Page 245 - Understanding Flight

App_Anderson 7/25/01 9:03 AM Page 232

232 APPENDIX

but the static pressure has not decreased. The fact that the air is moving
faster does not necessarily mean that the static pressure has decreased.
Let us now look again at the Ping-Pong ball in the jet of air. First,
one might reason that since the jet of air is not confined, if it had a
lower static pressure, the surrounding air would collapse the jet until
it had the same static pressure as the surroundings. This is reasonable,
since there would be a difference in (static) pressures and no barrier
to separate them. In fact the source of the jet of air has only increased
the dynamic and total pressures of the air. Likewise, one’s breath does
not have a decreased static pressure. Thus one must look for another
explanation for the Ping-Pong ball swinging together and the lifting of
the Bernoulli strip.
The answer lies in the Coanda effect and Newton’s laws discussed
in Chapter 2. Remember, the Coanda effect is the pheromone that
causes a flowing fluid such as air to wrap around a solid object.
When the ball is near the edge of the jet of air, the Coanda effect
causes an asymmetric flow of air around the ball, as in Figure A.2b
and momentum transfer causes a force to push the ball back in, just
like the lift on a wing.
The same is true with the Bernoulli strip. The Coanda effect causes
the air to bend over the paper strip. Newton’s first law says that this
requires a force on the air. Newton’s third law says that an equal and
opposite force is exerted on the paper. The paper is lifted. Our
incomplete understanding of its application causes most of the
problems with the applications of Bernoulli’s principle. We have been
led to assume that if air is flowing its static pressure has been lowered.
This of course is not necessarily so.
There are two other phenomena often attributed to Bernoulli’s
principle. The first is the situation where one blows between two Ping-
Pong balls hanging on strings as shown in Figure A.3. The result is
that they swing in toward each other. Here we just have the same
phenomenon as the Ping-Pong ball in the jet of air. But in this case
there are two balls instead of one.
A more interesting misapplication of Bernoulli’s principle is in
the explanation of the curve flight of a spinning baseball. Let us
start the discussion by examining the airflow around a nonspinning
ball in flight, as shown in Figure A.4a. In the figure the ball is