James Richard Usherwood

Many bird species fly in formation, reducing energy expenditure by exploiting the leader’s wake. Energy savings depend on precise follower positioning, updated from relative position or local flow cues. Prior work indicates that birds are sensitive to pressure-derived cues, which may allow followers to identify energy-efficient locations. We investigated whether a bio-inspired pressure-sensing scheme across the wingspan and nares region could predict aerodynamic efficiency and identify favorable following positions. In a wind-tunnel study of a single-leader-single-follower formation, we equipped a follower model with nares and spanwise pressure taps. Using mutual information theory and machine-learning models, we found that pressure and position inputs improved lift-to-drag prediction compared with position alone, and that the nares-inspired tap pair captured a substantial portion of the useful pressure information. These findings suggest that bio-inspired sensor placement can enhance aerodynamic sensing for improving efficiency and autonomy of formation-flying uncrewed aerial vehicles (UAVs).

The gaits of bipedal and quadrupedal animals are well characterized, while, with few naturally occurring tripedal gaits, little is understood regarding locomotion on three legs. Canine amputees provide a unique perturbation into the study of locomotion. This study assesses whether these individuals use distinct limb sequences, with clearly defined kinematics and kinetics, which could be considered discrete gaits. Data for a group of hind- and forelimb canine amputees across a range of speeds show that at higher speeds, three legged dogs use a three beat gallop-like gait: forelimb amputees use a rotary-like gallop; hindlimb amputees use both rotary- and transverse-like sequences. At low speeds, two strategies are observed: (i) maintenance of a three legged gallop with increased stride periods, and (ii) uncoupled walking, with an increased stride period on the ‘walking’ pair of legs and a constant stride period on the single leg, resulting in decoupling of fore- and hindlimb timings. The single limb is constrained by unsupported aerial time, maintaining a consistent period, striking more than once per cycle at lower speeds. Altered force distributions show high peak vertical forces in the single limb of forelimb amputees, but a relatively even distribution across all limbs in hindlimb amputees.

Tail fins, flukes, flapping wings and propellers are propulsors that generate thrust with foils. Efficient thrust production requires suitable foil design and operation. Simplified propeller theory allows the interactions between foil shape and kinematics to be addressed within the context of mechanical or ‘propeller’ efficiency. Efficient thrust at low forward speeds and/or high motor power is shown to require high foil area and/or lift coefficient. This offers an account for the relatively broad foils of the slow pike and birds requiring take-off at low speeds and high power. The efficiency benefit of high lift coefficients is consistent with the separated emarginate primaries forming multi-slat aerofoil sections observed not only in competent thermal soarers (vultures, storks, etc.) but also many game birds. Efficient thrust at high speeds requires relatively small foil area and/or low lift coefficients, consistent with the simple, high-aspect-ratio foils of tuna and swifts. The spectrum of relatively high-area to low-area foils does not imply a compromise between acceleration and efficiency; both ends of the scale may result in efficient propulsors, with high thrust capacity for given power supply, but with differences in form relating simply to speed.

Tail posture influences lift, drag, trim and stability for birds, yet the interaction between them as the tail spreads and pitches remains unclear, even during steady gliding. In this study, we investigated the aerodynamic consequences of tail morphing, exploring the interactions between weight support, drag, longitudinal trim and stability using data obtained from computational fluid dynamics simulations of high-fidelity, photogrammetry-derived geometry of a free-gliding barn owl. Assuming drag to be minimized over a range of speeds, the tail should be more spread and pitched at low speeds, and less so at high speeds. This influences the proportion of weight supported by the tail; in order to prevent net aerodynamic pitching moment and maintain longitudinal moment equilibrium, the relative position of the centre of gravity must shift. These effects shorten the negative static margin at higher speeds, making the model bird less unstable, limiting the reduction in pitch divergence doubling time that would otherwise have been coupled with the increase in speed. The drag-minimizing model owl is aerodynamically unstable at all speeds, but the feedback and control challenges of maintaining steady glides at high speeds are partially ameliorated and lower than would be predicted without a morphing airframe.

Swimming and flying animals produce thrust with oscillating fins, flukes or wings. The relationship between frequency f , amplitude A and forward velocity U can be described with a Strouhal number St , where St = 2 fA / U , where animals are observed to cruise with S t ≈ 0.2 –0.4. Under these conditions, thrust is produced economically and a reverse von Kármán wake is observed. However, propeller-driven craft produce thrust with steadily revolving blades and a helical wake. Here, the simplified aerodynamic geometry of lift-based thrust production is described, applicable to both oscillating and revolving foils. The same geometric principles apply in both cases: if the foil moves too slowly, it cannot produce thrust; if it moves too fast, it produces thrust with excessive power demand. Effective, economic thrust production by animals is not the result of oscillating foils or cyclic vortex shedding; rather, the selection of amplitude and frequency, and wake vortex structure, are corollaries of driving an efficient foil velocity with finite amplitudes. Observed Strouhal numbers for cruising animals appear too low for optimal mechanical efficiency; however, the deviation from optimal efficiency may be small, and there are physical and physiological advantages to relatively low amplitudes and frequencies for swimming and flapping flight.

