A fixed-wing aircraft is a kind of aircraft. An aircraft is a machine that can fly, but is heavier than air. Fixed-wing aircraft are sometimes called airplanes, aeroplanes or sometimes just "planes". All fixed-wing aircraft have wings that use forward airspeed to generate lift. Gliders are fixed-wing aircraft that do not have engines.
- Overall layout
- Wing types
- Image gallery
Many stories from antiquity involve flight, such as the Greek legend of Icarus and Daedalus, and the Vimana in ancient Indian epics. Around 400 BC in Greece, Archytas was reputed to have designed and built the first artificial, self-propelled flying device, a bird-shaped model propelled by a jet of what was probably steam, said to have flown some 200 m. This machine may have been suspended for its flight.
Some of the earliest recorded attempts with gliders were those by the 9th-century poet Abbas Ibn Firnas and the 11th-century monk Eilmer of Malmesbury; both experiments injured their pilots.
Leonardo da Vinci researched the wing design of birds and designed a man-powered aircraft in his Codex on the Flight of Birds (1502).
In 1799, Sir George Cayley set forth the concept of the modern aeroplane as a fixed-wing flying machine with separate systems for lift, propulsion, and control. Cayley was building and flying models of fixed-wing aircraft as early as 1803, and he built a successful passenger-carrying glider in 1853.
In 1856, Frenchman Jean-Marie Le Bris made the first powered flight, by having his glider "L'Albatros artificiel" pulled by a horse on a beach. In 1883, the American John J. Montgomery made a controlled flight in a glider. Other aviators who made similar flights at that time were Otto Lilienthal, Percy Pilcher, and Octave Chanute.
The first jet airliner, the de Havilland Comet, was introduced in 1952. The Boeing 707, the first widely successful commercial jet, was in commercial service for more than 50 years, from 1958 to 2010. The Boeing 747 was the world's biggest passenger aircraft from 1970 until it was surpassed by the Airbus A380 in 2005.
The most common configuration of a fixed-wing aircraft includes:
- A fuselage, a long, thin body, often cylindrical, and usually with tapered or rounded ends to make its shape aerodynamically smooth. The fuselage may contain the flight crew, passengers, cargo or payload, fuel and engines the aircraft is designed for or they may be attached to it. The pilots of manned aircraft operate them from a cockpit located at the front or top of the fuselage and equipped with controls and usually windows and instruments. An aircraft may have more than one fuselage, or it may be fitted with booms with the tail located between the booms to allow the extreme rear of the fuselage to be useful for a variety of purposes.
- A large horizontal wing with an airfoil cross-section shape. The wing deflects air downward as the aircraft moves forward, generating lifting force to support the aircraft in flight. The wing also stabilises the aircraft's roll (tilt left or right), and the wing-mounted ailerons control rotation about the roll axis. A wide variety of wing configurations (e.g., multiplane aircraft and delta wing planform) have been used.
- A vertical stabiliser a vertical surface mounted at the rear of the aircraft and typically protruding above it. The vertical stabilizer stabilises the aircraft's yaw (turn left or right) and mounts the rudder which controls its rotation along that axis.
- A horizontal stabiliser or elevator, or tailplane, mounted at the tail of the aircraft, near the vertical stabilizer. The horizontal stabilizer is used to stabilise the aircraft's pitch (tilt up or down) and mounts the elevators which provide pitch control. A fixed portion of the elevators may be omitted in which case it is termed an all flying tail. Some aircraft use a front-mounted canard instead of a rear-mounted horizontal stabilizer.
- Powered aircraft have one or more engines that provide thrust to push the aircraft forward through the air. The most common propulsion units are propellers (powered by reciprocating or turbine engines) and jet engines (which provide thrust directly from the engine and usually also from a large fan mounted within the engine).
- Landing gear, a set of wheels, skids, or floats that support the aircraft while it is on the surface. On seaplanes the bottom of the fuselage or floats (pontoons) support it while on the water. On some aircraft the landing gear retract during flight to reduce drag.
