Re: What other factors apart from shape influence how a plane stays in the air?

Hi Crystal!
Thank you very much for asking this question! This one of the areas where
the normal explanation put forth by science doesn't quite hold up, no pun
intended. The amount of lift force generated on an airplane with
Bernoulli's Principle has been calculated and found to be quite a bit less
than what is needed to keep a plane up in the air. Before I explain how an
airplane really stays up, I should probably go into Bernoulli a bit.
Bernoulli's Principle states (basically) that the faster a fluid (gas or
liquid, doesn't matter either way) is flowing, the lower its pressure.
Nature tends to abhorr (sic? Probably) a vacuum (or even a bit of lower
pressure) and tries to fill it. So anything under a low-pressure area will
get "pushed" or "sucked" into the space above it (sucked or pushed
depending on how you look at it, sucked is a little more intuitive but
pushed is a little more correct, but I digress). You can demonstrate this
at home. Take a strip of like printer or lined paper and blow over the top
of it. It will sort of flutter or gently raise itself, depending on the
weight of the paper and where you're blowing and how hard but I digress
again. Anyway, when air flows over an airplane's wing, it is split into two
streams, one that goes over the wing and one that goes under. The one that
goes over the top goes faster than the one on bottom so the pressure is
lower above the wing. (Note: A common misconception is that the air streams
arrive at the tail end of the wing at the same time. This is not true. In
fact, the one on the top arrives just a little faster than the one on
bottom.) According to the Bernoulli Lift subscribers, the wing (and
hopefully the airplane it's attached to :) ) get pushed up into that area
of lower pressure. Technically, this is a perfectly valid explanation of
why an airplane flies, but like I said earlier, it just doesn't generate
enough force. Here's a different explanation that does, when calculated,
generate enough force for an airplane to fly.
All you need to know for this explanation is (bad grammar? Probably)
Newton's three laws and something called the Coanda effect. The Coanda
effect is the tendency of air, water, or any even slightly viscous fluid to
stick to a surface it is flowing past or over. So air sticks just a little
bit to an airplane's wing and we get this nice little curved airstream. To
make the air stick to the wing, the wing must exert a (downward) force on
the air (Newton's First Law, yes yes?). So the air must then exert an
upward force on the wing (Newton's Third Symph...I mean...Law). Just a
little bit, certainly not enough to keep the airplane up, but it's there.
Most of the force that keeps an airplane up comes from the wing diverting
air downward and thereby being pushed upward by the air (Newton's Third
again). A pilot can increase lift by (a) increasing speed (so more air
flows under the wing generating more reaction force) or by (b) increasing
the "angle of attack". That's the angle of elevation a line drawn through
the middle of the wing (called the chord line) makes with the horizontal.
See attached image. If you increase the angle of attack, you get more of
the wing surface exposed to the air and, again, more reaction force. So the
wing diverting air downward is really what keeps a plane up in the air.
Sorry for my very long-winded explanation! :)
Hope this helps!
Spencer Bagley
P.S.- About the two images...the first one shows a Cessna Citation leaving
a deep trough in the clouds behind it where it diverted the air down. The
second one has some relevant wing and force diagrams.