Page 242 - Understanding Flight

App_Anderson 7/25/01 9:03 AM Page 229

APPENDIX

Misapplications of

Bernoulli’s Principle

ernoulli’s equation has mistakenly become linked to the concept
of flight. “Demonstrations” of Bernoulli’s principle are often
Bgiven as demonstrations of the physics of lift. They are truly
demonstrations of lift but certainly not of Bernoulli’s principle. As
discussed in Chapter 1, we are often taught only part of what is nec-
essary to understand the applications of Bernoulli’s equation. This
has been the source of a great many misconceptions that have been
enthusiastically propagated. When we are first introduced to
Bernoulli’s equation it is always in respect to a fluid flowing in a pipe
with a restriction. Since mass must be conserved, the flow through
the smaller cross section is faster. Since energy is conserved, the
faster-flowing fluid has a lower pressure. This is often all that we are
taught. And from this, most of us have come away with the belief that
if air is moving faster it has a lower pressure. On giving it deeper
thought we might rightfully assume that the lowered pressure is
measured perpendicular to the flow (the static pressure), since we
know that if we put our hand in the path of this faster-flowing air we
would feel an increase in pressure.
Because of this point of view, some very interesting things are
taught. First, take the example of the “Bernoulli strip” illustrated in

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