There are many different configurations of airplanes. An aircraft may have two or more fuselages, or additional pods or booms. Some aircraft have more than one horizontal or vertical stabilizer, while V-tail aircraft combine the horizontal and vertical stabilizers into a pair of diagonal surfaces. While all of the above items are essential - there have been aircraft flown that have dispensed with any one of the components listed, by modifying other components to fulfill the missing components function. A flying wing aircraft has no discernible fuselage structure and horizontal or vertical stabilizers, 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. Delta wing aircraft often dispense with the horizontal stabilizer and a few aircraft have even dispensed with the vertical stabilizer.
Most aircraft are largely symmetrical along a plane of symmetry, excepting the propeller and minor alterations to counteract the effects of the spinning propeller.
A number of controls allow pilots to direct aircraft in the air. The controls found in a typical fixed-wing aircraft are as follows:
- A yoke or joystick, which controls rotation of the aircraft about the pitch and roll axes. A yoke resembles a steering wheel, and a control stick is a joystick. The pilot can pitch the aircraft down by pushing on the yoke or stick, and pitch the aircraft up by pulling on it. Rolling the aircraft is accomplished by turning the yoke in the direction of the desired roll, or by tilting the control stick in that direction. Pitch changes are used to adjust the altitude and speed of the aircraft; roll changes assist the aircraft in turning in conjunction with the rudder. Control sticks and yokes are usually positioned between the pilot's legs; however, a sidestick is a type of control stick that is positioned on either side of the pilot.
- Rudder pedals, which control rotation of the aircraft about the yaw axis. There are two pedals that pivot so that when one is pressed forward the other moves backward, and vice versa. The pilot presses on the right rudder pedal to make the aircraft yaw to the right, and on the left pedal to make it yaw to the left. The rudder is used mainly to balance the aircraft in turns, or to compensate for winds or other effects that tend to turn the aircraft about the yaw axis. Several aircraft including the Ercoupe dispensed with rudder pedals by linking the rudders to the ailerons for simplicity.
- A Throttle or thrust lever for each engine. These control the power produced by the engines and hence airspeed. On piston-engine powered aircraft Engine Mixture Control levers will also be present.
- Brakes, used to slow and stop the aircraft on the ground, and sometimes for turns on the ground.
These were largely standardized during World War I - prior to which many aircraft manufacturers had their own systems.
Other controls can include:
- Flap levers, which are used to control the position of flaps on the wings.
- Spoiler levers, which are used to control the position of spoilers on the wings, and to arm their automatic deployment in aircraft designed to deploy them upon landing. The spoilers reduce lift for landing.
- Trim controls, which usually take the form of knobs or wheels and are used to adjust pitch, roll, or yaw trim. These are often connected to small airfoils on the trail edge of the control surfaces called 'trim tabs'. Trim is used to reduce the amount of pressure on the control forces needed to maintain a steady course.
- A tiller, a small wheel or lever used to steer the aircraft on the ground in conjunction with or instead of the rudder pedals (primarily found on larger aircraft).
- Undercarriage retraction levers, to raise or lower the undercarriage, for reduced drag while in flight.
- A parking brake, used to prevent the aircraft from rolling when it is parked on the ground.
The controls may allow full or partial automation of flight, such as an autopilot, a wing leveler, or a flight management system. Pilots adjust these controls to select a specific attitude or mode of flight, and then the associated automation maintains that attitude or mode until the pilot disables the automation or changes the settings. In general, the larger and/or more complex the aircraft, the greater the amount of automation available to pilots.
On an aircraft with a pilot and copilot, or instructor and trainee, the aircraft is made capable of control without the crew changing seats.
Instruments provide information to the pilot and the co-pilot. Flight instruments provide information about the aircraft's speed, direction, altitude, and orientation. Powerplant instruments provide information about the status of the aircraft's engines and APU. Systems instruments provide information about the aircraft's other systems, such as fuel delivery, electrical, and pressurisation. Navigation and communication instruments include all the aircraft's radios. Instruments may operate mechanically or electrically, requiring 12VDC, 24VDC, or 400 Hz power systems. An aircraft that uses computerised CRT or LCD displays almost exclusively is said to have a glass cockpit.
The Six Basic instruments (sometimes referred to as the six pack) include:
- An Airspeed Indicator, which indicates the speed at which the aircraft is moving through the surrounding air.
- An Altimeter, which indicates the altitude or height of the aircraft above mean sea level.
- A Heading indicator, (sometimes referred to as a "directional gyro (DG)"), which indicates the magnetic compass heading that the aircraft's fuselage is pointing towards. The actual direction the aircraft is flying towards is affected by the wind conditions.
