Since this thread has not come to a correct or satisfactory conclusion, but merely continued some incorrect views without objection, I hereby object!

 

(Harrumph!)

 

Rather than write an article and wade into the fray with my own poorly written descriptions and pictures, I refer those who want the final word to

 

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

 

Short summary: Lift occurs because flow is turned downward creating downwash. 

 

The web site also discusses several of the wrong theories and explains why they are wrong.

 

See among the pages available:

 

http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

 

Excerpt (red highlights added):

HOW IS LIFT GENERATED?

There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

Wander around the web site a bit, and be patient.  There is a lot to learn. 

 

Be sure to see:

 

http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

 

for the Newton vs. Bernoulli arguments.    Hint: both are right.  It depends on your perspective.

 

Since I like moving pictures, have a look at the moving visual presentation down the page a bit at

 

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

 

which shows the flow field around a wing at different angles of attack.  Change the angle of attack.  Move the probe around with the slider bars to see how pressure and velocity change around the wing, particularly close to the leading edge.  Also note the amount of up flow in front of the wing, particularly at higher angles of attack.  The flow moves up because the pressure above and in front of the wing the wing is lower than below and in front of the wing.  The low pressure field extends in front of the wing leading edge (subsonic flow, not supersonic flow).    See pressure diagrams around wings below. Pressures above the wing are low (“suction” compared to the free stream ambient static pressure) as are pressures below the wing, but the low pressure times area on the top is higher than the low pressure times area on the bottom, and thus the net force is up – lift.   Putting reflex shape and other shapes on the wing profile changes the pressure distribution, but the net effect is still the same – pressure difference between top and bottom yield lift.  The difference in pressure forces deflects the flow.  Action and reaction.  Note also that if the inside of your landing gear bays are pressurized with air leaking in from some higher pressure area, the pressure around the outside of the landing gear door will be lower, and doors will be pushed open as previously discussed.   This is particularly true for nose landing gear doors which are pressurized with cowl air, but I digress, as usual.   

 

 Fred

 

“With all thy getting, get thee understanding.”  - Malcolm Forbes.