How Birds Fly

When a bird is gliding, it flies the same way as an airplane. As the wings move through the air (blue lines), the special airfoil shape of the wings causes the air pressure above the wings to be lower than the pressure underneath. The difference in pressure is lift, a force that acts roughly perpendicular to the wing surface and keeps the bird from falling.

Flapping flight uses the same principle, but the movement of the wings is more complicated. There are three important motions in addition to the bird’s forward motion:

1. Flapping

2. Twisting

3. Folding

By flapping its wings down, together with the forward motion of the body, a bird can tilt the lift of its wings forward for propulsion. Why don’t birds simply move their wings up and down, without twisting and folding? Notice that the outer part of the wing moves down much farther than the inner part close to the body. Twisting allows each part of the wing to keep the necessary angle relative to the airflow. If part of the wing is angled lower than the airflow, there might not be enough lift. If part of the wing is pointed too high, there could be a lot of drag. The wings are flexible, so they twist automatically.

Wing folding isn’t essential – ornithopters fly without it – but it helps birds fly with less effort. To see why it is helpful, think about what happens during the upstroke. Because the wing is going up, the lift vector points backward, especially in the outer portion of the wing. The upstroke actually slows the bird down! By folding its wings (decreasing the wingspan) a bird can reduce drag during the upstroke.

In addition to the three basic movements described here, birds can do a lot of other things with their wings to allow them to maneuver in the air. Instead of using their tails for flight control, they move their wings forward and backward for balance. To make a turn, they can twist the wings or apply more power on one side. For slow flight, birds can flap their wings almost forward and backward instead of vertically; the upstroke and downstroke produce lift without forward body motion.

Since flapping wings are subject to unsteady flows – they not only move but accelerate through the air – they can produce more lift than fixed wings and are resistant to stalling. 

AIRPLANE BASICS

HOW A   JET ENGINE WORKS

HOW A PROPELLER WORKS

HOW A CONSTANT SPEED PROPELLER WORKS