An aeroplane can lift itself because the wing, angled slightly downwards towards the back, pushes air downwards as the wing is propelled forwards by the engine. In reaction, the wing is pushed upwards, generating lift, as predicted in the third law of motion formulated by Isaac Newton: that for every action, there is an equal and opposite reaction. The magnitude of the lift that is generated depends upon the shape of the aerofoil in cross-section, the area and shape of the lifting surface, its inclination relative to the airflow, and the airflow speed.

LiftThe lift developed on a wing or similar surface is directly proportional to the plan area exposed to the airflow but proportional to the square of the speed of the airflow. It is also approximately proportional to the inclination, or angle of attack, of the aerofoil relative to the airflow

for angles typically in the range of plus and minus 14°. At greater angles the airflow characteristics change rapidly, the

flow “breaks away”, and lift falls drastically. In these circumstances the aerofoil is said to have “stalled”.

As an aeroplane flies on a level course, the lift contributed by the wing and other parts of the structure counterbalances the weight of the plane. Within limits, if the angle of attack is increased while the speed remains constant, the plane will rise. If the angle of attack is decreased, that is, the wing is inclined downward, the plane will lose lift and start to descend. An aeroplane will also climb from level flight if its speed is increased, and it will dive if its speed is decreased.

During the course of a flight, a pilot frequently alters the speed and angle of attack of the aircraft. These two factors are often balanced against each other. For instance, if the pilot wishes to increase speed and yet maintain level flight, the angle of attack must be decreased to offset the extra lift that is provided by the increase in the speed of the aircraft.

In preparing to land, the pilot must ease the plane down and at the same time reduce its speed as much as possible. To compensate for the considerable loss of lift resulting from the decrease in speed, the pilot provides additional lift by altering the wing area, effective curvature, and angle of attack. This is done through the use of high-lift devices called flaps, large wing extensions located at the rear or trailing edge. Most flaps are normally retracted into the wing during cruising flight. If extra lift is wanted, the pilot extends the flaps outward and downward. Sometimes high-lift devices are provided at the front, or leading, edge of a wing.

Drag

Factors that contribute to lift in flight also contribute to undesirable forces called drag. Drag is the force that tends to retard the motion of the plane through the air. Some drag is a result of the resistance of the air to objects moving in it and is dependent upon the shape and smoothness of the surface. It can be reduced by streamlining the aircraft. Some designs also incorporate devices to reduce the drag owing to friction by maintaining the surface airflow in so-called “laminar” form.

Another form of drag, however, known as induced drag, is a direct result of the lift produced by the wing. Work has to be done to produce lift and the induced drag is the measure of this. The expenditure of energy appears in the form of eddies, or vortices, which form along the trailing edge and especially at the outer extremities, or tips, of the wing.

Aeroplane designers conceive aircraft with the highest possible ratio of lift to drag, which occurs when the drag resulting from the shape is equal to the induced drag resulting from the lift. The lift-to-drag ratio is limited by factors such as speed and acceptable weight of the airframe. A subsonic transport aircraft may have a lift-to-drag ratio of about 20, while that of a high-performance sailplane may be twice this. On the other hand, the extra drag that occurs when an aircraft flies at supersonic speed reduces the achieved lift-to-drag ratio to less than 10.

Aeroplane, heavier-than-air craft that is usually propelled mechanically and supported by the aerodynamic action of the airstream on fixed-wing surfaces. Other types of aircraft that are heavier than air include the glider or sailplane, which is similarly equipped with fixed-wing surfaces but is not self-propelled, and rotary-wing aircraft, which are mechanically driven and supported by overhead rotors, such as the Autogiro and Helicopters. Another type is the ornithopter, which is lifted and propelled by flapping wings. Toy-sized ornithopters have been developed, but large-scale experiments have been unsuccessful. For the history of heavier-than-air craft, see Aviation

The term “aeroplane” generally denotes craft operated from land bases, but it applies also to several other categories of aircraft, including the carrier-based plane, the seaplane, and the amphibian. The principal variation in configuration can be found in the landing apparatus.

The carrier-based plane is a type of land plane designed for use on an aircraft carrier, and is fitted with a tail hook that engages a cable stretched across the deck to arrest the plane after landing. The seaplane employs floats instead of the wheel gear of the land plane. In the variety of seaplane known as the flying boat, the fuselage is constructed as a hull, similar to that of a seagoing vessel, and serves to keep the plane buoyant. The amphibian is equipped with both wheel gear and hull or floats to permit operation with equal effectiveness on land and water.

