Natural fliers utilize passive and active flight control strategies to cope with windy conditions. This capability makes them incredibly agile and resistant to wind gusts. Here, we study how insects achieve this, by combining Computational Fluid Dynamics (CFD) analyses of flying fruit flies with freely-flying robotic experiments. The CFD analysis shows that flying flies are partly passively stable in side-wind conditions due to their dorsal-ventral wing-beat asymmetry defined as wing-stroke dihedral. Our robotic experiments confirm that this mechanism also stabilizes free-moving flapping robots with similar asymmetric dihedral wing-beats. This shows that both animals and robots with asymmetric wing-beats are dynamically stable in sideways wind gusts. Based on these results, we developed an improved model for the aerodynamic yaw and roll torques caused by the coupling between lateral motion and the stroke dihedral. The yaw coupling passively steers an asymmetric flapping flyer into the direction of a sideways wind gust; in contrast, roll torques are only stabilizing at high air gust velocities, due to non-linear coupling effects. The combined CFD simulations, robot experiments, and stability modeling help explain why the majority of flying insects exhibit wing-beats with positive stroke dihedral and can be used to develop more stable and robust flapping-wing Micro-Air-Vehicles.@en

To generate aerodynamic forces required for flight, two-winged insects (Diptera) move their wings back and forth at high wing-beat frequencies. This results in exceptionally high wing-stroke accelerations, and consequently relatively high acceleration-dependent fluid forces. Quasi-steady fluid force models have reasonable success in relating the generated aerodynamic forces to the instantaneous wing motion kinematics. However, existing approaches model the stroke-rate and stroke-acceleration effects independently from each other, which might be too simplified for capturing the complex unsteady aerodynamics of accelerating wings. Here, we use computational-fluid-dynamics simulations to systematically explore how aerodynamic forces and flow dynamics depend on wing-stroke rate, wing-stroke acceleration and wing-planform geometry. Based on this, we developed and calibrated a novel unsteady aerodynamic force model for insect wings with stroke accelerations. This includes improved versions of the translational-force model and the added-mass force model, and we identify a third novel component generated by the interaction of the two. This term reflects the delay in bound-circulation build-up as the wing accelerates. The physical interpretation of this effect is analogous to the Wagner effect experienced by a wing starting from rest. Here, we show that this effect can be modelled in the context of flapping wings as a stroke-acceleration-dependent correction on the translational-force model. Our revised added-mass model includes a viscous force component, which is relatively small but not negligible. We subsequently applied our new model to realistic wing-beat kinematics of hovering Dipteran insects, in a quasi-steady approach. This revealed that stroke-acceleration-related aerodynamic forces contribute substantially to lift and drag production, particularly for high-frequency flapping mosquito wings.

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When approaching a landing surface, many flying animals use visual feedback to control their landing. Here, we studied how foraging bumblebees (Bombus terrestris) use radial optic expansion cues to control in-flight decelerations during landing. By analyzing the flight dynamics of 4,672 landing maneuvers, we showed that landing bumblebees exhibit a series of deceleration bouts, unlike landing honeybees that continuously decelerate. During each bout, the bumblebee keeps its relative rate of optical expansion constant, and from one bout to the next, the bumblebee tends to shift to a higher, constant relative rate of expansion. This modular landing strategy is relatively fast compared to the strategy described for honeybees and results in approach dynamics that is strikingly similar to that of pigeons and hummingbirds. The here discovered modular landing strategy of bumblebees helps explaining why these important pollinators in nature and horticulture can forage effectively in challenging conditions; moreover, it has potential for bio-inspired landing strategies in flying robots.

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The evasive banked turn of a fly is among the most rapid flight maneuvers in nature, which it executes using small adjustments in its wingbeat pattern. It is suggested that, after open-loop turn initiation, flies control the bank dynamics using a PI controller based on sensory input from halteres; the yaw rotations are suggested not to be controlled throughout the maneuver, resulting in large sideslip at the turn's end. We tested these notions, by replaying banked turns of fruit flies on a newly-developed bio-inspired flying robot. Like insects, the robot steers by adjusting the motion of its flapping wings, and autonomous flight is achieved using on-board auto-pilot and sensors, including a haltere-like gyroscope. The robot's banked turns, controlled using a gyro-based PI-like controller, resembled those of fruit flies remarkably well, suggesting that fruit flies use a comparable controller based on haltere input. Yaw dynamics was also similar between the fruit flies and robot, whereby both rotated into the turn. This yaw movement reduced sideslip and might thus increase escape performance. Because the robot's yaw control was turned off, the yaw movement must have been produced passively. Using an aerodynamic model of flapping flight, we showed that a translation-induced coupled yaw torque caused this yaw movement. Because many flying animals tend to produce banked turns using flapping wings, the use of this mechanism might be more common in nature. @en

Insects are among the most agile natural flyers.Hypotheses on their flight control cannot always be validated by experiments with animals or tethered robots.To this end, we developed a programmable and agile autonomous free-flying robot controlled through bio-inspired motion changes of its flapping wings.Despite being 55 times the size of a fruit fly,the robot can accurately mimic the rapid escape maneuvers of flies,including a correcting yaw rotation toward the escape heading.Because the robot's yaw control was turned off,we showed that these yaw rotations result from passive,translation-induced aerodynamic coupling between the yaw torque and the roll and pitch torques produced throughout the maneuver.The robot enables new methods for studying animal flight,and its flight characteristics allow for real-world flight missions.2017

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