Kestrel-Inspired Morphing and Actuation: Bird Flight Insights and Technological Applications

The atmospheric boundary layer (ABL) can be a challenging environment to fly in, mainly due to its inherent turbulence. Small Uncrewed Aerial Vehicles (sUAVs) usually fly within the ABL and share this environment with flying animals such as birds, bats and insects. The performance of sUAVs, when any appreciable wind is present, is challenged by the dynamic inputs arising from the turbulence in the ABL. In contrast birds have evolved through millions of years and have techniques that enable them to cope far better than sUAVs in unsteady flow conditions. The difference in abilities between these two contrasting systems opens an opportunity to use avian strategies to improve the performance of sUAVs which is the focus of this research. Whilst bird flight can be studied through the observation and measurement of their biomechanics in outdoor flight it is more convenient to perform tests in a controlled environment such as a wind tunnel. The foundations of this research rely on motion-tracking flight data from two Nankeen kestrels (Falco cenchroides) collected during numerous wind-tunnel flight experiments when performing non-flapping wind-hovering flights. A wind-hover is a “hanging” flight where the bird remains essentially stationary (despite the turbulent wind inputs) usually scanning the ground below for prey. This is commonly performed by this bird species and requires a high level of agility. This work presents an investigation into the kinematics and morphing capabilities of kestrel wind-hovering flight, utilising the data set from the wind-tunnel study. Analysis revealed the importance of wrist extension (sweep) and other coupled kinematics for control and stability. Couplings between wings and tail were frequently observed, suggesting that kestrels balance aerodynamic forces between wing and tail motions. Behavioural differences were noted between the two birds highlighted the versatility of these birds in achieving stable flight through different morphing techniques. To further understand the kinematics, including couplings, a high-fidelity robotic kestrel was designed which closely mimicked the live birds' morphing motions, validated against real kestrel data. Wind-tunnel force and moment data provided insights into the control benefits of different wing and tail morphing degrees of freedom (DoF) for stability tailoring and control redundancy. Area-changing DoF, uncommon in aircraft design, showed potential for optimizing performance in different flight stages and enabling gust mitigation. In order to further understand why birds have greater levels of agility than equivalent-sized sUAVs measurements were made of the moments of inertia (MOIs) of a kestrel and compared with those of typical sUAVs. This knowledge, combined with a knowledge of the sUAVs control authorities (from control surface deflections) and similar avian control authorities provided insights into birds’ agility. Whilst sUAVs typically were found to exhibit higher control power than kestrels, the significantly lower moment of inertia of kestrel wings was found to be the main reason for birds’ greater agility. Finally, the research explored the feasibility of implementing bird-inspired morphing into sUAV platforms, including the effectiveness of symmetrical and asymmetrical wing sweep for pitch and roll control and stability tailoring. Overall, this research opens avenues for future morphing sUAV designs that leverage the agility and adaptability of birds, with potential applications for flight in complex urban environments and turbulent flow conditions. By implementing morphing, future aerial systems could achieve higher manoeuvrability and flight robustness, contributing to advances in surveillance, environmental monitoring, and search and rescue operations.<p></p>