Robotic Avian Wing Explains Aerodynamic Advantages of Wing Folding and Stroke Tilting in Flapping Flight

Abstract

Avian flapping strategies have the potential to revolutionize future drones as they may considerably improve agility, increase slow speed flight capability, and extend the aerodynamic performance. The study of live birds is time‐consuming, laborious, and, more importantly, limited to the flapping motion adopted by the animal. The latter makes systematic studies of alternative flapping strategies impossible, limiting our ability to test why birds select specific kinematics among infinite alternatives. Herein, a biohybrid robotic wing is described, partly built from real feathers, with more advanced kinematic capabilities than previous robotic wings and similar to those of a real bird. In a first case study, the robotic wing is used to systematically study the aerodynamic consequences of different upstroke kinematic strategies at different flight speeds and stroke plane angles. The results indicate that wing folding during upstroke not only favors thrust production, as expected, but also reduces force‐specific aerodynamic power, indicating a strong selection pressure on protobirds to evolve upstroke wing folding. It is also shown that thrust requirements likely dictate the wing's stroke tilting. Overall, the proposed biohybrid robotic flapper can be used to answer many open questions about avian flapping flights that are impossible to address by observing free‐flying birds.

Shoulder viscoelasticity in a raptor-inspired model alleviates instability and enhances passive gust rejection

“…The fact that the force and moment analysis from section 3.1 identifies a biologically-plausible equilibrium with such a limited tail area and tail moment arm is compelling, as many ornithopter designs require tail surface areas and moment arms in excess of what is often seen in birds and use longitudinal dihedral for pitch stability [64,65]. In addition, even ground-breaking bio-inspired designs with complex tail and wing morphing utilize a negative tail incident angle to achieve equilibrium, which is common in aircraft but not often seen in birds [66,67]. The biologically-plausible results may be largely permitted by the decoupled body and wing rotations.…”

Feather aerodynamics suggest importance of lift and flow predictability over drag minimization

“…( 1) When categorizing bird aerodynamics, the size reference used are either focused on the entire bird (KleinHeerenbrink et al, 2016;Pennycuick, 2008), or even more often the wings (Ajanic et al, 2023;Lentink and Kat, 2014) resulting in a Re of <100000-500000 (Norberg, 1990;Pennycuick, 2008). For the primary feathers that function as individual wing, Re is much lower since the chord is smaller, and for the feather of interest here, in the order of 7000.…”

Inviscid modeling of unsteady morphing airfoils using a discrete-vortex method

“…The normal force on the airfoil is obtained by integrating Eq. ( 21) over the chord line, and 1 2 ρU 2 re f c is used to normalize it. The different contributions to the normal force coefficient are broken down below:…”

“…Finally, the pitching moment coefficient (nose-up positive) is obtained by integrating the product of pressure force and position vector (distance from the pivot axis to the point where the force is applied) over the chord line. Using 1 2 ρU 2 re f c 2 to normalize it:…”

“…Drawing inspiration from diverse biological morphing strategies, 18 research on bio-inspired locomotion aims to revolutionize the future of micro aerial vehicles (MAVs) and autonomous underwater vehicles (AUVs). Ornithopters, which mimic the morphing and flapping capabilities observed in natural fliers, 1,3,10 and fish-like robots that reproduce the undulatory body motion of natural swimmers, 39 are expected to enhance the performance envelope of current technologies, improving maneuverability and versatility. However, to explore in depth the flow control capability of morphing techniques over fast-changing flows, in order to beneficially alter the unsteady vorticity field, tools are desired which provide rapid reliable predictions of flow behavior on diverse scenarios.…”