Key To Smooth Landing May Lie In Birds??humbs

The alula helps birds land gracefully.

Alula on a bird's wing

By Muriel Gottrop [GFDL or CC-BY-SA-3.0], via Wikimedia Commons
Alula on a bird’s wing.

We primates often like to think of ourselves as a superior species because of the convenience and adaptability of thumbs for such critical tasks as changing the television channel and grabbing another handful of popcorn. Turns out, birds have a thumb-like structure too, which may aid in their ability to gracefully turn in the air and swoop close to the ground for a smooth landing.

Researchers at Seoul National University in Korea released a study called “The Function of the Alula in Avian Flight” in a recent issue of the journal Nature, which explores the function of a structure called the alula in avian flight.

The alula is a small structure that is usually found on a joint between the ?rm?part of a bird? wing and the ?and?branch of the wing. It? made up of one bone and two to six feathers. The shape of the alula is comparable to a thumb in that it sticks up from the main wing just as our thumb sticks out from our hand (if you look closely at a picture of a bird with an outstretched wing, you can probably see it, pointing in a slightly different direction from the other wingtip feathers). Its location varies slightly between bird species, and it? not yet known whether this variation is linked to variation in the alula? size between species.

The scientists were curious what role the alula plays in flight patterns, particularly its use in slow flyers prone to coming in for landing at large angles to the ground. Previous research had indicated that alula had some lift enhancing properties and allowed birds to sink from the air at a sharp angle while deflecting some of the wind resistance to a quick downward drop.

Researchers examined the flight patterns of four captive magpies and measured their swoops from a perch to the ground. Then, lift and drag forces of the birds?wings were measured with the birds in a wind tunnel. They discovered that what the birds might actually be benefitting from could be the presence of a small vortex of air, which forms naturally at the tip of the alula feathers.

This isn? the first time that scientists have discovered animals using force from vortices to aid in their movement, according to one of the study? authors?harks also take advantage of vortices.

“Riblets [in shark skin] are known to decrease the friction drag on the shark skin so that the sharks can swim efficiently without too much effort,?said author Dr. Sang-im Lee. “And how the riblets do it? The riblets are basically tiny grooves on shark skin and they divide large vortices into smaller ones and at the same time make smaller vorticies formed away from the grooves. In terms of aerodynamics, large vorticies produce larger drag, and if the vorticies are trapped in the grooves, they are less likely to dissipate which also increases the drag.?lt;/span>

One thing that remained unclear from the study was whether birds adjust their flight patterns to compensate if they were to lose their alulas. When magpies molt, they temporarily lose their alula feathers, but the scientists could not tell from their observations whether the birds acted any differently in the midst of the molting process to compensate for a loss of the feathers. In a separate trial, researchers did clip a little length off alula feathers on one side of the birds?bodies and noticed a little disorientation when they were required to turn sharply.

“After many trials, it is possible that they eventually ?earn?to be more ?areful?when they are without the alula (which is actually a good thing to check),?said Lee. More research would be needed to understand how consciously the birds manipulate their flight pattern thanks to the structure.

The new information has implications beyond the nation? magpies, of course. Lee is hopeful that the findings would encourage engineers to examine the implications of an alula for manmade flying apparatuses, and reports that the research team is currently working on the design of a leading-edge device similar to the alula for the wings of unmanned aerial vehicles. The exchange of new information between biologists and flight engineers sometimes encounters a philosophical divide, however.

Traditionally, bird wings were thought of as two-dimensional structures with air flowing around them, but more recently biologists are realizing that the wings of birds are operate aerodynamically in a three-dimensional manner, as evidenced by features like a vortex-causing alula.

“People like to simplify things, and many simulation tools are still based on two-dimensional airfoil theories. But the bird wing does not work like that,?Lee said. “I think the biggest step for the engineers to overcome is to view bird flight as a three-dimensional phenomenon, rather than two-dimensional one.?lt;/span>

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