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230 APPENDIX

Bernoulli strip

Fig. A.1. The Bernoulli strip.

Figure A.1. The Bernoulli strip is a narrow piece of paper which one
blows across the top to produce lift. It rises into the airstream and is
clearly an example of lift on a wing. “The air goes faster over the top,
thus the pressure is lower and the paper rises.” Or at least so goes the
explanation.
Another common example of the physics of lift is that of a Ping-
Pong ball supported by a vertical jet of air (Figure A.2a). The
argument is that since the air is moving the static pressure is lower.
When the ball moves to the side, it comes into contact with the still
air that is of a higher pressure. The ball is pushed back into the
flow.
Before we explain what is missing in our understanding of
Bernoulli’s education, let us revisit the static port on an airplane.
An airplane has a small port somewhere on its side of the fuselage
where the static pressure is measured by the instruments, such as
the altimeter. This port provides a fairly accurate static pressure
measurement, even though air passes over it at a high speed. If one
watches the altimeter when the engine is started and the propeller
blows air across the static port, the indicated altitude does not
change. But the altimeter gives a very sensitive measure of
pressure. So what is wrong with our understanding of the Bernoulli
principle?
In aeronautics, Bernoulli’s equation is well understood. Ignoring
the change in altitude and compressibility of an airflow, one can
write Bernoulli’s equation as: