Flight Styles: Dynamic Soaring

Editors Note: This is the first in a series of short articles that will explore the various flight styles employed by different groups of birds. 

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When one sees an albatross up high above a swell it will almost always be rolled over to one side, exposing as much wing surface as possible to the wind. This immature Black-footed Albatross was photographed off of Newport, Oregon on 30 April 2010. (Photograph by Dave Irons)

Albatrosses, petrels, and shearwaters (the tubenoses) are highly adapted to lives spent mostly on the wing over vast windy expanses of water. For centuries humans have been awed by their capacity to cover great distances without exerting much apparent effort and yet, a clear understanding of the aerodynamics that make this possible is comparatively recent.  

In simple terms, tubenoses take advantage of the differences in wind speed (wind gradient) at various heights over the ocean's surface as well as the wind speed differences on the windward and leeward sides of each swell. Though we perceive wind speeds to be fairly constant, the ground speed of air masses vary with altitude. Friction near the ground or water surface translates to a loss of wind speed. Conversely, the wind speed increases with altitude as surface friction is reduced. Dynamic soaring is possible because seabirds are able to extract energy as they move between air masses with different wind speeds.  Radio controlled glider enthusiastic Joe Wurts, who is widely credited with being the first to understand and demonstrate how dynamic soaring works, offered this description.(http://illumin.usc.edu/189/dynamic-soaring)

Let's say the bird is flying at about 40 mph, and the wind is 10 mph. When the bird is going downwind, the airspeed 'of the Bird' is 40 mph, but the ground speed is 50 due to the 10 mph tailwind. Now the bird gets close to the ocean surface where the wind is less, let's say 5 mph. It keeps its groundspeed of 50 mph but since the tailwind is only 5 mph, it has 45 mph airspeed. It turns around, keeping the 45 mph airspeed, and now is heading upwind at 45 mph, with 40 mph groundspeed. It then climbs into the stronger headwind, and keeps the groundspeed of 40 mph, but with the wind of 10 mph, it is now at 50 mph airspeed. It then turns around and repeats the process.

Click on the link below to see Joe Wurts doing a demonstration of dynamic soaring with one of his radio-controlled gliders.

Joe Wurts Dynamic Soaring Video

As seabirds bank in and out of air masses with different ground speeds there is a net speed gain, which allows them keep moving along an invisible axis running down the middle of their arcing flight path with no additional input of propulsion (wingbeats). As birds bank up out of the calmer leeward side of the swell they use the added wind speed coming off the windward face of the wave to launch upwards and repeat this cycle using gravity and tailwind to pick up speed on the downward arc. When winds are heaviest, albatrosses and smaller tubenoses may go for long periods without a single wingbeat, while the smaller short-winged shearwaters, petrels, and storm-petrels often make a few quick flaps in between glides. Other birds, like White-throated Swifts (long arcing glides along rimrocks) and pelicans and gulls (gliding along nearshore waves), may be seen using their own versions of dynamic soaring, particularly during periods of heavy wind. 

Definition of terms:

Ground Speed -- The speed of an airborne object in relation to the ground. In the case of the birds and planes it is the combination of airspeed and windspeed. 

Air Speed -- The speed of an airborne object relative to the air it is moving through. 

Wind Shear -- A change in wind speed or direction along a straight line. 

1

Wonderful summary of dynamic soaring and I can’t wait to see the rest of this series.

2

BirdFellow – Birding services, social networking, and habitat conservation
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