Lift opposes weight, and is proportional to the square of the velocity of an airplane. As a plane goes faster, its lift increases, and its' lift force increases until it equals its weight. When lift equals weight, the plane can fly. In level flight, lift equals weight as the plane flies at constant velocity.
Aerodynamic Lift - The Common Explanation
Principle -The higher the fluid's velocity, the lower the fluid's pressure;
1) Air flowing over the top of the airfoil travels further than air flowing under the airfoil
2) Air flowing over the top of the airfoil is moving faster then air moving under the airfoil
3) Air pressure above the airfoil is lower than air pressure below the airfoil
4) The pressure differential causes the airfoil to rise
Aerodynamic Lift - The Physical Explanation
Typically, the Bernoulli principle is used exclusively to explain airplane lift/flight. These explanations show diagrams of airplane wings that are flat on the underside, but convex on the upper side. The diagrams typically show that air moving across the top of the wing has a further distance to travel than air moving across the bottom of the wing, and conclude that because air moving across the top of the wing must travel faster, the air pressure on top of the wing is less than the air pressure on the bottom of the wing.
There are several problems with this idea:
In reality, airplane wings are rarely shaped as the diagrams show. Most airplane wings are curved on both sides. In fact, in the case of delta-wing planes, the wings are essentially flat.
If the Bernoulli effect were all that held a plane up in the air, then what would happen if a plane flew upside-down, as stunt and aerobatic planes sometimes do? The Bernoulli model predicts that lower pressure on the surface of the wing that is normally the upper surface would now be drawing the plane earthward. According to this model, upside-down flight is impossible!
In order to create enough lift to hold the plane in the air, very large planes would have to have wings with highly convex surfaces. Yet we see that even wings of jumbo jets are only slightly curved instead of resembling hills.
Each time the plane accelerated, the Bernoulli effect should cause it to rise because of faster airflow over the wing. Yet this doesn't happen.
While it's true that air does move over the wing and does create an airfoil effect, this isn't enough of a force by itself to cause lift.
What contributes more substantially to lift is the force of the air diverted downward by the wing. According to "How Airplanes Fly: A Physical Description of Lift" (http://www.allstar.fiu.edu/aero/airflylvl3.htm) the total lift of the wing is directly proportional to the amount of the air diverted downward times the velocity of that air. If you think of a helicopter blade as a long, thin wing, you know what happens when you stand under it: you feel a massive rush of air downward! The equal and opposite reaction (Newton's third law) pushes the helicopter up in the air. A wing "catches" air in a way similar to how a sail catches air. The actual forces around an airplane wing are complex, but the net effect is a downward rush of air. Pilots change their elevation (go up or down) by changing the shape or angle (Angle of Attack, or "AOA") of the wing, which increases or decreases the downward rush of air.
A stall is an aerodynamic condition where the AOA increases beyond a certain point such that the lift begins to decrease. The angle at which this occurs is called the 'Critical AOA'. This Critical AOA is dependent upon the profile of the wing, its planform, its aspect ratio, and other factors, but is typically in the range of 8 to 20 degrees relative to the incoming wind for most airfoils.
Flow separation begins to occur at small AOA's while attached flow over the wing is still dominant. As AOA increases, the separated regions on the top of the wing increase in size and hinder the wing's ability to create lift. At the Critical AOA, separated flow is so dominant that further increases in AOA produce less lift and vastly more drag. (Note, airflow doesn't really separate from the wing, a vacuum does not magically emerge there. Rather, clean laminar flow gets pulled away by messy turbulent flow.)
Stalls depend only on AOA, not airspeed. Because a correlation with airspeed exists, however, a "stall speed" is usually used in practice. It is the speed below which the airplane cannot create enough lift to sustain the weight in 1g flight. In steady, level flight (1g), the faster an airplane goes, the less AOA it needs to hold the airplane up (i.e. to produce lift equal to weight). As the airplane slows down, it needs to increase AOA to create the same lift (equal to weight). As the speed slows further, at some point the AOA will be equal to the Critical (stall) AOA. This speed is called the "stall speed". The AOA cannot be increased to get more lift at this point and so slowing below the stall speed will result in a descent.
Change in wing's AOA (i.e., relationship of wing position to airflow direction)