The main goal of a wing is to create thrust with, an amount of force that is equal to the weight of the aircraft but pointing in the opposite direction, so the wing can lift the aircraft's weight. A wing flying through the air, creates lower air pressure above the wing and higher pressure under the wing.
I'd rather drop the word "lift" because some people think it's something other than thrust. Interesting is the fact that most aircraft's thrusters (propellers, jets) don't produce enough force to lift the aircraft up vertical. Instead of using these thrusters to lift the aircraft's weight, they transform their energy into the aircraft's speed, relative to the ambient air. Efficient thrust comes from accelerating a great weight of air, just a little bit. A wing moving through tons of air, can accelerate a lot of air mass downwards, but a propeller can accelerate far less weight. The energy of an aircraft's propeller is mainly transformed into:
- Driving the wings through the air. (Gearing down from high-speed-low-weight, to low-speed-high-weight, which is a more energy efficient way of producing thrust).
- Air resistance of all 'air conflicting aircraft parts'.
The rotary wings of a helicopter are something in between: a really big propeller, or a very small wing.
Imagine a flat & extremely thin wing, having a speed of say 200 km/h relative to the ambient air, and zero angle of attack. This "airfoil" obviously produces no lifting thrust. There is something happening though: drag. The air and the surface interact where they touch; the boundary layer. The air and the surface kind of stick together on an atomic level. This causes both the airfoil and the air to accelerate.
(note: there is no such thing as de-acceleration, that's another relative-type mistake. One more: "low air pressure"... isn't the opposite of high air pressure, it should be called "less air pressure")
The air, being a fluid, soon becomes turbulent by this local acceleration, getting random motions (well, not really random, it's just hard to see the logic).
Now, if the angle of attack is made positive, the air coming under the wing has a conflict with the wing. They're on the same path, but because they can't be at the same place at the same time, there will be a conflict. The wing is solid and the air is a fluid, a mixture of gasses. As these two try to push the other away, the air will become compressed but the solid surface will not (not noticeable). Air will flow away from this higher pressure area to any area of less pressure, which in this case is mainly downwards. The wing is pushed upwards and a bit backwards by the compressed air.
At the same time, the wing's top leaves a void behind, a vacuum space on top & behind the wing's airfoil. Vacuum space has an air pressure of zero point nothing. The ambient air has a pressure of about 1 KG per square centimeters, that is about 10.000 KG per square meter! Air flows from a higher to a lower pressure area, so it will travel to this vacuum with great acceleration! The "information" about air pressure travels through the air with the speed of sound, about 340 M/S. Air weighs only about 1,2 KG per cubic meter, so the vacuum isn't filled really fast. As long as the wing creates a vacuum, the pressure will be lower on top of the wing.
Both processes are continuously, so this wing with keep having higher pressure on the bottom and lower pressure on top.
An extremely thin wing isn't practical because a wing needs structural support, and there should be room in the wing for controls and such. Placing the support at the front bottom is the only good option, because it doesn't enlarge the frontal area there. The high pressure under the wing is lost by this shape.
Air flows. It doesn't flow corner wise. It (like all mass) "doesn't want" to accelerate, so if it has to, it will go as slowly as possible. The shape of the airfoil should be friendly to the air. If the airfoil is hard on the air, it will resist, and give that we call "air resistance".
The result of these few logic steps brought us a typical airfoil shape as result.
An illustration of an airfoil, showing (imaginationary-) air parcels around it.
What a wing does to the ambient air
The air on top of a wing is accelerated downward and backward.
The air on the bottom of a wing is accelerated downward and a bit forward.
In front of the wing is: higher pressure
On top of the wing is: lower pressure
On the bottom of the wing is: higher pressure
Looking at it in a Newtonian way: the wing accelerates air-mass mainly downwards, this action force on the inertia of mass gives a reaction force upwards: thrust (used for lift). (Inertia = the slowness of mass, not wanting to accelerate)
Looking at it from an air-pressure point of view: a wing creates a continues implosion on top (lower pressure), and with a higher than normal angle of attack it also creates a continues explosion on the bottom (higher pressure).
Extra information:
At the leading edge (the front of an airfoil), the air has to move away and is therefore of higher presence. This pressure "information" is "communicated" forwards by a pressure wave traveling through the air with the speed of sound (about 340 m/s). When flying with an airspeed of 330 m/s, the pressure wave from the leading edge is traveling forward with only 10 m/s (relative to the wing). This causes an enormous build-up of pressure; nearing the sound-barrier. The leading edge of most aircraft is round because the angle of attack must vary, and it's aerodynamic. A razor sharp leading edge prevents most pressure build-up, but can only be used with one angle of attack. A delta wing with a sharp leading edge can create vortex-lift, but that is an other story!.
