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.

The "squeezed into a smaller area" part comes from the classic example
of the effect in a venturi. If a (compressible) fluid flows from a fat
tube into a thin tube and back into a fat tube, it is being "squeezed
into a smaller area" when it's in the thin tube. The pressure in the
thin tube is lower.

As for the wing, a lot is left out of the explanation. Not all things
are equal, and you need to take that into account. For example,
although the path over the top is longer, at the end, the air is not put
back the way it was prior to passage. The air molecules are moving
downards. This is required by the way the trailing edge of the wing is
angled. It didn't start out that way, therefore force must be applied
to the molecules to make this happen. This can only come from the wing,
and that's what holds the wing up.

Symmetric airfoils generate lift too if they are at the proper angle of
attack. Thin symmetric airfoils generate lift, but the path over the
top and bottom is then nearly equal in length.

Hollow airfoils (think just the top surface of the wing, with the bottom
surface and some of the leading edge removed) will also hold a plane
up, and the path over the top and bottom is identical. What is
different (before and after) in all cases is that the air has acquired a
downward velocity, and this has to be balanced by an upward force
applied by the air to the wing.

Jose
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