Bernoulli

Saying that lift is caused by Bernoulli is like saying that moving a
stack of boxes is caused by the compression of flesh against
cardboard. It's an accurate description of a physical aspect of the
process, it's essentially to fully understanding what is going on, it's
a true in a very limited way, but, when you put that "cause" concept
in there, it is very, very, misleading.

Think of air as a spring. Its pressure is a form of stored energy.
(Remember that all the air we experience is kept in that potential
energy state by gravity.) Fill a container with air on the ground and
take it up to 18,000 feet and open a valve. Air will rush out and can
spin a little fan. It also takes energy to compress air.

Bernoulli's principle is just a subset of the law of conservation of
energy. Start a flow of fluid over a obstacle that changes the
direction and speed of the flow. For the principle to remain valid,
there must be no other energy inputs or drains from the system. This
is a key and seldom recognized point. It is also never true in the
real world.

The mass and velocity of the fluid at the beginning of the region in
which you are going to measure speed and pressure changes represents a
quantifiable amount of energy. It takes energy to make any part of the
flow speed up. For the total energy to remain constant, there has to
be a corresponding reduction in energy somewhere else in the system.
That reduction comes from pressure. Pressure and velocity remain in
balance.

If energy is added or subtracted from the system locally, the balance
predicted by Bernoulli does not need to be maintained. For example, it
the air is speeded up by a row of little engines and propellers,
pressure will not fall. Conversely, and here is where it falls apart
in the real world, if energy is drained out by the fluid being warmed
up due to friction, the energy taken out in the form of heat will not
need to be balanced by a corresponding rise in pressure.

The flow around an airfoil which results in lift creates a condition
in which flow increases above the wing and slows below it. Bernoulli
predicts that this will result in a pressure differential. Because of
the symmetries required by conservation of energy, the pressure
differential will be equal to the weight of the aircraft. To say that
this is the lift is where it is usually explained in a misleading way.
It is just a true (or false depending on your point of view) to say
that the pressure differential is the result of the lifting process.


--

Roger Long