Page 246 - Understanding Flight
P. 246
App_Anderson 7/25/01 9:03 AM Page 233
Misapplications of Bernoulli’s Principle 233
Force Force
Fig. A.3 Two Ping-Pong balls on strings.
traveling from right to left. The air splits around the ball and the
only force on the ball is the drag of the air.
Figure A.4b shows the effect on the air when the ball is spinning.
Here we have removed the effect of the ball traveling through the air.
The roughness of the ball causes air to be dragged around forming a
boundary layer moving in the direction of rotation. The stitching on
the ball enhances this boundary layer formation. The surface is
sometimes illegally roughened by the pitcher to enhance the effect.
When we put the airflows of Figure A.4a and A.4b together we find
that the airflow looks very much like that over a wing only upside
down in this example. The air that travels over the op of the ball meets
the oncoming flow around the ball and loses energy. This causes the
air to separate from the ball fairly early. The air that goes under the
ball is traveling in the same direction as the air around the ball. This
air does not lose energy as the air passing over the top of the ball did
and thus stays attached longer. The result of all this is that there is a
net upwash behind the ball and thus a downward force on the ball. So
a spinning ball will feel a sideways force when traveling through the
air. The backspin given to a dimpled golf ball causes it to experience
a lifting force in the same way.
There are other examples of the misapplication of Bernoulli’s
principle, but we think you get the idea. The next time you hear
Bernoulli given credit for some phenomenon, think it through and see
if you really believe what you are being told.