Dynamic Soaring


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Albatrosses are able to fly for hours, and maybe even days, without flapping their wings, because they have a neat trick that grabs energy from  wind.  They use this energy to stay in the air and moving, and so they don't have to flap their wings.  As a result, flying long distances doesn't take much energy for albatrosses. 

They can also store food for quite a while (right?).  With easy flying and the ability to store the food they find, albatrosses can make really long flights to search for prey.  Here's how they fly without flapping their wings.  You have to know some things about wind first.  There is a really cool movie that is loading at the bottom of the page right now, but you won't understand it unless you read this section first. Take the time to keep reading and by the time you're done, the movie should be ready.

What you are reading about is called DYNAMIC SOARING.

The ocean surface moves slower than the wind above it does.  So, the wind moving just above the ocean surface is slowed quite a bit by friction with the slow-moving ocean.  The higher you get from the ocean surface, the less the wind is slowed by the ocean.  So, if an albatross wanted to fly into the wind, it should fly close to the ocean surface, where the wind against the bird would be the slowest.  That is what they do, and they also do something else, and this is the interesting part.


Waved albatross in flight
Waved albatross flying downwind away from you.
Matt Smith-Kennedy, used with permission.


Picture an albatross that wants to fly to the west, but the wind is blowing from south to north.  The albatross is five meters in the air (a bit taller than your ceiling probably is).  It points itself to the north and glides quickly downwind.  It gains speed rapidly because it has the wind pushing it and because it is gliding gradually from a high position to a low position.  After a long downhill glide with the wind pushing it, the albatross turns west, which is the direction it wants to go.  It is moving very fast, just as you would be on a bike after a long downhill with the wind at your back.  Conveniently for the albatross, the wind moving from south to north is slowed so much by friction with the ocean that the bird's flight is not affected much by wind (like we said earlier).  Compared with the bird's speed, the wind is not blowing much at the surface.  So, the bird can fly a long distance, maybe a hundred meters (the length of a football field) on the energy it got from the long downhill glide.  It has not been flapping its wings during this time.  Finally it slows down enough that it needs more energy to keep going.  It could flap its wings to increase its speed, OR...

It could change the angle of its wings so that it rises up above the ocean surface where the wind is blowing more strongly, and grab itself some more wind energy by flying downwind again.  During the downwind glide they pick up enough energy to fly some to the west and also to rise up into the wind again at the end of their progress to the west.

This alternation of downwind gliding and cross-wind gliding is the usual way that albatrosses get around, and all without flapping their wings.  Now, really what they should do if they want to fly west in the situation above is to glide downwind to the northwest and upwind (just above the ocean surface) to the southwest.  This would have them moving mostly west even though the wind is moving to the north.

This technique is called dynamic soaring.  It's kind of hard to understand the first time through.   Click the link below to see the movie that has been downloading and remember what you've just read.  Pay attention to the arrows in the animation; they show wind direction and speed.  You'll see that wind arrows close to the ocean surface don't move as fast as the arrows higher above the surface.   That's a key point.  Review a few paragraphs up if you don't remember why. Here is the video:

Read above text while movie is loading.

 

This page last updated on July 10, 1998 12:06 PM