http://www.theaustralian.news.com.au/common/story_page/
0,5744,11654801%255E2702,00.html
Birds do it with spirals, scientists discover
Leigh Dayton, Science writer
December 11, 2004
BEES, bugs and butterflies do it, and now scientists know that birds do
it, too.
"It" being the creation of a spiralling current of air to suck their
wings up and generate lift.
According to a team of Dutch researchers, led by John Videler of
Groningen and Leiden universities, birds use a specific part of their
wing called the "hand-wing" in this "unconventional" way.
Previously, scientists believed birds kept aloft by using their wings
as aerofoils, or arched structures, to force air to flow at different
speeds above and below their wings.
But today, in the journal Science, Professor Videler and his colleagues
report that the complex structure of bird wings allows them to combine
both mechanisms in their aerial antics.
In a second article in Science, Ulrike Muller and David Lentink of
Wageningen University in the Netherlands note that engineers applied
the same phenomenon of spiralling air, or "leading-edge vortices", to
design fighter jets.
In 1996 biologists discovered that insects generate lift by producing
leading-edge vortices which they control by rapidly beating their wings.
Until now, researchers had no way of studying how air flows around the
wings of flying birds. Professor Videler's group side-stepped this
limitation by building a model of the wings of swifts -- extremely
agile fliers that catch insects in flight.
Because air and water flow the same way in some conditions, they "flew"
the wings in a water tunnel, discovering that the vortex spirals out
from the top of the wing toward the tip. Like a tornado, the air
pressure inside the vortex is low, sucking the wing upward.
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