Before World War II, flying boats were used for military transports and for intercontinental commercial service. These planes were limited to low flying speeds and to low landing speeds in water. With the advent of planes that fly and land much faster, to gain efficiency, large planes have been limited to land-based operation. The amphibian, even slower because of its double undercarriage, is less commonly employed than the land plane. For light sports planes,

amphibious floats are available. Generally resembling conventional floats, they have a recessed wheel located at the centre. The wheel does not extend far enough to add much drag to the float in the water, but it protrudes far enough to enable wheeled landings to be made on hard-surfaced runways or short-cut grass.

Particular types of heavier-than-air craft include the VTOL (vertical take-off and landing) and STOL (short take-off and landing) craft, and the convertiplane. The VTOL craft is an aeroplane that can rise vertically, move off horizontally, and then reverse the procedure for a landing. The term “VTOL” is limited to describing aircraft with performance similar to that of conventional aeroplanes but with additional vertical take-off and landing ability. Several means are used to lift VTOL aircraft off the ground. The direct downward thrust of jet engines is used in several designs, but the power required is high. Rotating wings and ducted fans are also

used for direct lift, but they introduce drag into the horizontal flight. Convertiplanes, combining the rotors of helicopters with the fixed wings of aeroplanes, show promise for short-distance commercial VTOL operation. They compete directly with helicopters, but can fly faster.

The STOL craft is an aeroplane that takes off and lands very steeply, thus requiring only a short runway. For a given payload, it is more efficient in terms of fuel consumption and power requirements than a VTOL craft. It is also capable of higher speeds and longer-range flights than a helicopter. In September 1999 a solar-powered aircraft completed its first test-flight in California. The aircraft has a wingspan of 75 m (247 ft) and flies without a pilot. In the future it is thought the plane could remain in continuous flight for up to six months at a time and would be employed for scientific tests and telecommunications projects. For lighter-than-air craft, see Airship; Balloon

The present-day conventional aeroplane may be divided into four components: fuselage, wings, tail assembly, and landing gear, or undercarriage.

A Fuselage

In the early days of aviation, the fuselage was merely an open framework to support the other components of the plane; the bottom of the airframe served as the landing gear. Subsequently, the need for greater strength and better performance resulted in the development of enclosed, box-like “strut-and-wire” fuselages that decreased drag, and also provided protection for pilot and passenger, as well as space for the payload. This “truss” structure was gradually superseded by the monocoque (literally, single shell) fuselage. The loads imposed on such a structure are carried primarily by the skin, rather than by an internal framework, as in the trussed structure. It is the most common fuselage presently in use. The outer shell also confers the possibility of pressurizing the internal volume for high-altitude flight.

Wings

Assembling an Aeroplane Here, at a McDonnell-Douglas assembly line, several large passenger planes are in production. In the foreground, the wing infrastructure is fastened to a body section. Further on in the assembly line, the tail section and engine mounts are added.Tom Carroll/Phototake NYC Although the single-winged plane, known as the monoplane, made its appearance in the first decade of powered flight, early aeroplane construction favoured the use of two wings (the biplane), and occasionally even three or four. Multiple-wing planes have the advantage of superior lift and relatively stronger construction, but the monoplane has lower drag. Once the cantilever principle of wing construction was developed, the dominance of the monoplane was assured, although it did not become the design of choice until the 1930s. Cantilever wings obtain their entire strength from internal structural elements. Cantilever construction is employed in most present-day aircraft, and external bracing is used only for some small, light planes.

The structure of a typical wing consists of a spar-and-rib framework enclosed by a thin covering of metal sheet. Treated fabric, or, infrequently, bonded plywood or resin-impregnated glass fibre are used for some small aircraft and sailplanes. The spar, or beam, extends from the fuselage to the wing-tip. One or more spars may be used in the wing, but the two-spar design is most common. The ribs, normally at right angles to the spars, give the wing its external shape. If the covering is of metal sheet, it contributes its own share of strength to the wing. This “stressed-skin” type of wing is used in all large planes, although there is an increasing use of high-strength reinforced plastic skins and structure.