- An Attitude indicator, sometimes called an artificial horizon, which indicates the exact orientation of the aircraft about its pitch and roll axes.
- A Vertical Speed Indicator, which shows the rate at which the aircraft is climbing or descending.
- A Turn Coordinator, or Turn and Bank Indicator which helps the pilot maintain the aircraft in a coordinated attitude while turning.
This section discusses the number of wings, which is commonly used means of distinguishing different types. A wing is a single unit that runs from one extremity (called a wingtip) to another. Any interruptions caused by a fuselage or engine is ignored for this purpose. This single wing unit generates lift by creating a high pressure area under it, and a low pressure area above it. When these meet at the wingtips, they mix (much like the water going down a drain), causing considerable drag. The more wings, and hence wingtips, an aircraft has, the more of this type of drag (Induced Drag) is created and so having the least number of wings possible is always the most efficient solution - however there are cases where more than one wing has been necessary.
Aside from the number of wings, the shape the wing forms when seen from above is also used as a distinguishing characteristic.
A monoplane is a fixed-wing aircraft with one main set of wing surfaces, in contrast to a biplane or triplane. Since the late 1930s it has been the most common form for a fixed wing aircraft. The earliest monoplanes were braced with wires running outside the wing, however lack of knowledge concerning the stresses a wing was subject to resulted in many failures, and in the United Kingdom the Royal Flying Corps banned their use.
The airfoil sections in use at the time were very thin, and could not have a cantilever structure installed within and so attempts were made to provide a stronger structure by adding an external truss - structurally a biplane, but lacking the lower wing. Unfortunately this adds a lot of drag. Junkers began experimenting with thicker airfoils, which had previously been ignored as being unlikely to be efficient. With the thicker airfoils, a cantilever structure contained entirely within the wing was possible, and it had the added bonus that thick airfoils were more efficient than the thin ones previously in use. The defeat of Germany (where all of the research into cantilever monoplanes was occurring) and conservatism in the aviation industry ensured that biplanes would continue to dominate for the next 20 years.
Monoplanes can be differentiated in where the wings attach to the fuselage: The actual point of attachment is called the wing root.
- low-wing, the wing lower surface is level with (or below) the bottom of the fuselage
- mid-wing, the wing is mounted mid-way up the fuselage
- shoulder wing, the wing is mounted above the fuselage middle
- high-wing, the wing upper surface is level with or above the top of the fuselage
- parasol-wing, the wing is located above the fuselage with structural support being typically provided by a system of struts, and, especially in the case of older aircraft, wire bracing. These were particularly popular in the late 1920s and 1930s.
- gull wing is similar to a parasol wing, but has the roots of the wing drop down to pass the structure through the fuselage reducing the number of struts needed. A variant of the gull wing is the inverted gull wing, in which the lower wing takes the form of a "W" from the front, with the middle peak attached to the fuselage. This has the advantages of shortening the undercarriage, and simplifying the wing root design and was most famously used on the Vought F4U Corsair of World War II.
A biplane is a fixed-wing aircraft with two superimposed main wings. The Wright brothers' Wright Flyer used a biplane design, as did most aircraft in the early years of aviation. The primary reason many early aircraft were biplanes was that it had a structural advantage over the monoplanes, since both wings formed a Pratt truss, which was immensely strong, however both the struts, and the extra lifting surface produced more drag and thus tended to be slower. Some later biplanes replaced the Pratt truss with a Warren truss which reduced the drag associated with the struts, however it wasn't enough to keep it competitive with cantilever monoplanes which superseded it for most purposes in the late 1930s. Its compact span for a given lifting area allows for great maneuverability and it is still used for aerobatics aircraft and crop dusting. A sesquiplane is a specific type of biplane, where the (usually) lower wing is significantly smaller than the upper wing.
Triplanes and Multiplanes
A triplane is a fixed-wing aircraft equipped with three vertically-stacked wing planes. Tailplanes and canard foreplanes are not included in this count unless they overlap with other wings, nor usually are airfoil fairing on axles. Only a very small number of triplanes were ever built, but the format does have the advantage of allowing an aircraft a high degree of maneuverability combined with a very good climb rate. Due to the drag however, as with biplanes, this type is obsolete and almost never used anymore except for recreations of early triplanes, such as the Fokker Dr.I and Sopwith Triplane. A multiplane is a fixed-wing aircraft with more than three wings, but is a rarely used format as all of the disadvantages of the triplane are even more pronounced.
