet to be explored.

Examples of such manoeuvres include well-studied modes like hovering, forward and turning flight, which have improved our understanding of flight mechanics and for engineers especially, fostered the design of micro-aerial vehicles (MAVs). 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. Insects elicit flight manoeuvres by drastically or subtly changing their wing and body kinematics. Vorticity from the forewingsвЂ™ trailing edge fed directly into the HW LEV to increase its circulation and enhance force production. Finally, wingвЂ“wing interaction was found to enhance the aerodynamic performance of the hindwings (HW) during backward flight.

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 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. Also, the backward velocity of the body in the upright position enhances the wings' net velocity in the US. 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. The upright body posture was used to reorient the stroke plane and the flight force in the global frame a mechanism known as вЂforce vectoringвЂ™ which was previously observed in manoeuvres of other flying animals. During backward flight, the dragonfly maintained an upright body posture of approximately 90В° relative to the horizon. The wing and body kinematics were reconstructed from the output of three high-speed cameras using a template-based subdivision surface reconstruction method, and numerical simulations using an immersed boundary flow solver were conducted to compute the forces and visualize the flow features. In this study, we investigated the backward free flight of a dragonfly, accelerating in a flight path inclined to the horizontal.

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