Kestrel-Inspired Morphing and Actuation

: Bird Flight Insights and Technological Applications

Abstract

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 equivalentsized 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.

Date of Award	13 May 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Shane P Windsor (Supervisor), Simon Watkins (Supervisor) & Abdulghani Mohamed (Supervisor)