Canard is an airframe configuration of fixed-wing aircraft in which the forward surface is smaller than the rearward, the former being known as the "canard", while the latter is the main wing. In contrast a conventional aircraft has a small horizontal stabilizer behind the main wing.
Canard designs fall into two main classes: the lifting-canard and the control-canard. In the lifting canard the weight of the aircraft is shared between the main wing and the canard wing. In the control-canard, most of the weight of the aircraft is carried by the main wing and the canard wing is used primarily for longitudinal control during maneuvering. Thus, a control-canard mostly operates only as a control surface and is usually at zero angle of attack.
A tandem wing aircraft is a fixed wing aircraft with two sets of wings, arranged one in front of the other rather than overlapping each other. NASA research has shown that they must be of different lifting characteristics otherwise a severe oscillation will develop, which has limited their use. A tandem wing can be distinguished from a canard by the location of the pitch controls (elevators) on the rear flying surface. A tandem wing may have the front wing larger than the rear, or the reverse. They have the advantage of normally using fewer struts than biplanes, but the induced drag of having multiple wings is still present, though interactions between the wings may reduce this over a conventional biplane.
A flying wing is a tailless fixed-wing aircraft which has no definite fuselage, with most of the crew, payload and equipment being housed inside the main wing structure.
The flying wing configuration was studied extensively in the 1930s and 1940s, notably by Jack Northrop and Cheston L. Eshelman in the United States, and Alexander Lippisch and the Horten brothers in Germany. After the war, a number of experimental designs were based on the flying wing concept, but the known difficulties remained intractable. Some general interest continued until the early 1950s but designs did not necessarily offer a great advantage in range and presented a number of technical problems, leading to the adoption of "conventional" solutions like the Convair B-36 and the B-52 Stratofortress. Due to the practical need for a deep wing, the flying wing concept is most practical for designs in the slow-to-medium speed range, and there has been continual interest in using it as a tactical airlifter design.
Interest in flying wings was renewed in the 1980s due to their potentially low radar reflection cross-sections. Stealth technology relies on shapes which only reflect radar waves in certain directions, thus making the aircraft hard to detect unless the radar receiver is at a specific position relative to the aircraft - a position that changes continuously as the aircraft moves. This approach eventually led to the Northrop B-2 Spirit stealth bomber. In this case the aerodynamic advantages of the flying wing are not the primary needs. However, modern computer-controlled fly-by-wire systems allowed for many of the aerodynamic drawbacks of the flying wing to be minimised, making for an efficient and stable long-range bomber.
Blended Wing Body aircraft have a flattened and airfoil shaped body, which produces most of the lift to keep itself aloft, and distinct and separate wing structures, though the wings are smoothly blended in with the body.
Thus BWB incorporates design features from both a futuristic fuselage and flying wing design. The purported advantages of the BWB approach are efficient high-lift wings and a wide airfoil-shaped body. This enables the entire craft to contribute to lift generation with the result of potentially increased fuel economy.
A lifting body is a fixed-wing aircraft configuration in which the body itself produces lift. In contrast to a flying wing, which is a wing with minimal or no conventional fuselage, a lifting body can be thought of as a fuselage with little or no conventional wing. Whereas a flying wing seeks to maximize cruise efficiency at subsonic speeds by eliminating non-lifting surfaces, lifting bodies generally minimize the drag and structure of a wing for subsonic, supersonic, and hypersonic flight, or, spacecraft re-entry. All of these flight regimes pose challenges for proper flight stability.
Lifting bodies were a major area of research in the 1960s and 70s as a means to build a small and lightweight manned spacecraft. The US built a number of famous lifting body rocket planes to test the concept, as well as several rocket-launched re-entry vehicles that were tested over the Pacific. Interest waned as the US Air Force lost interest in the manned mission, and major development ended during the Space Shuttle design process when it became clear that the highly shaped fuselages made it difficult to fit fuel tankage.