The size and shape of wings vary widely, depending on specific aerodynamic considerations. Wings of many supersonic planes have a high degree of sweepback (arrowhead tapering from the nose of the plane) and are as thin as possible, with a knife-like leading edge. Such a shape helps to reduce the shock of compression when the plane approaches the speed of sound. The structural importance of the wing is dramatically demonstrated by the development of the so-called “flying wing”, a craft in which fuselage and tail are almost entirely eliminated.

Tail Assembly

The conventional type of tail assembly consists of two basic surfaces, horizontal and vertical, each of which has movable sections contributing to control of the craft and fixed sections to provide stability. The leading section of the horizontal surface is known as the horizontal stabilizer, and the rear movable section is known as the elevator. Sometimes the whole surface can move and the elevator is eliminated. The stationary section of the vertical surface is called the fin, and the movable section, the rudder. Two vertical surfaces are used in some aircraft; in that case, a double rudder is used. The V-shaped tail combines the rudder and elevator functions in a single device. Tails vary in size according to the type of aircraft. In some supersonic aircraft the horizontal tail is replaced by a foreplane, or “canard”, located near the nose of the plane.

Landing Gear

Present-day landing gear is one of the most intricate of all aeronautical mechanisms. Its components include the shock strut, a hydraulic leg connecting the wheel with the wing or fuselage to absorb the shock of landing; the retracting mechanism, which raises and lowers the gear; the

wheels; and the wheel brakes. There are a number of types of undercarriage, but two are most commonly employed: the older two-wheel gear and the nose or tricycle gear, which is now usual. The former consists of two large wheels located forward of the centre of gravity of the plane with a small wheel at the tail. A tricycle gear consists of two large wheels or wheelgroups behind the centre of gravity and a third wheel, called the nosewheel, in front of the two main wheels. Landing is easier with the tricycle gear because braking and manoeuvring are improved and the danger of nosing over is diminished. Some large aircraft have more than two rear wheel groups. Other forms of landing gear include a caterpillar tread for handling heavy loads on poor landing fields, a swivelling gear for landing in crosswinds, and a combination ski-wheel gear for use on ice and snow.

Components of modern aircraft necessary for flight control include devices manipulated from the cockpit by the stick or wheel and by the rudder pedals, and instruments that provide the pilot with essential information.

A Mechanical Controls

Basic Movements of an Aeroplane Bridgeman Art Library, London/New York Expand The attitude of an aeroplane (its orientation relative to the horizon and to the direction of motion) is conventionally determined by three control devices, each of which provides for movement about a different axis. The three devices include the movable sections of the tail, which are the elevators and rudders; and the movable sections of the trailing (aft) edge of the wing, known as ailerons. The control surfaces are operated from the cockpit by means of a control stick or wheel column and rudder pedals. Stick control is used in smaller, lighter aeroplanes, and the wheel, with its greater leverage, is generally used in larger aircraft, as well as in some small ones

Elevators provide for pitching movement around the lateral axis. A backward pull on the control stick or wheel column raises the elevators, thereby depressing the tail and lifting the nose of the plane for a climb. Forward movement of the stick or column produces the opposite effect, making the plane dive.

Ailerons, usually placed far out on the wing, control rolling movement around the longitudinal axis. Leftward movement of the stick or wheel raises the left aileron and lowers the right, thereby banking the plane to the left. The reverse tilt occurs when the stick or wheel is moved to the right.

Rudders provide for turning movement around the vertical axis, in coordination with the ailerons, changing the course of the plane to the left or right. When the right rudder pedal is pressed, the rudder turns the plane to the right around the vertical axis. Pressing the left pedal produces a left turn.

To ensure easier and more dependable handling of all control surfaces, a number of secondary controls have been devised. Trim tabs are used on rudders, elevators, and ailerons as a means of adjusting the equilibrium, or trim, of the plane. Other

secondary controls include flaps (on trailing edges) and slots (on leading edges) to increase lift for take-off or drag for landing, or to improve various other flight characteristics. Spoilers are surfaces that normally lie flush with the wing but can be raised to present a flat surface to the airstream and “spoil” the lift of the wing. Somewhat similar surfaces are called air brakes and extend at right angles to the fuselage or undersurface of the wing to slow the speed of the plane. The control surfaces may be operated directly by pilot effort or by a hydraulic or electrical power system. In the latter case the pilot’s commands may be transmitted mechanically, by electrical signals (“fly-by-wire”) or by optical signals (“fly-by-light”).

