Bernoulli's Principle and Winged Flight 

Ben Stark                      Illinois Institute of Technology
                               IIT Center
                               Chicago IL 60616
                               (312) 567-3488

Objectives:

This science "mystery" was originally presented to third graders, but is 
probably appropriate for grades 3-12.  The objectives are (1) to see how the 
decrease in air pressure which occurs when air is moving (relative to air which 
is not moving or is moving more slowly) allows birds to fly.

Materials Needed:

wooden dowel

thread

masking tape

three ping pong balls (or styrofoam balls about the same size as the ping pong 
balls)

hand held hair drier

Strategy:

This lesson makes a perfect "science mystery."   It starts out with the wooden 
dowel.  Cut two pieces of thread of equal length (about one foot).  Tape one 
thread (at one end) to one of the balls and the other thread (at one end) to a 
second ball.  Then tape the other ends of the two threads to the dowel near one 
end; when this is done, the two balls should hang down from the dowel when it 
is held horizontally so that they are about 1/2 inch apart (horizontally) but at 
exactly the same level (height-wise).  Ask for a volunteer to hold the dowel 
horizontally (so the balls hang down) and another to blow (gently but 
firmly) through the space between the two balls.  Ask the children to predict 
what will happen to the balls (most probably will say that the balls will be 
pushed apart).  Actually, if the volunteer is careful to blow straight through 
the space between the balls and not on the balls themselves, the balls will be 
pushed together!  The increase in velocity of the air being blown through the 
space between the two balls relative to the speed of the (still) air on either 
side of the ball makes the air pressure lower between the balls than outside of 
them, and the balls are thus pushed together. 

Then take the hair drier and turn it on (the phenomenon and explanation are 
simpler if the air can be blown without heat, because then you don't have to 
worry about changes in temperature complicating things). Holding it vertically, 
place a ball above the stream of air and let it go.  The ball will float above 
the opening of the hair drier, and will be held quite well (laterally) within 
the stream of air.  The blowing air elevates the ball and places it within a 
stream of air moving relative to the air outside of the stream.  Thus, as above, 
the ball is trapped in a zone of low pressure, and the higher air pressure 
surrounding the stream of moving air keeps the ball from leaving the zone of low 
pressure. 

This is also the principle of bird flight.  The birds' wings are relatively flat 
on the bottom and convex on the top.  When a bird is moving forward through the 
air, then, the air flowing over the wing has farther to go in a given amount of 
time than the air beneath the wing (the shortest distance between two points is 
a straight line, i.e., beneath the wing; the distance above the wing is longer 
because it is not in a straight line).  Thus, the velocity of the air passing 
above the wing is greater than the velocity of the air passing beneath the wing, 
the air pressure is thus greater below the wing than above it, and the bird is 
pushed up.  The same principle of "lift" allows an airplane to fly.    

Performance Assessment:

Grading might be best achieved by assessment of a one page (or so) write-up of 
the experiment that includes accurate discussions of the results of the 
experiment as well as the principle behind it (the latter may be appropriate 
only for the upper grades).

References:

Encyclopedias (look under Bernoulli's principle)
Probably any high school or college general biology text