An aircraft is a vehicle which is able to fly by being supported by the air, or in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift (as with balloons, blimps and dirigibles) or by using the dynamic lift of an airfoil (as with vehicles that plane the air with wings in a straight manner, such as airplanes and gliders, or vehicles that generate lift with wings in a rotary manner, such as helicopters or gyrocopters)
Although rockets and missiles also travel through the atmosphere, most are not considered aircraft because they use rocket thrust instead of aerodynamics as the primary means of lift (A cruise missile may be considered to be an aircraft because it relies on a lifting wing).
The human activity which surrounds aircraft is called aviation. Manned aircraft are flown by an onboard pilot. Unmanned aerial vehicles may be remotely controlled or self-controlled by onboard computers. Target drones are an example of UAVs.
Classification by method of lift
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 fixed-wing aircraft or non-rigid balloons large enough to be called airships, so "airship" came to be synonymous with these aircraft. 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 fixed-wing aircraft 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, air must flow over the wing and generate lift. 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 mode 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
Airplanes or aeroplanes are technically called fixed-wing aircraft.
The forerunner of the fixed-wing aircraft is the kite. Whereas a fixed-wing aircraft 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 gliders. A glider designed by Cayley carried out the first true manned, controlled flight in 1853.
Besides the method of propulsion, fixed-wing aircraft are generally characterized by their wing configuration. The most important wing characteristics are:
* Number of wings – 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 (including 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.
Most fixed-wing aircraft feature a tail unit or empennage incorporating vertical, and often horizontal, stabilising surfaces.
Seaplanes are aircraft that land on water, and they fit into two broad classes: Flying boats are supported on the water by their fuselage. A float plane's fuselage remains clear of the water at all times, the aircraft being supported by two or more floats attached to the fuselage and/or wings. Some examples of both flying boats and float planes are amphibious, being able to take off from and alight on both land and water.
Some people consider wing-in-ground-effect vehicles to be fixed-wing aircraft, others do not. These craft "fly" close to the surface of the ground or water. An example is the Russian ekranoplan (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.
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 fixed-wing aircraft 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.
Other methods of lift
* A lifting body is the opposite of a flying wing. In this configuration the aircraft body is shaped to produce lift. If there are any wings, they are too small to provide significant lift and are used only for stability and control. Lifting bodies are not efficient: they suffer from high drag, and must also travel at high speed to generate enough lift to fly. Many of the research prototypes, such as the Martin-Marietta X-24, which led up to the Space Shuttle were lifting bodies (though the shuttle itself is not), and some supersonic missiles obtain lift from the airflow over a tubular body.
* Powered lift types rely on engine-derived lift for vertical takeoff and landing (VTOL). Most types transition to fixed-wing lift for horizontal flight. Classes of powered lift types include VTOL jet aircraft (such as the Harrier jump-jet) and tiltrotors (such as the V-22 Osprey), among others. A few examples rely entirely on engine thrust to provide lift throughout the flight. There are few practical applications. Experimental designs have been built for personal fan-lift hover platforms and jetpacks or for VTOL research (for example the flying bedstead).
* The FanWing is a recent innovation and represents a completely new class of aircraft. This uses a fixed wing with a cylindrical fan mounted spanwise just above. As the fan spins, it creates an airflow backwards over the upper surface of the wing, creating lift. The fan wing is (2005) in development in the United Kingdom.
Classification by propulsion
Heavier-than-air unpowered aircraft such as gliders (i.e. sailplanes), hang gliders and paragliders, and other gliders usually do not employ propulsion once airborne. Take-off may be by launching forwards and downwards from a high location, or by pulling into the air on a tow-line, by a ground-based winch or vehicle, or by a powered "tug" aircraft. 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). Some gliders can 'soar'- gain height from updrafts such as thermal currents. The first practical, controllable example was designed and built by the British scientist and pioneer George Cayley, who many recognise as the first aeronautical engineer.
Balloons drift with the wind, though normally the pilot can control the altitude, either by heating the air or by releasing ballast, giving some directional control (since the wind direction changes with altitude). A wing-shaped hybrid balloon can glide directionally when rising or falling; but a spherically shaped balloon does not have such directional control.