A straight wing is a wing planform in which the centre of lift across the wing forms a straight line from wingtip to wingtip, or nearly so. This was the dominant form of wing until early transsonic aircraft adopted swept wings to reduce transsonic drag. Modern fighters were able to readopt straight wings thanks to advances in both aircraft structures and aerodynamic high lift devices.
A swept wing is a wing planform is which the wings are angled backwards so that the tips are closer to the tail than the roots, resembling an arrow. This was done to reduce the drag associated when approaching supersonic speeds and is the form primarily used on airliners and other jet trasnports. A variant of the swept wing is the forward swept wing in which the wings are angled forwards. This has serious structural implications and so hasn't been used very much, but has been tried because a regular swept wing has poor stall characteristics. When an aircraft's lift is less than what is required for it to continue flying and stalls, ideally the nose should drop, which allows the aircraft to regain flying speed. In a swept wing aircraft, the normal result of a stall is for the nose to go up, making recovery difficult.
Variable geometry aircraft are 'fixed wing' aircraft where the wing configuration can be changed in flight.
A variable-sweep wing is an aeroplane wing that may be swept back and then returned to its original position during flight. While variable-sweep provides many advantages, particularly in takeoff distance, load-carrying ability, and the fast, low-level penetration role, the configuration imposes a considerable penalty in weight and complexity. The advent of relaxed stability flight control systems in the 1970s negated many of the disadvantages of a fixed platform. No new variable-sweep wing aircraft have been built since the Tu-160 (1980).
Variable camber wings aircraft changes the camber of the airfoil (1933), and varies the area and camber of the wing (1937). The various flaps and slats on the control surfaces of modern commercial airliners perform a similar function.
A variable-incidence wing has an adjustable angle of incidence (the angle between the wing and the fuselage) in order to reduce landing and take-off distances.
The necessary components add extra weight to the aircraft and increase maintenance costs. In some aircraft the benefits outweigh the costs, and variable-incidence functionality is incorporated into the design, most notably with the F-8 Crusader, although other designs have used it, such as the Martin XB-51. No modern aircraft has used this design since the F-8.
An oblique wing (also called a slew wing) is a variable geometry wing concept. On an aircraft so equipped, the wing is designed to rotate on center pivot, so that one tip is swept forward while the opposite tip is swept aft. By changing its sweep angle in this way, drag can be reduced at high speed (with the wing swept) without sacrificing low speed performance (with the wing perpendicular).
The delta wing is a wing planform in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ) and has many advantages over other configurations. The delta wing has more internal volume to carry fuel or internal weapons than a wing of a similar thickness to chord ratio, while also allowing wing loading to be reduced for the amount of drag produced in level flight. If its leading edge is raked back enough, it will escape the shock wave formed at the nose of the aircraft as transonic speeds are reached, reducing drag considerably, and the center of lift moves less than on conventionally configured aircraft, reducing trim drag. As the angle of attack increases, the leading edge of the wing generates a vortex which smooths the airflow, giving the delta a very high stall angle at the cost of high induced drag which allows a larger range in speed than a conventional wing intended for high speed flight. Pure delta-wings fell out of favour somewhat due to poor gust response characteristics at low altitudes (they get bounced around a lot and so must fly slower and higher) and advances in high lift devices, however many of the advantages have been retained by use of leading edge root extensions, which act in the same manner and many modern fighter aircraft, such as the JAS 39 Gripen and the Eurofighter Typhoon use a delta wing, often in conjunction with a canard.
Fixed-wing aircraft can be sub-divided according to the means of propulsion they use.
Unpowered fixed-wing aircraft
Gliders are fixed-wing aircraft that are intended primarily for unpowered flight. Sailplanes, hang gliders, and paragliders are gliders used mainly for recreation. After launch, further energy is obtained through the skillful exploitation of rising air in the atmosphere. Gliders that are used for the sport of gliding have high aerodynamic efficiency. The highest lift-to-drag ratio is 70:1, though 50:1 is more common. Glider flights of thousands of kilometres at average speeds over 200 km/h have been achieved. The glider is most commonly launched by a tow-plane or by a winch. Some gliders, called motor gliders, are equipped with engines (often retractable), and some are capable of self-launching. The most numerous unpowered aircraft are hang gliders and paragliders. These are foot-launched and are in general slower, smaller, and less expensive than sailplanes. Hang gliders most often have flexible wings given shape by a frame, though some have rigid wings. Paragliders have no frames in their wings. Military gliders have been used in war to deliver assault troops, and specialised gliders have been used in atmospheric and aerodynamic research. Rocket-powered aircraft and spaceplanes have also made unpowered landings.