Mathematical models based on inviscid flow theory are effective at predicting the aerodynamic forces on large-scale aircraft. Avian flight, however, is characterized by smaller sizes, slower speeds, and increased influence of viscous effects associated with lower Reynolds numbers. Therefore, inviscid mathematical models of avian flight should be used with caution. The assumptions used in such models, such as thin wings and streamlined bodies, may be violated by birds, potentially introducing additional error. To investigate the applicability of the existing models to calculate the aerodynamic performance of bird flight, we compared predictions using simulated wakes with those calculated directly from forces on the bird surface, both derived from computational fluid dynamics of a high-fidelity barn owl geometry in free gliding flight. Two lift models and two drag models are assessed. We show that the generalized Kutta–Joukowski model, corrected by the streamwise velocity, can predict not only the lift but also span loading well. Drag was predicted best by a drag model based on the conservation of fluid momentum in a control volume. Finally, we estimated force production for three raptor species across nine gliding flights by applying the best lift model to wake flow fields measured with particle tracking velocimetry.

The actions of the major human leg muscles are well established; however, the functions of these muscle actions during steady running remain unclear. Here, leg structures and mechanisms are considered in terms of their functions in meeting the task of a vehicle acting as an effective machine, supporting body weight during translation with low mechanical work demand and in supplying mechanical work economically. Legs are modelled as a sequence of linkages that predict muscle actions and reveal the varying muscle functions within the integrated leg. Work avoidance is achieved with isometric muscles and linkages that promote a sliding of the hip over the ground contact, resulting in an approximately horizontal path of the centre of mass. Economical work supply requires, for muscle with constrained power, shortening over the entire stance duration; this function is achieved by the hamstrings without disrupting the linkages resulting in work avoidance. In late stance, the two functions occur through coactivation of antagonistic muscles, providing one answer to Lombard’s paradox. Quadriceps and hamstring tensions result in opposing moments about both hip and knee joints, but by doing so perform the independent yet complementary roles of work avoidance during translating weight support and economical work supply.

The actions of the major human leg muscles are well established; however, the functions of these muscle actions during steady running remain unclear. Here, leg structures and mechanisms are considered in terms of their functions in meeting the task of a vehicle acting as an effective machine – in supporting body weight during translation with low mechanical work demand, and in supplying mechanical work economically. Legs are modelled as a sequence of linkages that predict muscle actions and reveal the varying muscle functions within the integrated leg. Work avoidance is achieved with isometric muscles and linkages that promote a sliding of the hip over the ground contact, resulting in an approximately horizontal path of the centre of mass. Economical work supply requires, for muscle with constrained power, shortening over the entire stance duration; this function is achieved by the hamstrings without disrupting the linkages resulting in work avoidance. In late stance, the two functions occur through coactivation of antagonistic muscles, providing one answer to Lombard’s paradox. Quadriceps and hamstring tensions result in opposing moments about both hip and knee joints, but by doing so perform the independent yet complementary roles of work avoidance during translating weight support and economical work supply.

A bird's wings are articulated to its body via highly mobile shoulder joints. The joints confer an impressive range of motion, enabling the wings to make broad, sweeping movements that can modulate quite dramatically the production of aerodynamic load. This is enormously useful in challenging flight environments, especially the gusty, turbulent layers of the lower atmosphere. In this study, we develop a dynamics model to examine how a bird-scale gliding aircraft can use wing-root hinges (analogous to avian shoulder joints) to reject the initial impact of a strong upward gust. The idea requires that the spanwise centre of pressure and the centre of percussion of the hinged wing start, and stay, in good initial alignment (the centre of percussion here is related to the idea of a ‘sweet spot’ on a bat, as in cricket or baseball). We propose a method for achieving this rejection passively, for which the essential ingredients are (i) appropriate lift and mass distributions; (ii) hinges under constant initial torque; and (iii) a wing whose sections stall softly. When configured correctly, the gusted wings will first pivot on their hinges without disturbing the fuselage of the aircraft, affording time for other corrective actions to engage. We expect this system to enhance the control of aircraft that fly in gusty conditions.

Human walking appears complicated, with many muscles and joints performing rapidly varying roles over the stride. However, the function of walking is simple: to support body weight as it translates economically. Here, a scenario is proposed for the sequence of joint and muscle actions that achieves this function, with the timing of muscle loading and unloading driven by simple changes in geometry over stance. In the scenario, joints of the legs and feet are sequentially locked, resulting in a vaulting stance phase and three or five rapid ‘mini-vaults’ over a series of ‘virtual legs’ during the step-to-step transition. Collision mechanics indicate that the mechanical work demand is minimized if the changes in the centre-of-mass trajectory over the step-to-step transition are evenly spaced, predicting an even spacing of the virtual legs. The scenario provides a simple account for the work-minimizing mechanisms of joints and muscles in walking, and collision geometry allows leg and foot proportions to be predicted, accounting for the location of the knee halfway down the leg, and the relatively stiff, plantigrade, asymmetric, short-toed human foot.