# Physics Photo of the Week

September 29, 2006
Flying Geese
Flying 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 aerodynamics.

The 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 below.

The wing (airfoil) drawing shows the 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 geese?  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 vortex-type 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.

Physics Photo of the Week is 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.  Please send any photos to dcollins@warren-wilson.edu.
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