悬停蜂鸟的升力产生
Aerodynamic theory and empirical observations of animals flying at similar Reynolds numbers (Re) predictthat airflow over hummingbird wings will be dominated by a stable, attached leading edge vortex(LEV). In insects exhibiting similar kinematics, when the translational movement of the wing ceases(as at the end of the downstroke), the LEV is shed and lift production decreases until the energy ofthe LEV is re-captured in the subsequent half-cycle translation. We here show that while the hummingbirdwing is strongly influenced by similar sharp-leading-edge aerodynamics, leading edge vorticity isinconsistent, varying from 0.7 to 26 per cent (mean 16%) of total lift production, is always generatedwithin 3 mm of the dorsal surface of the wing, showing no retrograde (trailing to leading edge) flow,and does not increase from proximal to distal wing as would be expected with a conical vortex (classIII LEV) described for hawkmoths. Further, the bound circulation is not shed as a vortex at the endof translation, but instead remains attached and persists after translation has ceased, augmented by therotation (pronation, supination) of the wing that occurs between the wing-translation half-cycles. Theresult is a near-continuous lift production through wing turn-around, previously unknown in vertebrates,able to contribute to weight support as well as stability and control during hovering. Selection for a planformsuited to creating this unique flow and nearly-uninterrupted lift production throughout the wingbeatcycle may help explain the relatively narrow hummingbird wing.