Wake analysis of a robotic avian wing with moult gaps in gliding flight

Birds periodically replace their flight feathers through moulting, a process needed to restore aerodynamic function that temporarily reduces the wing surface and flight performance. To mitigate these effects, birds adjust their wings, the angle of attack, and undergo mass loss to compensate for altered aerodynamics.Studying the aerodynamic impact of moult gaps in live birds is challenging, so we used a bio-hybrid robotic wing. The wing, built with jackdaw feathers (Corvus monedula), was tested at 17 angles of attack, from −13 to 17 degrees, flow speed U of 8 ms^−1 and Reynolds number around 53000.This work investigates how moult gaps alter vorticity distribution in the wake, drag D, and lift L, hence aerodynamic efficiency E = L/D in gliding flight. During moult, passerines lose up to 10–12% of wing area. According to the lift equation L=0.5ρSU^2C_L, if the wing area S is reduced and all other parameters, including fluid density ρ, remain equal, there will be a reduction in lift proportional to the loss of wing area, without taking into account the position of the moult gap that affects the shape of the wing and the lift coefficient C_L. Induced drag Dind = kL^2/0.5πb^2, where k is the induced drag factor, increases when moult reduces the wingspan b. Our work supports the idea that moult gaps significantly reduce aerodynamic efficiency, with the lift-to-drag ratio dropping from around 10 before moult, to a minimum near 7. The lowest lift-to-drag ratio occurred when the primaries forming the wing tip were missing (stage 4 and 3), as this both reduce the wingspan and compromise their function as aerodynamic slots. The position of the wing gap is reflected in the wake. Vorticity of opposite sign to that of the wing tip vortex, particularly in stage 1-2, alters the downwash distribution and increases induced drag. An inboard negative vorticity with strength comparable to the negative vorticity at the tip is observed. Separated wing tips, as in storks and raptors, help spread the vorticity shed at the wing tip to reduce drag, a function that is lost when the outermost primaries are missing (stage 4 and 3). Beyond avian flight, these findings could inspire solutions for mitigating wing damage, particularly in micro aerial vehicles where morphing wings may help maintain aerodynamic efficiency despite structural losses.