Kinematics and aerodynamics of avian upstrokes during slow flight by Kris E. Crandell and Bret W. Tobalske
Most modern research on the physics of flight in birds has focused largely on steady flight (at cruising speed in wind tunnels) and the power of the downstroke. Recently, however, Kris Crandell and Bret Tobalske began investigating how birds might be using aerodynamic forces produced during the upstroke (which is typically considered a “recovery” movement that doesn’t produce any force) to accelerate and get moving during slow, transitional modes of flight. The upstroke in most birds tends to occur in two distinct ways: either the bird will rotate their wrist and reverse their wing-tips, or they will flex their elbow, bringing their wing closer to their body. In birds like this adorable Diamond Dove, the tip-reversal upstroke appears to impart a significant force to the accelerating bird, shattering the long-held belief that the upstroke was an aerodynamically inactive and costly movement in avian flight!
Photo credit: Robert Niese and Kris Crandell (inset)