How Planes Fly Quiz
Four forces
Four forces act on a plane in flight. Weight pulls it down. Lift, made by the wings, pushes it up. Thrust, from the engines, drives it forward. Drag, the push of the air, holds it back.
To take off, a plane needs enough thrust to speed it up until the wings make more lift than the plane weighs. In steady, level flight the forces balance: lift equals weight and thrust equals drag.
Jet engines make thrust by sucking in air, squeezing it, burning fuel in it and blasting it out of the back. Propellers do it by pushing air backward with spinning blades.
How wings make lift
A wing is shaped and tilted so that it turns the air flowing past it downward. Pushing air down pushes the wing up, just as Newton's third law says: every action has an equal and opposite reaction.
The same flow also makes the air pressure lower above the wing than below it, which pulls and pushes the wing upward. Both are two ways of describing the same lift.
Tilting the wing up a little more, called raising the angle of attack, makes more lift, but only up to a point. Tilt too far and the air stops flowing smoothly over the top, and the wing stalls.
Steering and cruising
Pilots steer with hinged parts on the wings and tail. Ailerons on the wings roll the plane to bank left or right. The elevator on the tail tips the nose up or down. The rudder on the tail turns the nose left or right.
Big airliners cruise about 10 to 12 km up, where the air is thin. Thinner air means less drag, so the plane uses less fuel, and it's above most of the weather.
Flaps that slide out from the back of the wings add lift at low speed, which is why you see them extended during takeoff and landing.