Kites are aircraft that are tethered to the ground or other object (fixed or mobile) that maintains tension in the tether or kite line; they rely on virtual or real wind blowing over and under them to generate lift and drag. Kytoons are balloon kites that are shaped and tethered to obtain kiting deflections, and can be lighter-than-air, neutrally buoyant, or heavier-than air.
A propeller or airscrew comprises a set of small, wing-like aerofoils set around a central hub which spins on an axis aligned in the direction of travel. Spinning the propeller creates aerodynamic lift, or thrust, in a forward direction.
A tractor design mounts the propeller in front of the power source, while a pusher design mounts it behind. Although the pusher design allows cleaner airflow over the wing, tractor configuration is more common because it allows cleaner airflow to the propeller and provides a better weight distribution.
A contra-prop arrangement has a second propeller close behind the first one on the same axis, which rotates in the opposite direction.
A variation on the propeller is to use many broad blades to create a fan. Such fans are traditionally surrounded by a ring-shaped fairing or duct, as ducted fans.
Many kinds of power plant have been used to drive propellers.
The earliest designs used man power to give dirigible balloons some degree of control, and go back to Jean-Pierre Blanchard in 1784. Attempts to achieve heavier-than-air man-powered flight did not succeed fully until Paul MacCready's Gossamer Condor in 1977.
The first powered flight was made in a steam-powered dirigible by Henri Giffard in 1852. Attempts to marry a practical lightweight steam engine to a practical fixed-wing airframe did not succeed until much later, by which time the internal combustion engine was already dominant.
From the first controlled powered fixed-wing aircraft flight by the Wright brothers until World War II, propellers turned by the internal combustion piston engine were virtually the only type of propulsion system in use. (See also: Aircraft engine.) The piston engine is still used in the majority of smaller aircraft produced, since it is efficient at the lower altitudes and slower speeds suited to propellers.
Turbine engines need not be used as jets (see below), but may be geared to drive a propeller in the form of a turboprop. Modern helicopters also typically use turbine engines to power the rotor. Turbines provide more power for less weight than piston engines, and are better suited to small-to-medium size aircraft or larger, slow-flying types. Some turboprop designs (see below) mount the propeller directly on an engine turbine shaft, and are called propfans.
Since the 1940s, propellers and propfans with swept tips or curved "scimitar-shaped" blades have been studied for use in high-speed applications so as to delay the onset of shockwaves, in similar manner to wing sweepback, where the blade tips approach the speed of sound. The Airbus A400M turboprop transport aircraft is expected to provide the first production example: note that it is not a propfan because the propellers are not mounted direct on the engine shaft but are driven through reduction gearing.
Other less common power sources include:
* Electric motors, often linked to solar panels to create a solar-powered aircraft.
* Rubber bands, wound many times to store energy, are mostly used for flying models.
Air-breathing jet engines provide thrust by taking in air, burning it with fuel in a combustion chamber, and accelerating the exhaust rearwards so that it ejects at high speed. The reaction against this acceleration provides the engine thrust.
Jet engines can provide much higher thrust than propellers, and are naturally efficient at higher altitudes, being able to operate above 40,000 ft (12,000 m). They are also much more fuel-efficient at normal flight speeds than rockets. Consequently, nearly all high-speed and high-altitude aircraft use jet engines.
The early turbojet and modern turbofan use a spinning turbine to create airflow for takeoff and to provide thrust. Many, mostly in military aviation, use afterburners which inject extra fuel into the exhaust.
Use of a turbine is not absolutely necessary: other designs include the crude pulse jet, high-speed ramjet and the still-experimental supersonic-combustion ramjet or scramjet. These designs require an existing airflow to work and cannot work when stationary, so they must be launched by a catapult or rocket booster, or dropped from a mother ship.
The bypass turbofan engines of the Lockheed SR-71 were a hybrid design – the aircraft took off and landed in jet turbine configuration, and for high-speed flight the afterburner was lit and the turbine bypassed, to create a ramjet.
The motorjet was a very early design which used a piston engine in place of the combustion chamber, similar to a turbocharged piston engine except that the thrust is derived from the turbine instead of the crankshaft. It was soon superseded by the turbojet and remained a curiosity.
