An aircraft is a vehicle which is able to fly through the Earth's atmosphere or through any other atmosphere. Most rocket vehicles are not aircraft because they are not supported by the surrounding air. All the human activity which surrounds aircraft is called aviation.
Manned aircraft are flown by a pilot. Until the 1960s, unmanned aircraft were called drones. During the 1960s, the U.S. military brought the term remotely piloted vehicle (RPV) into use. More recently the term unmanned aerial vehicle (UAV) has become common.
Kinds of aircraft
Aircraft fall into two categories: lighter-than-air (aerostats), and heavier-than-air (aerodynes).
Lighter than air—aerostats
Aerostats use buoyancy to float in the air in much the same way that ships float on the water. They are characterized by one or more large gasbags or canopies, filled with a relatively low density gas such as helium, hydrogen or hot air, which is less dense than the surrounding air. When the weight of this is added to the weight of the aircraft structure, it adds up to the same weight as the air that the craft displaces.
Small hot air balloons called sky lanterns date back to the 3rd century BC and were only the second type of aircraft to fly, the first being kites.
Originally a "balloon" was any aerostat, while the term "airship" was used for large powered aircraft designs—usually fixed-wing—though none had yet been built. The advent of powered balloons, called dirigible balloons, and later of rigid hulls allowing a great increase in size, began to change the way these words were used. Huge powered aerostats, characterized by a rigid outer framework and separate aerodynamic skin surrounding the gas bags, were produced, the Zeppelins being the largest and most famous. There were still no aeroplanes or non-rigid balloons large enough to be called airships, so "airship" came to be synonymous with these monsters. Then several accidents, such as the Hindenburg disaster in 1937, led to the demise of these airships. Nowadays a balloon is an unpowered aerostat, whilst an airship is a powered one.
A powered, steerable aerostat is called a dirigible. Sometimes this term is applied only to non-rigid balloons, and sometimes dirigible balloon is regarded as the definition of an airship (which may then be rigid or non-rigid). Non-rigid dirigibles are characterized by a moderately aerodynamic gasbag with stabilizing fins at the back. These soon became known as blimps. During the Second World War, this shape was widely adopted for tethered balloons; in windy weather this both reduces the strain on the tether and stabilizes the balloon. The nickname blimp was adopted along with the shape. In modern times any small dirigible or airship is called a blimp, though a blimp may be unpowered as well as powered.
Heavier than air—aerodynes
Heavier-than-air aircraft must find some way to push air or gas downwards, so that a reaction occurs (by Newton's laws of motion) to push the aircraft upwards. This dynamic movement through the air is the origin of the term aerodyne. There are two ways to produce dynamic upthrust: aerodynamic lift, and powered lift in the form of engine thrust.
Aerodynamic lift is the most common, with aeroplanes being kept in the air by the forward movement of wings, and rotorcraft by spinning wing-shaped rotors sometimes called rotary wings. A wing is a flat, horizontal surface, usually shaped in cross-section as an aerofoil. To fly, the wing must move forwards through the air; this movement of air over the aerofoil shape deflects air downward to create an equal and opposite upward force, called lift, according to Newton's third law of motion. A flexible wing is a wing made of fabric or thin sheet material, often stretched over a rigid frame. A kite is tethered to the ground and relies on the speed of the wind over its wings, which may be flexible or rigid, fixed or rotary.
With powered lift, the aircraft directs its engine thrust vertically downwards.
The initialism VTOL (vertical take off and landing) is applied to aircraft that can take off and land vertically. Most are rotorcraft. Others, such as the Hawker Siddeley Harrier, take off and land vertically using powered lift and transfer to aerodynamic lift in steady flight. Similarly, STOL stands for short take off and landing. Some VTOL aircraft often operate in a short take off/vertical landing regime known as STOVL.
A pure rocket is not usually regarded as an aerodyne, because it does not depend on the air for its lift (and can even fly into space), however many aerodynamic lift vehicles have been powered or assisted by rocket motors. Rocket-powered missiles which obtain aerodynamic lift at very high speed due to airflow over their bodies, are a marginal case.
Aeroplanes or airplanes are technically called fixed-wing aircraft.
The forerunner of the aeroplane is the kite. Whereas an aeroplane relies on its forward speed to create airflow over the wings, a kite is tethered to the ground and relies on the wind blowing over its wings to provide lift. Kites were the first kind of aircraft to fly, and were invented in China around 500 BC. Much aerodynamic research was done with kites before test aircraft, wind tunnels and computer modelling programs became available.
The first heavier-than-air craft capable of controlled free flight were unpowered aeroplanes or gliders. A glider designed by Cayley carried out the first true manned, controlled flight in 1853.
Besides the method of propulsion, aeroplanes are generally characterized by their wing configuration. The most important wing characteristics are:
- Number of planes - Monoplane, biplane, etc.
- Wing support - Braced or cantilever, rigid or flexible.
- Wing planform - including aspect ratio, angle of sweep and any variations along the span. Includes the important class of delta wings.
- Location of the horizontal stabiliser, if any.
- Dihedral angle - positive, zero or negative (anhedral).
A variable geometry aircraft can change its wing configuration during flight.
A flying wing has no fuselage, though it may have small blisters or pods. The opposite of this is a lifting body which has no wings, though it may have small stabilising and control surfaces.
Seaplanes and Floatplanes differ in that a seaplane has the bottom of its fuselage shaped hydrodynamically and it sits directly on the water when at rest, while a floatplane has two or more floats attached below the rest of the aircraft so that the fuselage remains clear of the water at all times.
Some people consider wing-in-ground-effect vehicles to be aeroplanes, others do not. These craft "fly" close to the surface of the ground or water. An example is the Russian ekranoplan also nicknamed the "Caspian Sea Monster". Man-powered aircraft also rely on ground effect to remain airborne, but this is only because they are so underpowered - the airframe is theoretically capable of flying much higher. (Hovercraft are not considered to be aircraft, since they rely wholly on the pressure of air on the ground beneath, and have no aerodynamic lifting surface).
Rotorcraft
Rotorcraft, or rotary-wing aircraft, use a spinning rotor with aerofoil section blades (a rotary wing) to provide lift. Types include helicopters, autogyros and various hybrids such as gyrodynes and compound rotorcraft.
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.
Autogyros or gyroplanes have unpowered rotors, with a separate power plant to provide thrust. The rotor is tilted backwards. As the autogyro moves forward, air blows upwards through it, making it spin.(cf. Autorotation)
This spinning dramatically increases the speed of airflow over the rotor, to provide lift. Juan de la Cierva (a Spanish civil engineer) used the product name autogiro, and Bensen used gyrocopter. Rotor kites, such as the Focke Achgelis Fa 330 are unpowered autogyros, which must be towed by a tether to give them forward ground speed or else be tether-anchored to a static anchor in a high-wind situation for kited flight.
Gyrodynes are a form of helicopter, where forward thrust is obtained from a separate propulsion device rather than from tilting the rotor. The definition of a 'gyrodyne' has changed over the years, sometimes including equivalent autogyro designs. The most important characteristic is that in forward flight air does not flow significantly either up or down through the rotor disc but primarily across it. The Heliplane is a similar idea.
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. The whole aircraft would then have tilted forward to fly as a propeller-driven aeroplane using the duct as a wing (though this transition was never achieved in practice.)
Some rotorcraft have reaction-powered rotors with gas jets at the tips, but most have one or more lift rotors powered from engine-driven shafts.
