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Gliding birds lack a vertical tail, yet they fly stably rudderless in turbulence without needing discrete flaps to steer. In contrast, nearly all airplanes need vertical tails to damp Dutch roll oscillations and to control yaw. The few exceptions that lack a vertical tail either leverage differential drag-based yaw actuators or their fixed planforms are carefully tuned for passively stable Dutch roll and proverse yaw. Biologists hypothesize that birds stabilize and control gliding flight without rudders by using their wing and tail reflexes, but no rudderless airplane has a morphing wing or tail that can change shape like a bird. Our rudderless biohybrid robot, PigeonBot II, can damp its Dutch roll instability (caused by lacking a vertical tail) and control flight by morphing its biomimetic wing and tail reflexively like a bird. The bird-inspired adaptive reflexive controller was tuned in a wind tunnel to mitigate turbulent perturbations, which enabled PigeonBot II to fly autonomously in the atmosphere with pigeon-like poses. This work is a mechanistic confirmation of how birds accomplish rudderless flight via reflex functions, and it can inspire rudderless aircraft with reduced radar signature and increased efficacy. Editor's summary: Birds exhibit controlled gliding flight without a vertical tail, unlike the necessary rudders on airplanes that damp Dutch roll oscillations. To accomplish rudderless flight, Chang et al. developed a bioinspired aerial robot with morphing wings and tail named PigeonBot II. The robot consists of a biomimetic skeleton and real pigeon feathers that form wings that can spread and a tail that can spread, elevate, tilt, and deviate side to side like a bird. Initial experiments in a turbulent wind tunnel showed that reflexive tail tilting and deviation combined with wing morphing enabled stable flight by damping Dutch roll. Outdoor flight tests further demonstrated that the autonomous reflexive controller provided stability to the robot during take-off, cruise, and landing. —Melisa Yashinski [ABSTRACT FROM AUTHOR]

Flying animals of different masses vary widely in body proportions, but the functional implications of this variation are often unclear. We address this ambiguity by developing an integrative allometric approach, which we apply here to hummingbirds to examine how the physical environment, wing morphology and stroke kinematics have contributed to the evolution of their highly specialised flight. Surprisingly, hummingbirds maintain constant wing velocity despite an order of magnitude variation in body weight; increased weight is supported solely through disproportionate increases in wing area. Conversely, wing velocity increases with body weight within species, compensating for lower relative wing area in larger individuals. By comparing inter- and intraspecific allometries, we find that the extreme wing area allometry of hummingbirds is likely an adaptation to maintain constant burst flight capacity and induced power requirements with increasing weight. Selection for relatively large wings simultaneously maximises aerial performance and minimises flight costs, which are essential elements of humming bird life history.

Hovering flies generate exceptionally high lift, because their wings generate a stable leading edge vortex. Micro flying robots with a similar wing design can generate similar high lift by either flapping or spinning their wings. While it requires less power to spin a wing, the overall efficiency depends also on the actuator system driving the wing. Here, we present the first holistic analysis to calculate how long a fly-inspired micro robot can hover with flapping versus spinning wings across scales. We integrate aerodynamic data with data-driven scaling laws for actuator, electronics and mechanism performance from fruit fly to hummingbird scales. Our analysis finds that spinning wings driven by rotary actuators are superior for robots with wingspans similar to hummingbirds, yet flapping wings driven by oscillatory actuators are superior at fruit fly scale. This crossover is driven by the reduction in performance of rotary compared with oscillatory actuators at smaller scale. Our calculations emphasize that a systems-level analysis is essential for trading-off flapping versus spinning wings for micro flying robots.

Bird flight is a remarkable adaptation that has allowed the approximately 10 000 extant species to colonize all terrestrial habitats on earth including high elevations, polar regions, distant islands, arid deserts, and many others. Birds exhibit numerous physiological and biomechanical adaptations for flight. Although bird flight is often studied at the level of aerodynamics, morphology, wingbeat kinematics, muscle activity, or sensory guidance independently, in reality these systems are naturally integrated. There has been an abundance of new studies in these mechanistic aspects of avian biology but comparatively less recent work on the physiological ecology of avian flight. Here we review research at the interface of the systems used in flight control and discuss several common themes. Modulation of aerodynamic forces to respond to different challenges is driven by three primary mechanisms: wing velocity about the shoulder, shape within the wing, and angle of attack. For birds that flap, the distinction between velocity and shape modulation synthesizes diverse studies in morphology, wing motion, and motor control. Recently developed tools for studying bird flight are influencing multiple areas of investigation, and in particular the role of sensory systems in flight control. How sensory information is transformed into motor commands in the avian brain remains, however, a largely unexplored frontier. Key words: Aves, comparative biomechanics, neuromuscular control, visual guidance, wing morphing. Le vol des oiseaux est une adaptation remarquable qui a permis aux quelque 10 000 especes actuelles de coloniser tous les habitats terrestres de la planete, dont les regions polaires et de haute altitude, les iles eloignees, les deserts arides et bien d'autres. Les oiseaux presentent de nombreuses adaptations physiologiques et biomecaniques pour le vol. Bien que l'aerodynamique, la morphologie, la cinematique des battements d'ailes, l'activite musculaire et le guidage sensoriel associes au vol d'oiseau soient souvent etudies de maniere independante, en realite, ces systemes sont naturellement integres. S'il y a abondance de nouvelles etudes sur ces aspects mecanistes de la biologie aviaire, moins de travaux recents se sont penches sur l'ecologie physiologique du vol des oiseaux. Nous passons en revue la recherche a l'interface des systemes utilises dans le controle du vol et abordons plusieurs themes communs. La modulation des forces aerodynamiques pour repondre a differents defis est assuree par trois principaux mecanismes, soit la vitesse des ailes autour de l'epaule, la forme a l'interieur de l'aile et l'angle d'attaque. Pour les oiseaux qui battent des ailes, la distinction entre la modulation par la vitesse et par la forme regroupe des etudes variees sur la morphologie, le mouvement des ailes et le controle moteur. Des outils recemment mis au point pour etudier le vol des oiseaux influencent plusieurs champs d'etude, notamment le role des systemes sensoriels dans le controle du vol. La transformation de l'information sensorielle en commandes motrices dans le cerveau des oiseaux demeure toutefois une frontiere largement inexploree. [Traduit par la Redaction] Mots-cles: oiseaux, biomecanique comparative, controle neuromusculaire, guidage visuel, morphing des ailes., Introduction The flying abilities of birds are impressive; casual observation of their soaring, hovering, and manoeuvring can lead one to assume that the graceful and poised motions are almost effortless. [...]