The rotor of a helicopter, may, like a propeller, be powered by a variety of methods such as an internal-combustion engine or jet turbine. Tip jets, fed by gases passing along hollow rotor blades from a centrally mounted engine, have been experimented with. Attempts have even been made to mount engines directly on the rotor tips.
Helicopters obtain forward propulsion by angling the rotor disc so that a proportion of its lift is directed forwards to provide thrust.
Other forms of propulsion
* Rocket-powered aircraft have occasionally been experimented with, and the Messerschmitt Komet fighter even saw action in the Second World War. Since then they have been restricted to rather specialised niches, such as the Bell X-1 which broke the sound barrier or the North American X-15 which travelled up into space where no oxygen is available for combustion (rockets carry their own oxidant). Rockets have more often been used as a supplement to the main powerplant, typically to assist takeoff of heavily loaded aircraft, but also in a few experimental designs such as the Saunders-Roe SR.53 to provide a high-speed dash capability.
* The flapping-wing ornithopter is a category of its own. These designs may have potential, but no practical device has been created beyond research prototypes, simple toys, and a model hawk used to freeze prey into stillness so that it can be captured.
Classification by use
The major distinction in aircraft types is between military aircraft, which includes not just combat types but many types of supporting aircraft, and civil aircraft, which include all non-military types.
A military aircraft is any fixed-wing or rotary-wing aircraft that is operated by a legal or insurrectionary armed service of any type. Military aircraft can be either combat or non-combat:
* Combat aircraft are aircraft designed to destroy enemy equipment using its own armament.
* Non-Combat aircraft are aircraft not designed for combat as their primary function, but may carry weapons for self-defense. Mainly operating in support roles.
Combat aircraft divide broadly into fighters and bombers, with several in-between types such as fighter-bombers and ground-attack aircraft (including attack helicopters).
Other supporting roles are carried out by specialist patrol, search and rescue, reconnaissance, observation, transport, training and Tanker aircraft among others.
Many civil aircraft, both fixed-wing and rotary, 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. 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 military gliders were used during World War II to land troops.
Civil aircraft divide into commercial and general types, however there are some overlaps.
Commercial aircraft include types designed for scheduled and charter airline flights, carrying both passengers and cargo. The larger passenger-carrying types are often referred to as airliners, the largest of which are wide-body aircraft. Some of the smaller types are also used in general aviation, and some of the larger types are used as VIP aircraft.
General aviation is a catch-all covering other kinds of private and commercial use, and involving a wide range of aircraft types such as business jets (bizjets), trainers, homebuilt, aerobatic types, racers, gliders, warbirds, firefighters, medical transports, and cargo transports, to name a few. The vast majority of aircraft today are general aviation types.
Within general aviation, there is a further distinction between private aviation (where the pilot is not paid for time or expenses) and commercial aviation (where the pilot is paid by a client or employer). The aircraft used in private aviation are usually light passenger, business, or recreational types, and are usually owned or rented by the pilot. The same types may also be used for a wide range of commercial tasks, such as flight training, pipeline surveying, passenger and freight transport, policing, crop dusting, and medical evacuations. However the larger, more complex aircraft are more likely to be found in the commercial sector.
For example, piston-powered propeller aircraft (single-engine or twin-engine) are common for both private and commercial general aviation, but for aircraft such as turboprops like the Beechcraft King Air and helicopters like the Bell JetRanger, there are fewer private owners than commercial owners. Conventional business jets are most often flown by paid pilots, whereas the new generation of smaller jets are being produced for private pilots.
Experimental aircraft are one-off specials, built to explore some aspect of aircraft design and with no other useful purpose. The Bell X-1 rocket plane, which first broke the sound barrier in level flight, is a famous example.
The formal designation of "experimental aircraft" also includes other types which are "not certified for commercial applications", including one-off modifications of existing aircraft such as the modified Boeing 747 which NASA uses to ferry the space shuttle from landing site to launch site, and aircraft homebuilt by amateurs for their own personal use.
A model aircraft is a small unmanned type made to fly for fun, for static display, for aerodynamic research (cf Reynolds number) or for other purposes. A scale model is a replica of some larger design.