Photo of the Week
September 29, 2006
geese often fly in a "V" formation - honking loudly. There is
considerable physics at play that permits more efficient flight for the
geese that fly in this common formation. It all results from
physics of bird flight is similar to airplane flight -
except for the flapping of the wings.
A cross section of an airplane wing (similar to a bird wing) is shown
The wing (airfoil) drawing shows
relative flow of air around the bottom and top of the wing. A
wing moving forward in the air is equivalent to the air moving
backwards around a stationary wing. The camber (curvature) of
the top surface of the wing results in the deflection of the top air to
be deflected downward. This downward deflection may be thought of
as caused by the "suction" from the wing's top as
the result of lower pressure of air on the top surface.
Bernoulli's principle causes this lower pressure due
to the faster movement of air over the wing (the common
explanation). The net effect is a downward thrust of the air at
the trailing end of
the wing. The reaction to the downward deflection of air is the
lift force that keeps the airplane or bird aloft.
What does this have to do with the V-formation preferred by flying
Where the wings end, vortices are formed, called "wing-tip
vortices". See the diagram at right. This picture looks
from the rear of the aircraft (or bird). The arrows pointing down
represent the airflow deflected downward by the forward motion of the
airfoil that gives lift to the airplane. The downward flow of air
aft of the trailing edge of the wing causes a vortex at the wing-tips
as shown by the circulating arrows. When the air is deflected
downward, air from elsewhere has to come and replace it. The
shortest path is the adjacent air. As the adjacent air flows
toward the axis of the aircraft, lower air from outside the
path of the aircraft rises. The process continues leaving a
of wake from each wingtip of the airplane (or bird). The aviation
industry has extensively studied wing-tip vortices. They are
often persistent and present hazards to other aircraft, especially on
runways, limiting the frequency of take-offs and landings at busy
airports. The V-flying geese take advantage of the wing-tip
vortices. By flying not directly behind another goose, but off to
one side, the trailing goose can take advantage of the extra lift on
the "up" part of the trailing vortex from the goose in front. How
the geese decide who is the leader I will defer to those who study
animal behavior. I have noticed that these flying "V's" often
break-up with some birds deciding to go off in a different direction
forming a new "V" - all the while vigorously communicating with each
other with their characteristic honking.
Trailing vortices are also seen with automobiles in misty, rainy
weather. That is another PPOW for another time. A photo
contribution would be most welcome.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature an interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
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