Instruments

Flight Control Panel The cockpit of a Concorde jet shows the complexity of flight controls. Electronic and computerized equipment in the cockpit provides information regarding navigation, speed, altitude, landing, and engine performance.Albert Visage/Explorer/Photo Researchers, Inc.

Information required in flight is provided by various types of equipment, which may be divided into four general categories: power-plant instruments, flight instruments, landing instruments, and navigation aids. Power-plant instruments indicate whether the engines are functioning properly and include the tachometer, which shows the revolutions per minute of each engine, various pressure gauges, temperature indicators, and the fuel gauges. The primary flight instruments provide indications of speed (the air-speed indicator), direction (the magnetic compass and the directional gyro), altitude (the altimeter), and attitude (the rate-of-climb and turn-and-bank indicators and the artificial horizon). Several of the flight instruments, including the automatic pilot, utilize the gyroscopic principle.

Landing instruments needed in poor visibility are of two types, the instrument-landing system (ILS), providing direct signals to the pilot to ensure a safe landing, and the ground-controlled approach (GCA), a system employing radar equipment on the ground to guide the pilot solely by radio-telephonic advice. The ILS is widely used in civil aviation; the GCA system, in military aviation. Both systems may also use the standard approach lighting system (ALS), which guides the aeroplane the last few hundred metres of the airway route to the runway.

Flying the FlyerOrville Wright mans the controls of the Wright Flyer in 1908, five years after he made the world’s first successful, sustained flight. The Wright brothers’ experiments with heavier-than-air flight had launched Flyer I on December 17, 1903, near Kill Devil Hill in Kitty Hawk, North Carolina. The first flight lasted about 12 seconds, and the plane travelled 36.5 m (120 ft) at an altitude of roughly 3 m (9.9 ft) and an airspeed of 48 km/h (30 mi/h). Wilbur Wright made a longer, 59-second flight later on the same day.

Ornithopter Design Leonardo da Vinci designed several flying machines. This one, called an ornithopter, simulated the motion of a bird flying.

Assembling an Aeroplane Here, at a McDonnell-Douglas assembly line, several large passenger planes are in production. In the foreground, the wing infrastructure is fastened to a body section. Further on in the assembly line, the tail section and engine mounts are added

Henson and Stringfellow's "Aerial Steam Carriage" One of the biggest difficulties faced by early would-be pilots was finding an engine that was both powerful and light. Many models, such as the Henson and Stringfellow’s “Aerial Steam Carriage” (shown above) might have flown as early as 1845 with adequate engines. Unfortunately, the only engines available were steam engines, which were too weak or too heavy for successful flight. It was not until the arrival of the compact, relatively lightweight petrol engine that planes were able to get off the ground.

Culver Pictures/Courtesy of Gordon Skene Sound Collection. All rights reserved.Amelia EarhartIn 1932 Amelia Earhart became the first woman to make a solo flight across the Atlantic Ocean. In 1937 she and a navigator, Frederick Noonan, attempted a flight around the world. Towards the end of their journey they disappeared somewhere over the central Pacific Ocean; their fate remains a mystery. Here, Amelia Earhart speaks of the aircraft’s rapid transformation from a novel invention to an ordinary part of everyday life.

Precursor to the famous Sopwith Camel fighter of World War I, the Sopwith Pup was a light, manoeuverable aeroplane. It travelled at speeds of 185 km/h (115 mph) and was among the first planes to use the new aileron wing design. Ailerons are hinged flaps on the tips of wings used to turn, or bank, the plane.Dick Hanley/Photo Researchers, Inc. During the period before World War I the design of both the aeroplane and its engine showed considerable improvement. Pusher biplanes—two-winged aeroplanes with the engine and propeller behind the wing—were succeeded by tractor biplanes, with the propeller in front of the wing. Only a few types of monoplanes were used.

Throughout World War II, aircraft became increasingly crucial factors in military strategy and battles. The need to produce high-performance military aeroplanes as rapidly as possible during the war served as the impetus for many advances in aircraft design and production techniques. Here, World War II military aircraft fly in formation.

