An aircraft is a vehicle which is able to fly through the air (or through any other atmosphere). All the human activity which surrounds aircraft is called aviation. Aeroplanes are fixed-wing aircraft. They generally use some form of propulsion, to provide thrust that moves the craft forward through the air. Gliders are an exception. They have no engine but gain their initial thrust from some launch mechanism, and then gain energy from gravity and thermal currents. For a glider to maintain its forward air speed and lift, it must descend in relation to the air (but not necessarily in relation to the ground). The first practical example of which was designed and built by the British scientist and pioneer George Cayley who is universally recognised as the first aeronautical engineer. The forerunner of the aeroplane is the kite. Kites depend upon the tension between the cord which anchors it to the ground and the force of the wind currents. Much aerodynamic work was done with kites before test aircraft, wind tunnels and now computer modelling programs became available.
Most low-speed aeroplanes have a straight wing. For flight near or above the speed of sound, a swept wing is usually used, where the wing angles backwards towards the tips (though forward sweep is occasionally experimented with). A notable variation is the delta wing, which is shaped like a triangle: the leading edge is sharply swept, but the trailing edge is straight; an especially graceful form this is the double-curved ogival delta found on Concorde. Another variation is the crescent wing, seen for example on the Handley Page Victor, which is sharply swept inboard, with reduced sweep for the outboard section. A variable-geometry wing, or swing-wing, can change the angle of sweep in flight. It has been employed in a few examples of combat aircraft, such as the F-111, Panavia Tornado, F-14 Tomcat and B-1 Lancer, among others.
Helicopters have powered rotors. The rotor is driven (directly or indirectly) by an engine and pushes air downwards to create lift. By tilting the rotor forwards, the downwards flow is tilted backwards, producing thrust for forward flight. Compound rotorcraft have wings which provide some or all of the lift in forward flight. Compound helicopters and compound autogyros have been built, and some forms of gyroplane may be referred to as compound gyroplanes. Tiltrotor aircraft (such as the V-22 Osprey) have their rotors horizontal for vertical flight, and pivot the rotors vertically like a propeller for forward flight. The Coleopter had a cylindrical wing forming a duct around the rotor. On the ground it sat on its tail, and took off and landed vertically like a helicopter.
Combat aircraft like fighters or bombers represent only a minority of the category. Many civil aircraft have been produced in separate models for military use, such as the civil Douglas DC-3 airliner, which became the military C-47/C-53/R4D transport in the U.S. military and the "Dakota" in the UK and the Commonwealth. Even the small fabric-covered two-seater Piper J3 Cub had a military version, the L-4 liaison, observation and trainer aircraft. In the past, gliders and balloons have also been used as military aircraft; for example, balloons were used for observation during the American Civil War and World War I, and cargo gliders were used during World War II to land troops.
Combat aircraft themselves, though used a handful of times for reconnaissance and surveillance during the Italo-Turkish War, did not come into widespread use until the Balkan War. During World War I many types of aircraft were adapted for attacking the ground or enemy vehicles/ships/guns/aircraft, and the first aircraft designed as bombers were born. In order to prevent the enemy from bombing, fighter aircraft were developed to intercept and shoot down enemy aircraft. Tankers were developed after World War II to refuel other aircraft in mid-air, thus increasing their operational range. By the time of the Vietnam War, helicopters had come into widespread military use, especially for transporting, supplying, and supporting ground troops.
Civil aviation broadly divides into commercial and general activities, however there can be some overlap in practice, General aviation is a catch-all covering other kinds of private and commercial use. The vast majority of flights flown around the world each day belong to the general aviation category, which covers a wide range of activities such as business trips, civilian flight training, recreational balloon flying, firefighting, medical transport (medevac) flights, and cargo transportation on freight aircraft, to name a few.
