Re: Misconceptions: Physics of Flight

[John Denker <jsd@MONMOUTH.COM>]

At 01:17 PM 8/11/99 -0400, Lois Breur Krause wrote:
> as i picture the system, there's this unsuspecting mass
> of air, made up of discrete particles.

Yes, the air is made of discrete particles -- but be careful.  When you say
"discrete" many people will misunderstand and think you said
"noninteracting" which would be a big mistake.  The air particles interact
much more strongly with each other than with the wing.  It is a much better
approximation to think of the air as a *fluid* not a bunch of loose particles.

>  a wing moves through it. the wing
> forces air particles out of its way.  those that are pushed up are
> deflected at a steeper angle than those that are pushed down, because of
> the shape of the leading edge.

Sorry, no, the shape of the leading edge is by no means the determining
factor.  In fact, the shape of the *trailing* edge is more important!  Why
do you think they control the airplane with ailerons on the trailing edge?

> as the wing passes, the air particles come
> together again behind it.  the particles that were deflected upwards had
> farther to go, and moved faster, not because of the difference in length of
> the top surface of the wing vs. the bottom surface, but because they went
> UP farther than the others went down.  the mental picture i have is that
> the individual molecules move up and down, or down and up again, not
> horizontally.

Of course they do move up and down, but that's not the whole story.  Their
horizontal velocity changes quite significantly, and this is central to
understanding what is going on.

> i must be wrong somewhere.  i always thought this was a simple thing. y'all
> make it sound so complicated! ;-)

Fluid dynamics is complicated.  There's no use pretending it's not.

> but i'm still wrestling with how a plane can fly upside down, except that
> air is matter, has mass and takes up space... does it have to be going
> faster to maintain altitude if it's upside down, to cause enough lift to
> overcome gravity?

Airplanes fly just fine inverted.  They produce lift by the same mechanism.
 For more on this, see
  http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec_inverted_camber