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A fancy for flight

9 DEC 2004

You can learn a lot by letting a big, heavy bird land on your hand. You might notice, for example, that the bird lands softly, coming to a stop without slamming into your hand, or losing altitude.

Bird drawing with arrows showing air flow around wings.Don't try this with a 747! If one of these lumbering flying machines just slowed to a stop, it would crash. To stay aloft, airplanes land at high speeds, then roll to a halt.

(left) Here's how the vortex is formed along the swept-back section of outer wing. Lines show how the inner wing creates conventional lift. Photo: Courtesy John J. Videler.

still of bird wing movie(right) Here's the vortex, looking from straight behind the bird. The wings are swept way back, as shown in the diagram. The wing tip (white) is a bit in front of the little tornado. 252KB movie courtesy John J. Videler. Link will open in new window.

So how do birds fly up to a branch and land smoothly and precisely? It turns out that they use a completely different kind of lift -- which not only works at slow speeds, but even helps birds brake to a stop.

This week, John J. Videler, a marine zoologist at Groningen University in the Netherlands, and his colleagues described how a bird called the common swift creates lift with a "leading-edge vortex" (LEV). Think of an LEV as a horizontal tornado that forms above a sharp, swept-back wing as it cuts through the air.

The vortex is a low-pressure zone. Like the low-pressure zone formed above conventional wings, it makes lift. The LEV explains some of the speed and agility of fighter planes. Until now, it had been seen in insects, but never in birds.

Fighter jet takes off from aircraft carrier at sea. On the F-15 Tomcat fighter plane, the leading-edge vortex aids maneuverability. Photo: Argonne National Laboratory

Videler had noticed those nice, soft landings While messing around with captive falcons, and they started him wondering: How did birds do what a 747 cannot?

To answer this question, his crew of researchers stuck a model of a bird wing in water containing shiny particles. They lit the particles with a laser and made a movie as the water moved across the wing.

Why "fly" through water? Because it's easier to track the movement of water rather than air. At any rate, water can represent air when the experimental conditions (size of objects and speed of flow) make the ratio of inertia to viscosity (stickiness) the same in both fluids. More than that, we can't understand... so we hope you don't need to, either!

You do need to know that most birds have two-part wings, with different shapes, and apparently different functions. The close-in "arm wing" is rounded on front, humped on top, and sharp on the back -- just like most airplane wings. Further away, the "hand wing" is flatter on top and extremely sharp on the front. The hand wing resembles the wing of a fighter plane, and it is also often swept back -- angled -- toward the rear.

Bird in flight, wings spread wide open for air to pass over.
Wings on some high-performance jets can change angle to alter the leading-edge vortex. Wings that are nearly straight out make more lift. Swept-back wings make more drag (air friction). Acrobatic birds also take advantage of the LEV; changing wing angle gives them the ratio of lift and drag they need for flying and snatching insects in mid-air. Original photo/diagram: NASA

A sideways tornado answers a lot of questions about how birds fly.

Lift is not a drag
The study demonstrates a new relationship between aircraft engineering and biology. While high-performance aircraft have used the leading-edge vortex for many years, this is the first evidence for the LEV in bird wings, Videler says. "We're looking a"t the wing from a different perspective; they have two different parts, which operate in different ways."

The LEV not only creates lift, especially at slow speeds, but also confers another benefit that helps the swift perform insectivorous aerobatics. While conventional lift is chiefly an upward force, the LEV can also produce drag, which allows sudden steering. "The LEV can be used for controlling flight," says Videler. "It's very suited for that because there is no time delay, the forces are produced instantaneously. That's very useful if you want to maneuver very quickly."

Now, anything that helps a bird catch mosquitoes is okay by us.

Graph shows that high wing angle creates higher flight.
Wings on some high-performance jets can change angle to alter the leading-edge vortex. Wings that are nearly straight out make more lift. Swept-back wings make more drag (air friction). Acrobatic birds also take advantage of the LEV; changing wing angle gives them the ratio of lift and drag they need for flying and snatching insects in mid-air. Diagram: NASA

But enjoy this irony: When German inventor Otto Lilienthal tried to fly by studying birds in the late 1800s, he figured out how they get conventional lift. Not only did the great inventor miss the LEV effect, but he almost refused to see it, Videler believes: "In his original drawings of the cross-section of the hand wing, he cheats, he turns them into conventional profiles."

But give the man credit: The conventional wing profile does make lift, and the Wright Brothers took notice. "After that," Videler says, "people took the idea ...and started building aircraft." Fair enough. 747s do fly, after all. But, Videler laments, "Biologists never looked again at bird wings properly. They believed that hand wings would operate in the conventional way, and they never worried that they had it wrong."

Or half wrong, anyway.

Next step: Does the leading-edge vortex also make lift when a wing flaps?
--David Tenenbaum


Bibliography
Leading-Edge Vortex Lifts Swifts, by John J. Videler, Eize J. Stamhuis and G. David E. Povel, Science Dec. 10, 2004.


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