Smaller and older propeller aircraft make use of reciprocating engines (or piston engines) to turn a propeller to create thrust. The amount of thrust a propeller creates is determined by its disk area - the area in which the blades rotate. If the area is too small, efficiency is poor, and if the area is large, the propeller must rotate at a very low speed to avoid going supersonic and creating a lot of noise, and not much thrust. Because of this limitation, propellers are favoured for aircraft which travel at below mach .5, while jets are a better choice above that speed. They may be quieter than jet engines (though not always) and may cost less to purchase maintain and so remain common on light general aviation aircraft such as the Cessna 172. Larger modern propeller aircraft such as the Dash 8 use a jet engine to turn the propeller, primarily because an equivalent piston engine in power output would be much larger and more complex.
Jet aircraft are aircraft propelled by jet engines, which are used because the aerodynamic limitations of propellers do not apply to jet propulsion. These engines are much more powerful than a reciprocating engine for a given size or weight and are comparatively quiet and work well at higher altitude. Most modern jet aircraft use turbofan jet engines which balance the advantages of a propeller, while retaining the exhaust speed and power of a jet. This is essentially a ducted propeller attached to a jet engine, much like a turboprop, but with a smaller diameter. When installed on an airliner, it is efficient so long as it remains below the speed of sound (or subsonic). Jet fighters and other supersonic aircraft that do not spend a great deal of time supersonic also often use turbofans, but to function, air intake ducting is needed to slow the air down so that when it arrives at the front of the turbofan, it is subsonic. When passing through the engine, it is then re-accelerated back to supersonic speeds. To further boost the power output, fuel is dumped into the exhaust stream, where it ignites. This is called an afterburner and has been used on both pure jet aircraft and turbojet aircraft although it is only normally used on combat aircraft due to the amount of fuel consumed, and even then may only be used for short periods of time. Supersonic airliners (e.g. Concorde) are no longer in use largely because flight at supersonic speed creates a sonic boom which is prohibited in most heavily populated areas, and because of the much higher consumption of fuel supersonic flight requires.
Jet aircraft possess high cruising speeds (700 to 900 km/h, or 400 to 550 mph) and high speeds for take-off and landing (150 to 250 km/h). Due to the speed needed for takeoff and landing, jet aircraft use flaps and leading edge devices to control of lift and speed. Many also use thrust reversers to slow down the aircraft upon landing.
Solar-powered aircraft use solar cells to generate energy for electric motors which in turn drive propellers. On 8 July 2010, the manned Solar Impulse became the first solar-powered aeroplane to fly through an entire night.
In World War II, the Germans deployed the Me 163 Komet rocket-powered aircraft. The first fixed-wing aircraft to break the sound barrier in level flight was a rocket plane – the Bell X-1. The later North American X-15 broke many speed and altitude records and laid much of the groundwork for later aircraft and spacecraft design. Rocket aircraft are not in common usage today, although rocket-assisted take offs are used for some military aircraft. Recent rocket aircraft include the SpaceShipOne and the XCOR EZ-Rocket.
Ramjet aircraft and scramjet aircraft
A ramjet is a form of jet engine that contains no major moving parts and can be particularly useful in applications requiring a small and simple engine for high-speed use, such as missiles. The D-21 Tagboard was an Mach 3+ reconnaissance drone that was cancelled in 1971. The SR-71's engines ran 80% as ramjets at high speeds.
Scramjet aircraft are in the experimental stage. A scramjet has a very simple engine design. It works by air being forced into one side of a tube-like engine. That air is ignited by fuel, causing it to come out hotter and faster on the other side. This engine requires high speed in order to work, but it is suitable for the speeds at which it travels. The NASA X-43 is an experimental unmanned scramjet with a world speed record for a jet-powered aircraft – Mach 9.7, nearly 12,000 kilometres per hour (7,500 mph) at an altitude of about 36,000 metres (118,000 ft). The X-43A set the flight speed record in 2004.
Fixed-wing aircraft Facts for Kids. Kiddle Encyclopedia.