The Way of a Bird

Aircraft designers have studied the flight of birds only superficially. But with slide rule and logarithm they have come close, independently, to the mechanisms that keep the bird on the wing. The masters of machines that can outfly any bird for speed or distance must admit that a bird is, in a structural sense, a small and amazingly efficient living airplane. John H. Storer explains all this in a new book, The Flight of Birds (Cranbrook Institute of Science; $2.50).

With a series of slow-motion pictures, Storer proves that birds use all the aerodynamic tricks that man builds into his airplanes--and a few more besides. A bird's "propellers," explains Storer, are the big feathers at the ends of its wings. They are perfect airfoils with thick leading edges and thin trailing edges. When the bird flaps its wings downward, the "prop feathers" separate, twist to assume the proper "angle of attack," and act like propeller blades. They generate a forward force that pulls the wing forward, and the bird with it.

At the end of the downstroke, the wing is far forward. The bird pulls it back and up. This "rowing" motion against the air gives the bird an extra forward drive.

An airplane is supported by the reaction between its stationary wings and the air that strikes them as the plane moves horizontally. A bird is supported in the same way. The broad inner portions of its wings, which move less than the tips, are kept at any angle of attack that gives them maximum lift.

Slots & Flaps. The lift in an airplane's wings can be increased by increasing the angle of attack (i.e., the angle at which it meets the air stream). If the angle becomes too great, the air stream does not flow smoothly over the wing; it breaks into turbulent eddies. The wing loses most of its lift, and the stall that results can throw the airplane, into a disastrous spin. The danger of stalling can be lessened by slots behind the leading edge of the wing. The slots feed thin layers of air to the wing's upper side and suppress the dangerous turbulence.

The wings of many birds are also slotted so that the angle of attack (and the lift) may be increased without risking a stall. They have a movable feather called an "alula," which usually rests against the leading edge. When the bird needs extra lift from its wings (i.e., for a quick, high-angle climb), it increases its wings' angle of attack. Then it opens a slot by moving the alula. A thin stream of air rushes over the wing, preventing a stall.

An airplane will also stall when it flies too slowly. Birds, like planes, are equipped with "flaps": movable sections which can be protruded from the trailing edges of the wings. When slowing down for a landing, birds often spread their tails at a proper angle of attack. The tail acts exactly like airplane flaps, providing extra lift and keeping the bird from stalling.

Helicopters & Catapults. Some birds are helicopters. A hummingbird's wings are pivoted so that they can beat horizontally, always keeping their leading edges forward in the stroke. When the hummingbird hovers beside a flower, its fast-beating wings, just like the spinning blades of a helicopter's rotor, provide lift only. Wood ibis are helicopters only on the takeoff. When frightened, they whir up nearly vertically; then they settle into normal flight when they feel that they are out of danger.

Other birds are gliders. Turkey buzzards and pelicans, for example, have "power," but they use it only on the take-off or during emergencies. Most of the time they soar on motionless wings, riding on "up-currents" of air. Such birds (like men's gliders) have long, narrow wings, the most efficient shape for gliding. The wings of an albatross, probably the most efficient glider, have an "aspect ratio" of 11 to 1, i.e., its wings are eleven times as long as they are broad. A magpie, which, like a fighter plane, needs good maneuverability and seldom glides, has a low aspect ratio (about 2 to 1).

Some birds use a catapult take-off like the scouting planes carried on cruisers--e.g., egrets give themselves initial flying speed by a powerful push from their legs. Other birds need a long take-off run. Condors head into the wind like airplanes and run along the surface of the land, flapping their wings frantically until they at last reach flying speed. Andean Indians sometimes trap condors by tricking them into landing on bait encircled by low stone walls. Then the foolish condor is helpless, like a bomber that has made an emergency landing on a small field and cannot take off again.

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