The airplane has six main parts—fuselage, wings, stabilizer (or tail plane), rudder, one or more engines, and landing gear. The fuselage is the main body of the machine, customarily streamlined in form. It usually contains control equipment, and space for passengers and cargo. The wings are the main supporting surfaces. Modern airplanes are monoplanes (airplanes with one wing) and may be high-wing, mid-wing, or low-wing (relative to the bottom of the fuselage). At the trailing edge of the wings are auxiliary hinged surfaces known as ailerons that are used to gain lateral control and to turn the airplane.

The lift of an airplane, or the force that supports it in flight, is basically the result of the direct action of the air against the surfaces of the wings, which causes air to be accelerated downward. The lift varies with the speed, there being a minimum speed at which flight can be maintained. This is known as the stall speed. Because speed is so important to maintain lift, objects such as fuel tanks and engines, that are carried outside the fuselage are enclosed in structures called nacelles, or pods, to reduce air drag (the retarding force of the air as the airplane moves through it).

Directional stability is provided by the tail fin, a fixed vertical airfoil at the rear of the plane. The stabilizer, or tail plane, is a fixed horizontal airfoil at the rear of the airplane used to suppress undesired pitching motions. To the rear of the stabilizer are usually hinged the elevators, movable auxiliary surfaces that are used to produce controlled pitching. The rudder, generally at the rear of the tail fin, is a movable auxiliary airfoil that gives the craft a yawing (turning about a vertical axis) movement in normal flight. The rear array of airfoils is called the empennage, or tail assembly. Some aircraft have additional flaps near the ailerons that can be lowered during takeoff and landing to augment lift at the cost of increased drag. On some airplanes hinged controls are replaced or assisted by spoilers, which are ridges that can be made to project from airfoils.

Airplane engines may be classified as driven by propeller, jet, turbojet, or rocket. Most engines originally were of the internal-combustion, piston-operated type, which may be air- or liquid-cooled. During and after World War II, duct-type and gas-turbine engines became increasingly important, and since then jet propulsion has become the main form of power in most commercial and military aircraft. The landing gear is the understructure that supports the weight of the craft when on the ground or on the water and that reduces the shock on landing. There are five common types—the wheel, float, boat, skid, and ski types.

Early attempts were made to build flying machines according to the principle of bird flight, but these failed; it was not until the beginning of the 20th cent. that flight in heavier-than-air craft was achieved. On Dec. 17, 1903, the Wright brothers produced the first manned, power-driven, heavier-than-air flying machine near Kitty Hawk, N.C. The first flight lasted 12 sec, but later flights on the same day were a little longer; a safe landing was made after each attempt. The machine was a biplane (an airplane with two main supporting surfaces, or wings) with two propellers chain-driven by a gasoline motor.

The evolution of the airplane engine has had a major effect upon aircraft design, which is closely associated with the ratio between power load (horsepower) and weight. The Wright brothers' first engine weighed about 12 lb (5.4 kg) per horsepower. The modern piston engine weighs about 1 lb (0.4 kg) or less per horsepower, and jet and gas-turbine engines are much lighter. With the use of jet engines and the resulting higher speeds, airplanes have become less dependent on large values of lift from the wings. Consequently, wings have been shortened and swept back so as to produce less drag, especially at supersonic speeds. In some cases these radically backswept wings have evolved into a single triangular lifting surface, known as a delta wing, that is bisected by the fuselage of the plane. Similar alterations have been made in the vertical and horizontal surfaces of the tail, again with the aim of decreasing drag.

For certain applications, e.g., short-haul traffic between small airports, it is desirable to have airplanes capable of operating from a runway of minimum length. Two approaches to the problem have been tried. One, the vertical takeoff and landing (VTOL) approach, seeks to produce craft that take off and land like helicopters, but that can fly much faster. The other approach, short takeoff and landing (STOL), seeks to design more conventional aircraft that have reduced runway requirements. The lessened lift associated with swept-back wing designs increases the length of runway needed for takeoffs and landings. To keep runway lengths within reasonable limits the variable-sweep, or swing, wing has been developed. A plane of this type can extend its wings for maximum lift in taking off and landing, and swing them back for travel at high speeds.

A proposed variant of the swing wing, in which one wing sweeps to the rear and another forward, produces an arrangement that causes a minimum shock wave at supersonic speeds. It is thought that if this modification were applied to supersonic transport (SST) designs it would somewhat lessen their objectionable noise levels. No solution has been proposed to lessen their high fuel consumption, however. Recent developments in fan-jet engines, in which a turbine powers a set of vanes that drive air rearward to augment thrust, have made supersonic flight possible at low altitude. Much research has also gone into reducing the noise and air pollution caused by jet engines.

See aerodynamics; airport; aviation; autogiro; glider; seaplane.