An airplane is a powered, flying machine that uses aerodynamic laws to stay in the air. To fly, the plane must balance four forces – lift, thrust, drag, and weight. Airplanes use wings to create lift, which pushes them upwards, engines to generate thrust, which moves them forwards, and rudders to turn them in the direction of travel. A pilot, or flight crew, operates the aircraft from a cockpit in the front of the fuselage.

Airplanes are built from wood or metal and are equipped with windows and controls. Some are powered by internal combustion, while others use hydrocarbon jet engines. All require a large fuel tank for propellant, which is stored under the wings. The plane’s body, or fuselage, is long and thin with tapered or rounded ends for an aerodynamically smooth shape. It holds the cockpit, passengers, cargo, and fuel. The fuselage may be made of one piece or have multiple sections that are joined together.

The main section of the fuselage, called the empennage, contains the wings, tailplane and elevators that act horizontally, and the vertical fin and rudder. In most fixed-wing aircraft, the empennage also includes flaps that can change the plane’s pitch (angle of attack) for maneuvering and reducing the effects of wind at different speeds. In rotary-wing aircraft, the tailplane and elevator are located at the rear of the fuselage.

Aerodynamically, the shape of the wing is very important. The wing must be of a very thin cross-section to provide the required amount of lift with a relatively small area. A wing must also be of a very long span from side to side, with a short chord, to be structurally efficient. At transonic speed, it helps to sweep the wings backwards or forwards (aerodynamically known as wing incidence) to reduce drag caused by supersonic shock waves.

Several scientific accounts of how wings generate lift have been developed. Most treated the air as a perfect fluid with zero viscosity, which simplified the underlying mathematics. However, they failed to explain why the parcels of air moving across the top surface of the wing follow the downward curve of the wing’s airfoil.

The answer turns out to be a complex combination of factors, including Newton’s third law of motion: every action has an equal and opposite reaction. The higher pressure above the wing generates the lift, while the deflected downward air at the lower surface of the wing exerts an equal and opposite force on it.

Another interesting aspect of lifting air is that it tends to be stronger at lower speeds. This is because the air is less dense at lower temperatures, and therefore has a smaller radius of curvature. The lift generated is proportional to the square of the velocity.

Those puffy white lines that you see in the sky when you look at an airplane are actually water condensation from the engine exhaust. The water vapor is cooled by the cooler air of the upper atmosphere, causing it to expand. As it does so, it forms those trails that we call contrails.