Passive aeroelastic deflection of avian primary feathers*

Bird feathers are complex structures that passively deflect in response 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. We predicted that 1) feathers would deflect under aerodynamic load, 2) bending would result in lateral redirection of force, and 3) twisting would alter spanwise $\alpha $ ``washout'' and delay the onset of stall. We found that bending resulted in the redirection of lateral forces toward the base of the feather on the order of 10 percent of total lift. In comparison to the airfoil which stalled at $\alpha =$13.5 deg, all feathers continued to increase lift production with increasing angle of attack to the limit of our range of measurements ($\alpha =$27.5 deg). These results demonstrate that feather flexibility may provide passive roll stability and delay stall by twisting to reduce local $\alpha $ 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.