An airplane's wings make lift, the force that holds the plane up in the air. Lift is the part of the air's force on a wing that points at right angles to the flow of air. Lift can be explained in two ways that describe the same force. One way looks at the difference in air pressure above and below the wing, while the other looks at the wing pushing air downward.

Air pressure under a lifting wing is greater than the pressure above it, and the difference pushes the wing up. Lift is also made when a wing turns the air. The air is bent in one direction and the wing is pushed in the opposite direction, as Newton's third law says. A wing pushes air downward, and the more downwash a wing makes, the harder it pushes the mass of air down.

A plane has to move for its wings to make lift, and a plane that is not moving through the air makes no lift. Lift only works where there is air. Planes that fly faster get more lift: lift grows with the square of the plane's speed. Because air gets thinner as a plane climbs, a plane must fly faster at high altitude to keep the same lift.

Flaps and slats let a plane keep enough lift while flying slowly at takeoff and landing. A slat is a movable part on the front edge of a wing, and a flap is a movable part on the back edge. On takeoff and landing, flaps tilt down to give the wings more lift. Flaps also add drag, and that helps a plane slow down for landing. After the wheels touch down, spoilers pop up on top of the wings. They spoil the lift, which presses the plane onto the runway so its brakes work better.

Lift pushes a plane up, weight pulls it down, thrust pushes it forward and drag pulls it back: these are the four forces of flight. Drag is the part of the air's force on a plane that points along the flow, against the plane's motion. A plane needs enough lift from its wings to beat its weight, and enough thrust from its engines to beat its drag.

Thrust comes from pushing gas backward. The engine pushes gas to the rear, and the gas pushes the engine forward. A turbofan has a big fan at the front. Some of the air goes through the engine's core and some goes around it, and both streams add thrust. Some airplanes use propellers, instead of jets, to push them through the air.

The Wright brothers, Orville and Wilbur, made the first flight of a powered airplane in 1903. They picked Kitty Hawk, North Carolina, for their tests. They tested gliders there for three years before they tried a plane with an engine. The first flight, on December 17, 1903, lasted only 12 seconds, and the Wright Flyer went 120 feet.

Words to know

• downwash: air pushed down
• flaps: back wing parts
• slat: front wing part
• drag: air pushing back
• turbofan: engine with fan