LAUSR

Abstract The flapping wing is one of the most widespread propulsion methods found in nature. The current study focuses on the estimation of unsteady aerodynamic forces on freely flying birds through analysis of wingbeat kinematics and near wake flow measurements using long duration time-resolved particle image velocimetry. Three distinct bird species have been investigated: a shorebird (Western sandpiper, Calidris mauri), a songbird (European starling, Sturnus vulgaris) and a boobook owl (Australian boobook, Ninox boobook), corresponding to different flight strategies. Using long-time sampling data, several wingbeat cycles have been analyzed. Drag and lift were estimated using the momentum equation for viscous flows, revealing a highly unsteady behavior. The owl experienced the highest total drag over the cycle whilst the sandpiper was shown to exert minimal drag compared to the other two birds. The unsteady drag term was found to have a crucial role in the balance of drag (or thrust), particularly during the transition phases. The lift estimation presents a similar distribution over the wingbeat cycle, where both the owl and the sandpiper feature high values of unsteady lift compared to the starling. The large lift variation generated by the sandpiper is presumably used to achieve its high-performance migratory flight. For owls, the large variation of the lift appears to be essential for weight support while flying at slow speeds. The aerodynamic performance of the starling appears to be relatively low which matches their limited migratory behavior. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag and maximize lift during flapping flight by incorporating unsteady motion that interacts with the wake flow dynamics.