Air power played a significant role in World War II. The vintage Fairey Swordfish, shown here, contributed to Allied naval victories in 1940 and 1941. This torpedo bomber, although not as advanced as some of the other bombers in use at the time, crippled an Italian fleet at Taranto and inflicted severe damage on the German battleship Bismarck.Ian A. Griffiths/Robert Harding Picture Library The most significant development of all was jet propulsion. It was originally proposed by Frank Whittle in Britain. However, it was Germany that developed and flew the first jet-propelled aircraft, the Heinkel He 178, powered by an HeS 3B engine developed by Hans von Ohain. The aircraft first flew in August 1939, just one week before the outbreak of war.

PROPULSION

Two basic means are used to provide the thrust for an aeroplane in flight: propellers or jet propulsion. In a propeller-driven aeroplane either a piston-driven internal-combustion engine or a turboprop engine is utilized to drive the propeller, which thrusts the air backwards because it has aerofoil-shaped blade sections cutting through the air in a screw-like fashion. In jet propulsion, the forward thrust is provided by the discharge of high-speed gases through a rear-facing nozzle. Rocket engines, working on a similar principle, are occasionally used.

An aircraft engine must satisfy a number of major design requirements, including high reliability, long life, low weight, low fuel consumption, and low frontal area. The most important factor is reliability. Long life is mainly an economic consideration, of special interest in commercial aviation. The relative importance of the other three requirements depends upon the type of plane that the engine is intended to propel. Low weight and low fuel consumption are naturally interdependent because the fuel itself is a weight factor. Low frontal area is desirable as a means of minimizing the drag caused by the engine.

Piston Engines

The piston engine used in most propeller-driven aircraft is one of two types, the reciprocating engine and the rotary engine. In the reciprocating engine, heat energy is utilized to move pistons operating within cylinders. Cylinder arrangement is generally in-line, horizontal-opposed, or radial, and either air-cooling or liquid-cooling systems are used. Nearly all aircraft reciprocating engines are petrol operated. In general, the advantages of the reciprocating engine are reliability and fuel economy. The rotary engine replaces the pistons by a single rotating one and hence has fewer ports. It is claimed to produce lower vibration. Some engines of this type are becoming available for use in small aircraft.

The compound engine consists of a reciprocating engine combined with an exhaustgas turbine- that drives a supercharger, an air compressor in the intake system of the engine. The supercharger compensates for the decreasing density of the atmosphere at higher altitudes. The chief advantage of the compound engine over the basic reciprocating engine is its high power at altitude. The compound engine served as the chief engine in United States military aircraft during World War II, before the advent of jet propulsion. British high-performance reciprocating engines of that era used mechanically driven superchargers.

Jet Engines

Most non-reciprocating aircraft engines operate on the principle of jet propulsion, and include the turbojet, the turboprop, the ramjet, and the rocket engine. The turbojet and its modifications, the turbofan and turboprop, are gas turbine engines, in which the air that enters the intake of the engine is first compressed in a compressor. Fuel is then added to burn

with the oxygen in the air, increasing the gas temperature and volume. The high-pressure gases are then directed through a turbine, which drives the rotating assembly of the engine. In the case of the turbojet the expansion is partial and the residual gas, which is now at intermediate pressure, is accelerated by expansion through a rear-facing nozzle, to produce a high leaving velocity and, with it, the desired thrust.

Turboprop and turbofan engines extract most of the gas energy in the turbine, the residual jet thrust being of secondary magnitude. Turboprop engines are efficient for medium-sized planes at speeds up to about 480 to 640 km/h (300 to 400 mph). At higher subsonic speeds the turbofan is the preferred engine, as the performance of a propeller drops to a low level of efficiency. Turbofan engines use less fuel and are quieter than turbojets, but at higher, supersonic, speeds the high exhaust velocity of the turbojet is necessary.

The ramjet engine is a jet engine in which the air compression needed for combustion is obtained from the speed of forward motion alone. As in the turbojet, its total power output is delivered as the jet thrust of its expelled gases. Although the ramjet can be applied to piloted aircraft, its present rate of fuel consumption is so prohibitively high that it is used only in guided-missile applications.

The rocket engine carries its own oxidant as well as its fuel and, like the ramjet, has its chief application in guided missiles. A solid-propellant rocket is used for rocket-assisted take-off, supplementary initial power for heavily loaded aircraft.

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