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

Partly overlapping feathers form a large part of birds wing surfaces, but in many species the outermost feathers split, making each feather function as an independent wing. These feathers are complex structures that evolved to fulfil both aerodynamic and structural functions. Yet, relatively little is known about how the profile shape and microstructures of feathers impact aerodynamic performance. Here we determine, using fluid dynamic modelling, the aerodynamic capabilities of a section of the primary flight feather forming the leading edge of the split wing tip of a Jackdaw (Corvus monedula). Our findings demonstrate that the feather section exhibits a relatively high performance, with lift comparable to manmade aerofoils and plates with larger camber at higher Reynolds number. However, there is a drag penalty associated with the feather shaft. The models vortex shedding behaviour results in stable lift, with small fluctuations, compared to manmade aerofoils. Notably, the aerodynamic pitch torque around the shaft varies with angle of attack. This, when combined with the built-in pitch-up twist of the feather implies a passive pitch control mechanism for the feather. Taken together, our findings suggest evolutionary adaptations of the flow around the feather, which could be of interest when designing micro-air vehicles and wind turbines.