A new study from researchers at the University of Oxford and the University of California, Davis, shows how a Harris's hawk adjusts its wing and tail configuration as it glides through a narrow opening, shifting its aerodynamic stability in the process. The work, published March 4 in Journal of the Royal Society Interface, offers fresh insight into how birds manage the tradeoff between stability and maneuverability and suggests principles that could be applied to future UAV control systems.
Lead author Kiran Weston and Professor Graham Taylor of the Oxford Flight Group teamed with doctoral student Huanglun (Adam) Zhu and assistant professor Christina Harvey in the UC Davis Department of Mechanical and Aerospace Engineering. The team used motion capture imaging in Oxford's flight hall to record a Harris's hawk gliding from one perch to another, then introduced a pair of soft poles into the flight path to create a narrow gap. The obstacle encouraged the bird to briefly tuck its wings as it passed through, providing a natural test of how its aerodynamics change during the maneuver.
After capturing the bird's motion, the researchers reconstructed the hawk's wing and tail shapes at several key points in the flight. They then created physical models of these configurations using resin-based 3D printing, supported by the UC Davis Engineering Student Design Center. The models were tested in the wind tunnel at the UC Davis College of Engineering to measure how each shape affected lift, drag and stability.
The experiments revealed that when the hawk tucks its wings to slip through the gap, its flight regime shifts from aerodynamically unstable to stable. In aeronautics, an unstable configuration can respond rapidly to control inputs and is highly maneuverable, similar to a jet fighter aircraft. A stable configuration, by contrast, resists perturbations and helps maintain a steady course with less active control.
This ability to switch between stability states suggests that birds manage their flight in a fundamentally different way from most human-built aircraft, which are typically designed to operate either stably or, with the aid of active control systems, in a controlled-unstable mode, but not to toggle between the two during a single maneuver. By morphing their wings and tail, birds like Harris's hawks can fine-tune both their aerodynamic forces and their inherent stability to match the demands of the environment.
The findings point toward new approaches for drone design, in which small UAVs might change their shape or control surfaces to rapidly adapt between agile obstacle avoidance and efficient, steady cruising. Understanding how living flyers coordinate body morphing with changes in stability could help engineers design more capable autonomous aircraft for use in cluttered spaces such as forests, urban canyons or indoor environments.
UC Davis has recently completed its own dedicated flight research facility, the Center for Animal Flight and Innovation, which includes a flight hall equipped with motion capture and high-speed video systems. The new center will expand studies of bird flight and related aerodynamics, with a view to transferring biological insights into engineered systems, including next-generation drone technology. Construction and equipment for the facility were funded by the U.S. Army Combat Capability Development Command Army Research Laboratory.
Research Report:Stability shifts in gliding flight: hawks morph from an unstable to stable state when navigating a gap
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