Wings, Slats, and Flaps: Keeping Airplanes in the Air
To understand how an airplane’s wings keep it in the sky, we first have to understand the forces influencing all aircraft - thrust, drag, lift, and weight. Thrust is the force that propels the aircraft forward provided by its engines, both piston and jet turbine. Drag is the force trying to pull the aircraft back, generated as the aircraft displaces the air in front and around it. Lift is the force generated by the airplane’s wings and the flow of air over and below them, while finally weight is the force of the Earth’s gravity trying to pull the plane back to the ground.
An airplane’s wings generate lift as they move through the air, but how exactly? They do so for two reasons: Bernoulli’s principle, and the shape of the wing itself, also known as the airfoil. Bernoulli’s principle, named after the Swiss mathematician that discovered and described the phenomenon in 1738, states that as air speeds up, its pressure drops. An aircraft airfoil is shaped deliberately so that air going over the wing will be going faster than the air going under it. This means that the faster air going over the airfoil exerts less pressure than the high-pressure, slower-moving air going underneath it. This results in lift that pushes the airfoil, and the rest of the airplane, up.
Typically, a plane’s airfoil will be designed to provide enough lift to keep the plane level while flying at its cruising speed. But what about when a plane is taking off and landing? In those situations, the plane’s flaps and slats come into play to alter its airfoil and generate more lift.
During takeoff and landing, the flaps located on the back of the wing extend downward from the trailing edge, altering the shape of the wing’s airfoil and generating more lift. This altered shape also generates more drag; this is useful for slowing down during landing, but it also means takeoff requires more thrust than normal. Slats also work on the same principle of altering the wing’s shape but are located on the leading edge of the aircraft. They are also used during takeoff and landing.