How do birds fly? This is a question that humans have tried to answer for thousands of years. From watching birds we know that flapping the wings up and down somehow makes them fly, and yet when man has tried to mimic this flapping motion it has never resulted in flight.
Scientists are only just starting to understand how birds fly; the following is a simplified explanation.
There are two natural forces that a bird must overcome so that it can fly:
The bird must generate a force, called lift, that pushes it away from the ground, and another force called thrust that pushes it forward through the air.
Intuitively, the downward flap of a wing beat should create lift, but then why doesn't the upward flap do the opposite? The answer is partly explained by how birds soar and glide.
When a bird is soaring it does not flap its wings and yet it is creating lift so as to remain aloft. This is the amazing, counter-intuitive part: lift is created not by flapping but by air flowing over the surface of the wing.
If we take a slice through a bird's wing its shape is like a teardrop, which is called an aerofoil. When an oncoming stream of air hits the leading edge of an aerofoil it splits into two air streams, one passing over the top of the aerofoil and the other underneath. The air streams below the aerofoil bunch together forming a higher pressure region whilst those above spread apart to form a lower pressure region. The difference in pressure above and below the aerofoil creates lift.
Thrust is the force required to overcome drag and drive the bird forwards. The counter-intuitive discovery that lift is created by air flow over the wing prepares us for an equally complicated explanation for how birds create thrust.
If we watch a large bird that has a slow wing beat, such as a member of the crow family, we can see that the wing is not simply flapped up and down. On the downstroke, the wing tip moves downwards and slightly forwards and on the upstroke the tip also moves backwards. This gives the impression that the bird is swimming or rowing through the air. Unfortunately, it is not quite so simple.
Towards the end of the downstroke, the air beneath the wing causes the feathers to twist into a vertical position (see illustration). Each of these flight feathers is also shaped as an aerofoil. With the front facing surface of the feather corresponding to the top surface of the aerofoil (in the illustration above), air passing over the surfaces of the twisted feather creates a forwards pushing thrust.
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