Meeting Abstract
While water and air are fundamentally different media, diverse species locomote effectively in both. As a prominent example, roughly 40 species of birds across five extant clades have co-opted their wings for use in underwater propulsion, here termed “aquatic flight”, while retaining their aerial flight. During aquatic flight, these species flex the wrist and elbow joints of their wings, substantially reducing the effective span and area of their wings relative to in aerial flight. To elucidate the function of this behavior, we investigated the aero- and hydrodynamic performance of the flexed and extended wing postures on pairs of wings from ten common murres (Uria aalge). We used a propeller model to emulate flapping in air and water and a wind tunnel to emulate gliding. We hypothesized that the flexed posture would produce greater ratios of vertical-to-horizontal force (an efficiency metric) across all conditions, but that the total vertical force produced by this posture would be insufficient for weight support. During emulated gliding, flexed wings did achieve greater ratios of vertical-to-horizontal force when all angles of attack (0-60 deg) were considered. However, during emulated flapping, extended wings achieved greater ratios of vertical-to-horizontal force and greater coefficients of vertical force at both aerial and aquatic Reynolds numbers. Extended wings produced 1.5-6X more vertical force than flexed wings, but even extended wings were insufficient for weight support — consistent with the poor slow-flight performance of murres. It is therefore unclear why birds use a flexed wing during flapping of aquatic flight. Perhaps steady-state models fail to capture the performance of the flexed-wing posture or the use of a flexed wing for aquatic flight is compulsory due to limitations on factors outside of propulsor shape (e.g. structural or muscular constraints). (NSF IOS 1838688).
