Stability and control benefits of coupled wing and tail morphing in kestrel wind-hovering flight explored using a robot replica

Birds control flight differently to aircraft, morphing their wings and tail to modify forces rather than relying on hinged control surfaces. Some species, such as kestrels, are more manoeuvrable than similar-sized fixed-wing uncrewed air vehicles and can fly in more turbulent wind conditions. We propose that birds achieve this advantage partly through morphing and explore specific benefits that morphing might provide versus conventional control surfaces. Focusing on kestrel flight, we examined the aerodynamic effects of morphing using a high-fidelity robot replicating the predominant motions of wind-hovering kestrels. Wind-tunnel testing assessed the impacts of wing extension, tail spread and tail incidence on force production and stability. The findings illustrate flexibility in control inputs combinations for achieving required flight forces and the ability to trim for different levels of stability through area-changing degrees of freedom. Coupled wing and tail extension improved lift while decoupling lift and pitching moment modulation, reflecting strategies used in real kestrels. Surprisingly, wing and tail morphing offered no greater control authority than conventional control surfaces, but when combined with low wing inertia, yielded superior manoeuvrability. These findings highlight the potential of combining avian-inspired morphing with low-inertia wing designs to enhance the manoeuvrability and performance of small aircraft.