456 and, provided that the force generated is greater than the gravitational force as a result of the body's mass, the body will be raised into the air.

In aircraft the fixed wings supply only lift, and horizontal propulsive force (thrust) is supplied by engines. In insects, wings are movable and supply thrust as well as lift. Further, insect wings are of uniform thickness; that is, they do not have curved upper and flat lower surfaces. Thus, to develop both lift and thrust during their stroke, wings both move up and down and change their angle of attack.

When an insect wing begins to accelerate through air, a starting vortex (circulation of air, as in a whirlwind) is developed at the wing's trailing edge (Figure 14.10B). This vortex, in turn, creates a second vortex (bound vortex) that moves clockwise round the wing, backward over the upper surface, and forward over the lower surface. This causes the air flow to speed up on the wing's upper surface, whereas on the lower surface air flow is slowed. These differences in air speed generate lift.

The relative values of lift and thrust will change throughout the wing beat (Figure 14.11). In the middle of a downstroke, the rapid downward movement of the wing operating in conjunction with the already moving horizontal stream of air over the wings (assuming the insect is in forward flight) will result in a positive angle of attack (i.e., the air will strike the underside of the wing) and give rise to a strongly positive lift. Concurrently, because the wing is pronated (its leading edge is pulled down), there will be a slightly positive thrust (Figure 14.11B). During an upstroke, the angle of attack becomes slightly negative (pressure of air above is greater than pressure of air below the wing) causing slightly negative lift, yet increasing the positive thrust (Figure 14.11C). The angle at which an insect holds its body in flight also results in positive lift, though this amounts to less than 5% of the total lift in the desert locust and about 20% in some Diptera.

For many years it was assumed that the lift generated by the wing movements described above was sufficient for insect flight. However, calculations showed that conventional (steady-state) aerodynamic theory, which is based on rigid wings moving at constant velocity, cannot account for the production of lift in most insects. Ellington's and Dickinson's groups (see Ellington, 1995, 1999; Ellington et al., 1996; Dickinson et al., 1999; Dickinson and Dudley, 2003; Sane, 2003) studied the aerodynamic performance of both the wings of tethered insects (bumble bees and hawk moths) and mechanically powered model wings. As a result, it is now considered that lift in most flying insects is produced by three interactive mechanisms: delayed (dynamic) stall, rapid wing rotation, and wake capture (Figure 14.12). Delayed stall operates during the main "transla-tional" part of a stroke (both up and down), whereas the other two mechanisms occur during stroke reversal, that is, when the wings twist and reverse direction (Dickinson et al., 1999).

Delayed stall refers to the condition in which for a brief period wings can be held with a high angle of attack, thus producing a leading-edge vortex. The latter creates a region of low pressure above the wings, thus augmenting lift. The vortex appears at the beginning of the downstroke and forms a conical spiral, getting larger as it moves toward the wing tip, generating lift equivalent to about 1.5 times the weight of the hawk moth (Ellington et al., 1996). The tipward flow of the vortex also increases stability. Toward the end of a stroke, the vortex is shed as the wings twist and reverse direction.

Dickinson et al. (1999), while confirming the importance of delayed stall, showed that rapid wing rotation and wake capture also contribute to the generation of lift using their large-scale model of the Drosophila wing. Rapid circulation of air in the boundary layer is induced around a spinning object, for example, a ball. If the ball is thrown with spin,

FIGURE 14.11. (A) Changes in the angle at which a wing is held in flight relative to direction of movement. Arrows indicate the angle at which air strikes the wing. Numbers indicate chronological sequence of wing positions during a stroke; and (B,C) Magnitude of lift and thrust approximately midway through downstroke and upstroke, respectively. [A, after M. Jensen, 1956, Biology and physics of locust flight. III. The aerodynamics of locust flight, Philos. Trans. R. Soc. Lond. Ser. B 239:511-552. By permission of The Royal Society, London, and the author. B, C, after R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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