Dynamic Soaring - Bird

Bird

Some seabirds dynamically soar by repeatedly diving into the valleys of ocean waves, and then wheeling back up into the air. Albatrosses are particularly adept at exploiting the technique and they use it to travel many thousands of miles using very little energy from flapping. When the bird pulls up into the wind out of the still air in the lee of a wave, it suddenly becomes exposed to a head wind, so the speed of the air over its wings increases. It then turns in the other direction and, with the wind behind it, dives back into the shelter of a wave. This also results in an increase in its air-speed. So by repeating this "wheeling" pattern, the bird can continue flying almost indefinitely without having to put in much effort besides steering. In effect it is harvesting energy from the wind gradient.

Lord Rayleigh first described dynamic soaring in 1883 in the British journal Nature:

"…a bird without working his wings cannot, either in still air or in a uniform horizontal wind, maintain his level indefinitely. For a short time such maintenance is possible at the expense of an initial relative velocity, but this must soon be exhausted. Whenever therefore a bird pursues his course for some time without working his wings, we must conclude either

1. that the course is not horizontal,
2. that the wind is not horizontal, or
3. that the wind is not uniform.

It is probable that the truth is usually represented by (1) or (2); but the question I wish to raise is whether the cause suggested by (3) may not sometimes come into operation."

The first case-described above by Rayleigh is simple gliding flight, the second is static soaring (using thermals, lee waves or slope soaring), and the last is dynamic soaring.