What is Lift? The motion of a wing moving through the air causes the lift force that counteracts the effects of the weight force. In doing so, lift keeps the aircraft in the air instead of crashing to the ground. Sources of Lift The airfoil is the cross-sectional design of a wing; the familiar teardrop shape that you think of when you look at a wing from the side. A common misconception is that lift is caused by one factor, explained by Bernoulli's Principle. According to Bernoulli's Principle, the pressure of a fluid (in this case, air) decreases as its speed increases. When air hits the leading edge (rounded edge) of the airfoil, the airflow above the wing travels faster than the flow below the wing. Therefore, the air pressure below the wing is higher than the air pressure above the wing. Because of this difference in pressure, there is a force known as lift, that pushes the wings upward. [View the explanation] However, this is not the only source of lift. The wings have the ability to deflect air downwards. This deflection comes from the fact that the airflow going over the airfoil has to curve up then down. The actual shape of the airfoil pulls the airflow so that it follows its curves. If the airflow follows the curves, then the air is accelerated downwards after it passes by the wing. According to Newton's Third Law of Motion, as the wing pushes the airflow downwards, the airflow pushes against the wing upwards! Although this is a major reason why there is any lift at all on an airplane, many people fail to recognize this as a source of lift. However it is important to keep in mind that each of these "sources" of lift are actually all different aspects of the same lift creating process. Angle of Attack The angle of attack is the angle at which the wings meet the airflow. This is an adjustable factor, and it is directly related to the lift, up to a certain point. The angle of attack is another factor in deflecting air down. As we all know, this deflection will cause a reactive force: lift. Up to a certain point, increasing the angle of attack (usually within 3-15°) increases the amount of lift. But once you get over that, the air cannot flow smoothly over the wings and they break off into mini whirlpools of air. This greatly reduces the lift force, and increases the drag force leading to what is known as a stall, where the plane falls straight toward the ground. Lift Factors A faster speed generates more lift. There is more air flowing across the wing, and therefore more air is being deflected. However during takeoff and landing, pilots want to fly as slow as possible. This is where special high lift devices are needed to create more lift to compensate for the lift lost when the airspeed decreased. Flaps and slats are moveable devices that increase the surface area of the wing during takeoff and landing to create more lift. The increased surface area means that more air "encounters" the wing and is deflected. A slot is an opening on a wing that evenly flows the air around a wing, even at higher than normal angles of attack so that the plane does not stall. This steeper angle also contributes to the creating of extra lift to keep the airplane in the air at low speeds.