How does a plane fly?

Flight begins with the wing. Its cross-section, the aerofoil, is curved on top and set at a slight angle to the oncoming air. As the aircraft moves forward, the wing deflects the airflow downward and, in the same motion, the air over the upper surface speeds up and its pressure drops. The higher pressure beneath and the lower pressure above produce a net upward force: lift. The faster the wing moves, the more lift it generates, in proportion to the square of the speed. That is why an aircraft must accelerate along a runway before it can leave the ground, and why it can carry more when it is flying fast in dense, cold air than when it is slow or in thin air.

The wing only works if something keeps it moving, and that is the job of the engines. A propeller accelerates a wide column of air backward; a turbofan does the same with a large fan and adds a hot exhaust jet. In both cases the reaction to pushing air backward drives the aircraft forward. Thrust must overcome drag, which comes from the friction of air over the surfaces, the pressure of the air being pushed aside and the cost of generating lift itself. Once thrust exceeds drag the aircraft accelerates; once they are equal it holds a steady speed.

An aircraft is controlled by shaping the airflow rather than by pushing against anything solid. Hinged ailerons near the wingtips move in opposite directions to roll the aircraft and bank it into a turn. The elevator on the tailplane pitches the nose up or down, changing the wing's angle of attack and therefore its lift. The rudder on the fin yaws the nose left or right and keeps turns coordinated. Flaps extend from the trailing edge to add lift at low speed for takeoff and landing, and spoilers rise from the wing to dump lift and slow the aircraft on the ground.

A flight ties these elements together. On the runway the engines run up to full power and the aircraft accelerates until the wings can lift it, typically at 250 to 300 km/h for an airliner. Climbing, the pilot holds a nose-up attitude so that lift exceeds weight. In cruise, everything settles into balance. On approach, the flaps come out, power is reduced and the aircraft descends along a shallow path, touching down at the lowest safe speed before brakes, spoilers and reverse thrust bring it to a halt.