The elevated aerodynamic performance of insects has been attributed in part to the generation and maintenance of a stable region of vorticity known as the leading edge vortex (LEV). One explanation for the stability of the LEV is that spiraling axial flow within the vortex core drains energy into the tip vortex, forming a leading-edge spiral vortex analogous to the flow structure generated by delta wing aircraft. However, whereas spiral flow is a conspicuous feature of flapping wings at Reynolds numbers (Re) of 5000, similar experiments at Re=100 failed to identify a comparable structure. We used a dynamically scaled robot to investigate both the forces and the flows created by a wing undergoing identical motion at Re of ~120 and ~1400. In both cases, motion at constant angular velocity and fixed angle of attack generated a stable LEV with no evidence of shedding. At Re=1400, flow visualization indicated an intense narrow region of spanwise flow within the core of the LEV, a feature conspicuously absent at Re=120. The results suggest that the transport of vorticity from the leading edge to the wake that permits prolonged vortex attachment takes different forms at different Re.

Most insects are thought to fly by creating a leading-edge vortex that remains attached to the wing as it translates through a stroke. In the species examined so far, stroke amplitude is large, and most of the aerodynamic force is produced halfway through a stroke when translation velocities are highest. Here we demonstrate that honeybees use an alternative strategy, hovering with relatively low stroke amplitude (Ϸ90°) and high wingbeat frequency (Ϸ230 Hz). When measured on a dynamically scaled robot, the kinematics of honeybee wings generate prominent force peaks during the beginning, middle, and end of each stroke, indicating the importance of additional unsteady mechanisms at stroke reversal. When challenged to fly in low-density heliox, bees responded by maintaining nearly constant wingbeat frequency while increasing stroke amplitude by nearly 50%. We examined the aerodynamic consequences of this change in wing motion by using artificial kinematic patterns in which amplitude was systematically increased in 5°increments. To separate the aerodynamic effects of stroke velocity from those due to amplitude, we performed this analysis under both constant frequency and constant velocity conditions. The results indicate that unsteady forces during stroke reversal make a large contribution to net upward force during hovering but play a diminished role as the animal increases stroke amplitude and flight power. We suggest that the peculiar kinematics of bees may reflect either a specialization for increasing load capacity or a physiological limitation of their flight muscles.bee flight ͉ flight in heliox ͉ stroke amplitude ͉ unsteady mechanisms ͉ wingbeat kinematics I n 1934, August Magnan and André Sainte-Lague (1) concluded from a simple mathematical analysis that the flight of bees was ''impossible.'' Since this time, bees have symbolized both the inadequacy of aerodynamic theory as applied to animals and the hubris with which theoreticians analyze the natural world. Although the assumptions used by Magnan and SainteLague have since proven erroneous (2), conventional fixed-wing aerodynamic theory is indeed insufficient to explain the flapping flight of bees and other small insects. In particular, the performance of insect wings, when tested under steady conditions in wind tunnels, is too low to account for the forces required to sustain flight (3). However, a number of more recent studies have demonstrated that wings perform much better when started from rest or rotated continuously around their base (4-6) due to the formation of a leading-edge vortex (LEV). Instead of shedding to initiate stall, the LEV remains attached throughout each stroke, presumably because of the transport of vorticity by span-wise flow (7-9). Whereas the delayed stall forces are greatest at midstroke, flapping wings generate additional forces during stroke reversals. These forces, which result from the rapid rotation of the wing, added mass effects, and the influence of the wake shed from previous strokes, are very sensitive to the precise p...

Helicopter Seed Lift The “helicopter” seeds of maple trees and other similar autorotating seeds detach from their parent tree under windy conditions and gyrate as they are dispersed by the wind. The reproductive success of the tree depends on the flight performance of its seeds. Autorotating seeds are known to generate high lift as they slowly descend through the air, but the means by which they do so is unclear. Lentink et al. (p. 1438 , see the cover) have elucidated the aerodynamic mechanism for high lift in autorotating seeds using a robot model seed and a three-dimensional flow measurement technique. Maple seeds and a hornbeam seed create a prominent leading-edge vortex that is similar to the flow structures that are responsible for the high lift generated by the wings of hovering insects and bats. Thus, both animals and plants have converged on an identical aerodynamic solution for generating lift.