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All aircraft that are producing lift, i.e. flying, have wingtip vortices. These vortices, kinda like horizontal tornadoes, rotate counter to each other when view from the rear. They tend to sink and move away from the centerline of the aircraft. They are most pronounced when the airplane is heavy, slow and in the landing configuration.

Needless to say, wingtip vortices and the turbulence and loss of control that they may cause a following, particularly smaller, aircraft are a major hazard to be avoided during takeoff and landing.

In this photograph these sinking, trailing wingtip vortices have sunk behind the aircraft flying just above a stable stratiform layer of clouds which are disturbed by the vortices hence rendering them visible.

The existence of the curls at the edges of the corridor are indeed caused by the wingtip vortices.

However, the central depressed area is due to what makes a wing fly. The vortices are a side-effect of this process but not the principal component.

The common misnomer in what "makes" a wing fly is lift. And indeed "lift" is the net effect of the process.

But more accurately, wings fly by accelerating air downward.

The equation F=MA applys. The upward force acting against the wing (i.e.lift) is a result of the mass of the air which is accelerated downward by the wing.

This is easily understood by anyone when one considers how a propeller works, or a helicopter rotor. It is easy to both visualize and experience the motion of air involved. But is less well understood when applied to a wing.

Essentially, a wing is a "linear propeller".

Each second of level flight, as the plane moves forward, it accelerates the mass of air it passes through downward.

Some of the air which passes below the wing is compressed by the wing and thus thrust downward. The air directly above the wing is rarified due to the wings shape and orientation relative to the direction of motion, and thus a volume of air above the wing is also accelerated downward.

That is also why symmetric wings on aerobatic planes still work - they are oriented by the aircraft's attitude such that the accelerate the air downward - and why "normal" wings with a flat bottom and curved top will still make the plane fly when it is upside down.

The total mass of all the air moved by the wing - multiplied by the acceleration of that air (which yields the speed to which the air is propelled downward) exactly equals the weight of the aircraft in level flight.

The vortices are a side-effect of the difference in air pressure between the top and bottom surfaces. So the explanation that there is high pressure below the wing and low pressure above it are indeed correct.

But the common description - which is that the air on the bottom of the wing lifts it, and the air above of the wings "sucks it upward" (no pun intended) - doesn't accurately describe the process.

The accurate physical description is the acceleration of the mass of air downward. That is the net effect of what has happened to the atmosphere after the aircraft has passed.

Short Version - An aircraft flys by pushing air downward.

You've proven that the http://FreeRepublic.com board has its aerodynamic experts! Thanks for your comments. And it looks likes NASA's insights into the vortices, wakes, and "Twin Tornados" are now part of http://ChamorroBible.org/gpw/gpw-20041010.htm web page.

Have you seen what the http://fark.com readers did with the C-17 Globemaster Photo? They had a field day! It seems like whatever scene the C-17 Globemaster is placed in, it still looks impressive: http://forums.fark.com/cgi/fark/comments.pl?IDLink=1160585