EXPERIMENTS with a scaled-up replica of the wings of the common fruit fly enabled Michael Dickinson of the University of California at Berkeley and his colleagues to describe the dynamics of insect flight. Before the data collected from robofly, "we didn't really know how the damn things could stay in the air," he says.

Hovering Fly. The wingstroke of an insect is divided into four stages: two translational phases (upstroke and downstroke), when the wings sweep through the air with a high angle of attack, and two rotational phases (pronation and supination), when the wings rapidly rotate and reverse direction.

Data collected from robofly allowed the researchers to calculate the total force exerted and its direction (red arrows) and to derive the magnitude of the lifting forces generated as a vector (blue arrows). The analysis revealed three ways flying insects achieve lift--delayed stall, rotational circulation and wake capture.

DOWNSTROKE. In this example, as a fly moves from right to left during a downstroke of its wings (top),blue arrows indicate the direction of wing movement and red arrows the direction and magnitude of the forces generated in the stroke plane.

During this phase, the insect has at its disposal two means of generating lift. Delayed stall (1) causes the formation of a leading-edge vortex that reduces pressure over the wing. Rotational lift (2) is created when the insect rotates the angle of its wings (dotted line), creating a vortex similar to that of putting "backspin" on a tennis ball. At its completion (3), the maneuver also results in a powerful force propelling the insect forward.

UPSTROKE. As the insect drives its wing upward, it has the option of using another mechanism to gain lift--wake capture. This gains an insect added lift by recapturing the energy lost in the wake. As the wing moves through the air, it leaves whirlpools, or vortices, of air behind it (4). If the insect rotates its wing (dotted line), the wing can intersect its own wake and capture its energy in the form of lift (5).

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