Passive pitch induced by a spreading pigeon tail

Aspect ratio is among the most influential parameters that impact the lift and drag generation of finite-span wings, in particular their dependency in the angle of attack. During landing, birds may use their tail in a rather complex way by rapidly changing both its angle of attack (pitch motion) and spread angle (spread-fold movement), which is a proxy for aspect ratio. Generally, the pitch motion is assumed to be primarily active, we reevaluate this by estimating the passive aerodynamic effects induced by the tail's spread-fold movement. Using a biohybrid robotic tail with real feathers, we present here the lift and drag characteristics of a pigeon tail for different spread angles. Corroborating the energetic landscape for these lift-drag characteristics, we predict whether further spreading the tail passively induces an upwards or downwards pitch as a function of spread angle and flow velocity. Our results suggest a new passive control pathway for landing in birds and their robotic counterparts.