Passive aeroelastic deflection of avian primary feathers

Abstract

Bird feathers are complex structures that passively deflect as they interact with air to produce aerodynamic force. Newtonian theory suggests that feathers should be stiff to effectively utilize this force. Observations of flying birds indicate that feathers respond to aerodynamic loading via spanwise bending, twisting, and sweeping. These deflections are hypothesized to optimize flight performance, but this has not yet been tested. We measured deflection of isolated feathers in a wind tunnel to explore how flexibility altered aerodynamic forces in emulated gliding flight. Using primary feathers from seven raptors and a rigid airfoil, we quantified bending, sweep, and twisting, as well as α (attack angle) and slip angle. We predicted that (1) feathers would deflect under aerodynamic load, (2) bending would result in lateral redirection of force, (3) twisting would alter spanwise α 'washout' and delay the onset of stall, and (4) flexural stiffness of feathers would exhibit positive allometry. The first three predictions were supported by our results, but not the fourth. We found that bending resulted in the redirection of lateral forces more toward the base of the feather on the order of ∼10% of total lift. In comparison to the airfoil which stalled at α = 13.5 • , all feathers continued to increase lift production with increasing angle of attack to the limit of our range of measurements (α = 27.5 • ). We observed that feather stiffness exhibited positive allometry (∝ mass 1.1±0.3 ), however this finding is not statistically different from other hypothesized scaling relationships such as geometric similarity (∝ mass 1.67 ). These results demonstrate that feather flexibility may provide passive roll stability and delay stall by twisting to reduce local α at the feather tip. Our findings are the first to measure forces due to feather deflection under aerodynamic loading and can inform future models of avian flight as well as biomimetic morphing-wing technology.

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

“…Left uncorrected, this would result in excessively high angles of attack and unfavourable flow conditions. Since the FM torque is always directed pitch down, it suggests the torque counteracts the built-in pitch-up twist in the feather, as also suggested in (Norberg, 1985;Oorschot et al, 2020). Furthermore, the change in slope of the torque curve will affect the de-twist rate of the feather, having a higher change with α for higher α and a lower change with α for lower α.…”

“…The vanes are built from the barbs and barbules where the barbules are arranged in a hook and groove system which connects adjacent barbs (Figure 2) and creates a cohesive aerodynamic surface (Ennos et al, 1995; Feo et al, 2015; Lucas and Stettenheim, 1972; Sullivan et al, 2019). The shaft then transfers the forces from the barbs to the wing and body, while the feather bend, twist and sweep in a complex manner (Oorschot et al, 2020). In addition to generating aerodynamic forces these feather structures must be strong enough to transfer the forces to the body without breaking or buckling, e.g.…”

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

“…Furthermore, the choice of using real feathers, instead of our previously developed artificial ones, [ 58,59 ] led to a further similarity between the synthetic and natural wings: real feathers deform aeroelasically in a way expected to control and improve the flow over the wing. [ 60,61 ] The multiple cores in the downstroke wake tip vortex of our flapper (Figure 3C,D), caused by the slotted wingtip, are testament to such a phenomenon. [ 62,63 ] While we here mimicked the jackdaw wing, alternative wing designs can easily be mounted due to the modular design of our drive mechanism, providing a flexible platform for future comparative studies of wing morphology.…”

Check-Valve Design in Enhancing Aerodynamic Performance of Flapping Wings

“…There are several ways for lift improvement; for instance, by increasing the wing foil thickness and the camber [8], by adding wing corrugation [9], by enlarging the stroke angle [10], using wing rotation mechanisms [11], using morphing wings [12][13][14], by changing the wing rotation mechanisms [11], using morphing wings [12][13][14], by changing the wing materials [15][16][17], or by adjusting the wing stiffness along the chord-wise/span-wise direction [18]. Not many studies have been performed for the enhancement of lift through mimicking feathers [19][20][21][22][23][24][25] in the upstroke. The adaptive flight control of Caltech's Microbat by using check valves [23], the Festo's BionicSwift using of aerodynamic feathers similar to overlapping shingles on the roof [26,27], and the ventilated design [22] are the prominent research works in lift enhancement.…”