tiny-tim said:
Ah! You're talking about the spin that a pitcher puts on a baseball, that makes it curve in flight!
That's Magnus effect. One explanation is the thin layer of air attached to the spinning ball, creates a direct differential in air speed relative to the ball, accelerating some air perpendicular to the path of the ball which causes the ball to accelerate in the opposing direction, which causes the curve. The first link below mentions this and includes a diagram of "top spin", the net effects are air is accelerated upwards, so the ball is accelerated downwards. The other explanation is that the layer of air attached to the ball is too thin to be directly responsible for "lift", but causes the air flow around the ball to detach on the forward moving side sooner than the backwards moving side of a ball. The second link mentions this and includes a diagram of "back spin", air is accelerated downwards and the ball is accelerated upwards.
Airspeed diffferential explanation:
http://en.wikipedia.org/wiki/Magnus_effect
Seperation of air stream explanation:
http://www.geocities.com/k_achutarao/MAGNUS/magnus.html
Like the flow over an aeroplane wing - the flow over the top of the wing is faster than the flow under it, and that creates a difference in air pressure, which lifts the wing.
Using the air itself as a frame of reference, the horizontal flow over the top of the wing is slower than the horizontal flow below the wing. However, with the air as a frame of reference, the dominant component of air flow is downwards (corresponding to lift), with a smaller component of air flow forwards (corresponding to drag).
Unlike the spinning ball, where separation of air streams seems to be the more likely explanation of spinning ball "lift", wings simply have an effective angle of attack which when combined with air speed, accelerates the air downwards. For a "normal" wing design, most of this downwards acceleration of air occurs above the wing, partly because the low pressure area above the wing "steals" some of the air flow that would go under the wing. However there are wings with flat tops, curved bottoms that fly just fine, as in this case of some pre-shuttle prototypes for space re-entry vehicles called flying bodies:
M2-F2 with F104 chase plane.jpg
From this web site:
http://www.dfrc.nasa.gov/Gallery/Photo/index.html
The M2-F2 was a glider prototype for the M2-F3 which was powered by a rocket engine and achieved speeds of mach 1.6, so in spite of it's appearance, it wasn't a high drag wing. The main change to the M2-F3 was to add a 3rd vertical stabilizer at the back.