A summary of posts I've made in previous threads:
After visiting a large number of web sites, my conclusion is that lift is a combination of Coanda effect, "void effect" and simple deflection, all of which result in the "downwards" acceleration of air. Coanda effect explains how laminar flow follows a convex suface. "Void effect" explains how turbulent flow follows a convex surface. Concave surfaces simply deflect airflow. The curvature of air flow accelerates the air and generates lift. "Void effect" explains how drag is developed "behind" a wing, while direct forward deflection of air accounts for the drag in front of a wing, along with friction along the surface of a wing.
Except for a special class of airfoils, most of the air flow over and under a wing is turbulent, with only a portion of the air flow being laminar near the leading edge. For most wings, the flow transitions from laminar to turbulent flow above and below a wing, detaching during the transition, but reattaching after the transition. This happens in the first 30% of the chord length or sooner on a "normal" airfoil, and between the first 30% to 70% of chord lengh for a "laminar" airfoil (by definition). In some cases, rough surfaces and/or turbalators are used to cause the transition to occur at a specific position on an air foil. In the case of gliders, an "oil flow test" is done to visualise this transition. A bead of oil is placed on the leading edge of the wings, the glider is flown for a while at a fixed speed, then landed and the oil pattern observed. It's common practice to do this in glider magazine reviews.
http://www.standardcirrus.org/Turbulators.html
Oil flow testing is also done in wind tunnels:
http://www.hisacproject.com/news.html
At this web site, pages 4 and 5 discuss how little air flow is laminar over many wings, and how "laminar" air foils increase laminar flow to 30% or more over the chord length of a wing. In the case of gliders, laminar "bubbles" result in either more drag or less lift so the laminar air flow is deliberately broken up sooner than it normally would via rougher surfaces or turbulators (this is mentioned in the article). The laminar section starts mid way down page 4:
http://www.dreesecode.com/primer/airfoil4.html
"All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30% and 75% of chord."
http://www.aviation-history.com/theory/lam-flow.htm
The next website does a descent job of explaining lift, but with a bit too much emphasis on Coanda effect, ignoring void effect and turbulent flow, but towards the end of this web page, there's a diagram of a wind blowing over a roof, and although the air downwind of the roof is turbulent, it's also at lower pressure, due to void effect. Since both laminar and tubulent air flows contribute to lift, both cases should have been covered better than it was at this web page:
"The physical cause of low or high pressure is the forced normal (perpendicular)
acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect.":
http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html
videos
Assuming this next video isn't GGI, it appears to be a series of pictures of a flame aimed at various angles over an glowing (from the heat) airfoil at a fixed angle of about 45 degrees. As the flame angle is made more horizontal, the effective angle of attack becomes higher. What I call "void" effect is more evident here, as the flame flow is detaches from the aft end of the airfoil at low effective angle of attack. At higher effective angle of attack, the flame flow detaches from the "upper" surface of the airfoil, but it's still accelerated (curved) "downwards", while below the airfoil there is significant direct deflection. About 28 seconds into this video (you can hold it at this position), the downwards curvature of the flame over the "top" of the wing is still evident, in spite of the large amount of apparent detachment.
http://www.youtube.com/watch?v=hkJaTTIiXSc&fmt=18
Next is a link to a small wind tunnel video, considered a "2d" airflow (equivalent to a 3d wing with infinite wingspan). Air speed is slow, chord length is small, so the Reynolds number is quite low, and the air flow is much more laminar and the angle of attack before stall is much higher than it would be if everything were scaled up to a faster speed and a larger size. The smallish wind tunnel also prevents any significant upwards or downwards flow of air, so the air flow is not the same as it would be in an open environment. Wind tunnels that are much larger than the wing or model being tested, such as the one linked to above showing oil flow testing are much closer to "real world" environment. The transition into the stalled condition is very abrupt. In the segment annotated as "stall", there's virtually no lift, but near the end of the video, that starts off "flow attached", then "stall", there's still significant lift although there is a stall.
http://www.youtube.com/watch?v=6UlsArvbTeo&fmt=18
For this model, the stalling angle of attack is fairly small:
http://www.youtube.com/watch?v=5wIq75_BzOQ&fmt=18
Another wind tunnel, slow air speed, short chord, but not as much as the first video. Again the nature of the wing tunnel (proably drawing air inwards from the right), prevents the air flow from remaining deflected, and skews what would happened in an open environment:
http://www.youtube.com/watch?v=TGUSmdFmXDg&fmt=18
Regarding equal transit times, here are a couple of links to pictures of a flat top, curved bottom, pre-shuttle prototype:
M2-F2 glider with F104 chase plane:
m2-f2.jpg
M2-F3 rocket powered model (reached a speed of Mach 1.6) with B52:
m2-f3.jpg[/QUOTE]
)
