Lift is the force generated by the wings. In straight and level flight, lift generated by the wings counteracts the airplane's weight. Two principles that allow a wing to make lift. They are Newton's Third Law and Bernoulli's Principle.
Newton's Third Law of Motion
In the 17th century, Sir Isaac Newton developed three laws of motion, which describe how objects in motion behave. His third law explains how a wing generates a portion of its lift. It states that every action has an equal and opposite reaction. This explains why a rifle recoils or a rocket launches into space.
If the wings push air down, the air will push the wings back in the opposite direction, up. Newton's third law accounts for about 10-20% of the total lift produced by the wing. Bernoulli's Principle accounts for rest.
Bernoulli's Principle
To understand the Bernoulli's Principle, the first step is to understand a venturi. A venturi is simply a tube with a restriction placed inside of it. Bernoulli used venturis to study fluids in motion, measuring velocities and pressures inside. He found when the air first enters the venturi, it has its normal pressure and velocity. When the air hits the restriction, however, it squirts through the narrower passage, causing its velocity to increase and its pressure to decrease. As the air exits the venturi, its pressure and velocity return to normal.
Bernoulli's principle states that as air velocity increases, its pressure decreases. This principle is utilized in the generation of lift, as explained below.
Airfoils
An airfoil is a device that produces a force when in motion relative to the surrounding air. The wing is an airfoil, as is the propeller. The wing is shaped to generate lift. If you stand at the wingtip of an airplane and look down the wing, you will see this shape. The front of the wings, its leading edge, is rounded. The top of the wing is more curved, while the bottom is flatter. This curvature is referred to as the wing's camber. It then tapers into a pointed trailing edge.
As the wing moves through the air, the air moving over the more curved upper surface, gets squirted over the top of the wing. As a result, its velocity increases and its pressure decreases, according to Bernoulli's Principle. An area of low pressure then exists above the wing. Since fluids seek lower pressure, the higher pressure air under the wing tries to move toward the lower pressure air above the wing. The wing gets in the way, and the resulting force is lift.
Relative Wind and Angle of Attack
When you walk or run, you will feel a breeze blowing in your face. If you stick your hand outside a moving car you also feel the air moving. This is relative wind, and it is generated by anything that moves through air. An airplane, like any other object, develops relative wind opposite its direction of motion.
The angle of attack is the angle formed between the relative wind and the wing's chord line, which is the imaginary line connecting the mid point of the wing's leading edge to the wing's trailing edge. It is the angle between which way the airplane is moving and where the wings are facing.
The amount of lift generated by the wing depends on angle of attack and airspeed. In level flight at high airspeeds, the wing produces enough lift to support the weight of the airplane at a low angle of attack. At lower airspeeds, a higher angle of attack is required to maintain level flight.
Critical Angle of Attack
An airplane wing can only generate lift up to a certain critical angle. At angles of attack above this critical angle, the air no longer flows smoothly around the wing. Instead, the air flows around the wing in a turbulent pattern, and the wing stops producing lift. This situation is known as a wing stall.
While high angles of attack are most commonly encountered at lower airspeeds, such as during takeoff and landing, a stall angle of attack is possible at any airspeed and any aircraft orientation. The airplane can have a low, high, or stalled angle of attack even as it travels straight up or straight down.
Design Maneuvering Speed
One important airspeed, which is not marked on your airspeed indicator, is the design maneuvering speed, or VA. If the airplane is flown at a speed faster than VA, then a sudden full movement of the controls could overstress and damage the airframe. A sudden and full control movement at speeds below VA should result in the aircraft reaching it's critical angle of attack and slowing down, prior to becoming so stressed that the airframe could be damaged. This concept applies well to flight in turbulence. VA is often used as a do not exceed speed in rough air.
Angle of Incidence
At normal cruising speed, a certain angle of attack is required to carry the weight of the airplane. Aircraft designer's often have the wings mounted to the airframe at a slight angle. There's no reason to have the whole airplane pointing up, just because the wings need to be facing a little upward. Whether it be for better comfort, visibility, or some other design purpose, the angle at which the wings are mounted to the airframe is the angle of incidence. The most important thing to remember about angle of incidence is not to confuse it with angle of attack.
Pilots control the angle of attack, but the angle of incidence is set by the design and construction of the airplane.