Hi,

The title of this post implies that I know why airplanes fly. I
don't, not completely at least. But I do know that I have read a lot
of "official" explanations that are just plain wrong.

Before I begin my exposition about what keeps the plane afloat, I
would like anyone who care to participate in this discussion to do a
couple of simple experiments. This will get us all on the same page
(no pun intended, heh):

We are all familiar with the "blow-over-sheet-of-paper" trick to
illustrate Bernouilli's principle. That trick is actually has more
going on than Bernouilli's princinple, but I am going to avoid talking
about it until we can all at least agree on the concepts of voids and
pressures.

Let us do an experiment that uses not one but two sheets of paper.

EXPERIMENT 1:

Take two sheets of paper. Superpose one on top of the other on a
desk, perfectly aligned. Then carefully grab the edges of the top
sheet with both hands, gripping the edges between your palms, but
making sure to keep the top sheet as close to the bottom sheet as
possible, including the edges. The closer, the better. It helps to
grab long-wise, not short-wise. Try to grab as much edge as possibe.
Now take a breath...

In one quick motion, yank up the top sheet. Watch what happens to the
bottom sheet. It follows the top sheet for a brief moment.

EXPERIMENT 2:

Do the same as EXPERIMENT 1, but be creative. Instead of simply
yanking upward, move in a nice fluid-but-fast motion all around the
room. If you are careful, you should be able to make some nice,
gracious curves, keeping the bottom sheet intact. Some of you might
find it hard to believe, but with the right contraption, you could
actually keep two pieces of cardboard stuck together like this,
dragging one with the other all over the room, even though there is no
glue or any other adhesive binding the two.

There is one important lesson to be learned from these experiments,
especially the 1st. Bernoulli's Principle has nothing to do with
this. Bernoulli's principle has to do with air flow that is
*coplanar* with the surface under discussion. Bernoulli's principle
has to do with gases that are flowing in a direction that is
perpendicular to the normal vector of the surface over which it flows.
When you yank the top paper to lure the bottom paper, you are moving
in a direction that is *colinear* to this normal vector. Simply
stated, if you do not move sideways *at all*, but only outward, away
from the paper, you will STILL cause the bottom sheet to follow.

I will leave it to the reader to explain why the bottom sheet follows
the top sheet.

-Le Chaud Lapin-