kd5sak wrote:
"Richard Lamb" wrote in message
nk.net...
TRUTH wrote:
"Jim Macklin" wrote in
news:uX8Lf.104268$4l5.39451@dukeread05:

Bernoulli theory:

So how do these equations relate to our two-dimensional airfoil? Look
again at
the Clark Y and notice that an airfoil is a curved shape. While the bottom
is
relatively flat, the top surface is thicker and more curved. Thus, when
air
passes over an airfoil, that flow over the top is squeezed into a smaller
area
than that airflow passing the lower surface. The Continuity equation tells
us
that a flow squeezed into a smaller area must go faster, and the Bernoulli
equation tells us that when a flow moves faster, it creates a lower
pressure.

I don't quite understand the "squeezed into a smaller area". I Understood
that the flow over the top surface had to travel further (thus faster) over
the longer curved distance to get from the leading edge to the back of the
airfoil. I am just a lay person and do not even play an aeronautical
engineer on TV so I may be totally mistaken.

You are, but don't feel bad. It is a common misconception even still
taught by some flight instructors. The truth is, there is nothing
connecting molecules of air together. It does not matter that a
molecule above the wing has to travel farther in order to 'catch up' to
one below the wing. It never met the lower molecule and cares nothing
about it. :-)

Airplane wings use the curved upper surface to displace air which,
because it is slightly sticky, follows the surface of the wing. If you
hold a water glass sideways under a stream of water you will see the
water curve around the glass all the way to the bottom. Air flowing
over a wing does the same thing. As you probably learned in basic
physics, though, gases like air maintain a constant total pressure. Air
is being accelerated in one direction over the wing, so pressure is
being increased in a single direction. We call this dynamic pressure.
It is the pressure you feel when you blow on your hand. If dynamic
pressure in one direction is increased and total pressure must remain
constant, then the pressure in all other directions must be decreased.
We call the molecules moving in all these other directions the static
pressure.

It is like cars in a parking lot, all moving in different directions.
If most of them reach a road and start moving in a single direction,
then there must be fewer cars moving in other directions. Since most of
the air particles are being accelerated in a single direction then
there must be fewer of them moving in any other direction. This creates
an area of low pressure above the wing. Air above the wing moves into
this low pressure area and is in turn accelerated behind and down off
the trailing edge of the wing. Newtonian physics tell us that if there
is acceleration in one direction there must be an equal and opposite
reaction in the other. We call that lift. The amount of lift generated
is computed by an equation involving the air density, speed of the
wing, area of the wing, and something called the lift coefficient which
is basically how much air can be displaced by the wing.

Thus, wings generate lift by accelerating air over the top of the wing
and then down off the trailing edge. People don't realize it does this
because they see pictures of air streams taken in wind tunnels, where
the fan continues to blow the air straight backward behind the wing. In
actual flight, however, the wing is simply forcing a huge volume of air
straight down. You can see this when an airplane flies low over water;
the ripples in the water are almost directly below the airplane.

Really, a wing is just a big fan blade, only instead of spinning
around it moves in a straight line. You do not stand at the edge of the
fan to catch the breeze it generates. You stand behind it. You also
know that the air blown by a fan comes from in front of the fan. You
can hold strips of paper in front of a fan and watch them being sucked
toward the fan. Well, the wing is just a fan blade. A great big fan
blade, to be sure, but that is all that it is.

We call it Bernoulli's principle because Bernoulli was the first to
notice that if you accelerate a fluid in one direction that pressure in
the other directions is reduced. One method of accelerating a fluid is
to force it through a tube that narrows, which is what Bernoulli did.
Wings do not really do that, although you commonly see science
popularizers showing air flowing through a Bernoulli tube and then
removing half the tube and calling it a wing. The fact is, air is not
really being compressed in that way at all. It is simply being
accelerated over the top of the wing by the front part of the curved
surface. That is why lift is greatest at the point where the wing is
thickest. Nevertheless, Bernoulli's equations work well for predicting
lift even though the method of accelerating the air is slightly
different than forcing it through a narrow tube. It is the same
principle, just differently implemented.

The Wright Brothers actually found that wings generate somewhat more
lift than would have been first predicted by Bernoulli. Their first
wings were too thick with a greatly exaggerated curve in order to
generate maximum lift. What they discovered through trial and error,
though, was that although such a wing generated a great deal of lift it
also could not generate more by increasing the angle of attack -- the
angle with which it meets the air. Instead, what they got by increasing
angle of attack was complete separation of air flow from the wing and
lift went to 0, what we call a stall. This is one reason the Wrights
never rebuilt the first Flyer after it was destroyed shortly after
making its first flights. They realized that the machine would never be
able to climb very rapidly and that it would always be prone to
suddenly falling out of the sky because of stalls. They considered the
thing to be extremely dangerous and went back to the drawing board.
Also, of course, they destroyed it to keep it out of the hands of
potential competitors like Curtis. A shame, really.

Or, you can just take the simple explanation and say that the air has
to travel further over the wing in order to generate lift. It is wrong,
but it works well enough for laymen.