QUESTION:
In a recent chat about Aero Design, and I saw this answer you gave about
lift  and inverted flight.  I am a little confused.  Could you explain it
a little more.

>According to Bernoulli's principle the
>air has to go across the airfoil to create lift. So if a plane is upside down,
>does that reverse the principle?
>Holly, if you think of lift as going above the wing then flying upside down
>the lift will still be above the wing, but towards the ground.

My question is, if the lift is going in the opposite direction "towards
the ground," why doesn't the airplane get pulled downward and crash? What
keeps the airplane flying?

ANSWER from  Jeff Samuels  on October 24, 1997:
You are right about Bernoullie's principle.  The trick is to get the air
flowing faster over the wing's upper surface than the lower surface. 
Normally, a wing does this two ways. First, the upper surface of the wing
is longer than the lower surface (we call this a "cambered" airfoil).
Second, the wing is tilted leading edge up and trailing edge down (we call
this "angle of attack") which helps to force the air going over the upper
surface to travel farther and therefore faster.  To fly upside down with
lift still pointing upwards, you need to tilt the upside-down wing even
farther (leading edge up even more) to get the air to still flow faster
over the top.  This means you now have to fight against effect of the
camber.  Planes designed to fly upside down, like stunt planes, sometimes
have wings without camber.  That is, their airfoil shape has the same
upper and lower surface shape, so the wing can fly equally well right side
up or upside down.

ANSWER from Frank Quinto on October 17, 1997:
Using Bernoulli's principle, as flow speeds up - pressure is reduced.
Normally airplane wings have more curvature (more curvy) on the top
surface than the bottom surface.  More curvature on the top surface makes
the flow speed-up on top of the wing and reduces the pressure and lift
occurs.

To get more lift, the airplane can increase the attitude of the wing
relative to the incoming airflow.  The technical term for this is the
increase in angle-of-attack.  This increase in wing attitude causes the
flow to go faster, thereby getting greater lift.  Therefore,  whatever
side is getting the faster flow that is the side the lift will occur.

To clarify my answer to Holly/North High School, when you are normally
flying the lift goes upward because the curve side is on top.  If you are
flying upside down, the curve top side is now faces the ground and the
lift of the wing will be towards the ground.  This means that most
airplanes cannot fly upside down or they would crash into the ground. 
Some airplanes have wings that have curvature on both the top and bottom
surfaces.  These types of wings are usually on aerobatics (air show)
airplanes.  Since they have curvature on either side of the wing, they can
generate lift even when flying upside down.  When an air show airplane is
flying upside down, the pilot must make sure that bottom surface (which is
now the top surface while flying upside down) has the faster flow to get
lift.  I hope this clarifies things, if not ask again.  Thanks for your
interest in aeronautics.