/air"playn'/, n.
1. a heavier-than-air aircraft kept aloft by the upward thrust exerted by the passing air on its fixed wings and driven by propellers, jet propulsion, etc.
2. any similar heavier-than-air aircraft, as a glider or helicopter.
Also, esp. Brit., aeroplane.
[1870-75, for an earlier sense; alter. of AEROPLANE, with AIR1 r. AERO-]

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Fixed-wing aircraft that is heavier than air, propelled by a screw propeller or a high-velocity jet, and supported by the dynamic reaction of the air against its wings.

An airplane's essential components are the body or fuselage, a flight-sustaining wing system, stabilizing tail surfaces, altitude-control devices such as rudders, a thrust-providing power source, and a landing support system. Beginning in the 1840s, several British and French inventors produced designs for engine-powered aircraft, but the first powered, sustained, and controlled flight was only achieved by Wilbur and Orville Wright in 1903. Later airplane design was affected by the development of the jet engine; most airplanes today have a long nose section, swept-back wings with jet engines placed behind the plane's midsection, and a tail stabilizing section. Most airplanes are designed to operate from land; seaplanes are adapted to touch down on water, and carrier-based planes are modified for high-speed short takeoff and landing. See also airfoil; aviation; glider; helicopter.

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also called  aeroplane  or  plane 

      any of a class of fixed-wing aircraft that is heavier than air, propelled by a screw propeller or a high-velocity jet, and supported by the dynamic reaction of the air against its wings.

      The essential components of an airplane are a wing system to sustain it in flight, tail surfaces to stabilize the wings, movable surfaces to control the attitude of the plane in flight, and a power plant to provide the thrust necessary to push the vehicle through the air. Provision must be made to support the plane when it is at rest on the ground and during takeoff and landing. Most planes feature an enclosed body ( fuselage) to house the crew, passengers, and cargo; the cockpit is the area from which the pilot operates the controls and instruments to fly the plane.

Principles of aircraft flight and operation

      An aircraft in straight-and-level unaccelerated flight has four forces acting on it. (In turning, diving, or climbing flight, additional forces come into play.) These forces are lift, an upward-acting force; drag, a retarding force of the resistance to lift and to the friction of the aircraft moving through the air; weight, the downward effect that gravity has on the aircraft; and thrust, the forward-acting force provided by the propulsion system (or, in the case of unpowered aircraft, by using gravity to translate altitude into speed). Drag and weight are elements inherent in any object, including an aircraft. Lift and thrust are artificially created elements devised to enable an aircraft to fly.

      Understanding lift first requires an understanding of an airfoil, which is a structure designed to obtain reaction upon its surface from the air through which it moves. Early airfoils typically had little more than a slightly curved upper surface and a flat undersurface. Over the years, airfoils have been adapted to meet changing needs. By the 1920s, airfoils typically had a rounded upper surface, with the greatest height being reached in the first third of the chord (width). In time, both upper and lower surfaces were curved to a greater or lesser degree, and the thickest part of the airfoil gradually moved backward. As airspeeds grew, there was a requirement for a very smooth passage of air over the surface, which was achieved in the laminar-flow airfoil, where the camber was farther back than contemporary practice dictated. Supersonic aircraft required even more drastic changes in airfoil shapes, some losing the roundness formerly associated with a wing and having a double-wedge shape.

      By moving forward in the air, the wing's airfoil obtains a reaction useful for flight from the air passing over its surface. (In flight the airfoil of the wing normally produces the greatest amount of lift, but propellers, tail surfaces, and the fuselage also function as airfoils and generate varying amounts of lift.) In the 18th century the Swiss mathematician Daniel Bernoulli (Bernoulli, Daniel) discovered that, if the velocity of air is increased over a certain point of an airfoil, the pressure of the air is decreased. Air flowing over the curved top surface of the wing's airfoil moves faster than the air flowing on the bottom surface, decreasing the pressure on top. The higher pressure from below pushes (lifts) the wing up to the lower pressure area. Simultaneously the air flowing along the underside of the wing is deflected downward, providing a Newtonian equal and opposite reaction and contributing to the total lift.

      The lift an airfoil generates is also affected by its “angle of attack”—i.e., its angle relative to the wind. Both lift and angle of attack can be immediately, if crudely, demonstrated, by holding one's hand out the window of a moving automobile. When the hand is turned flat to the wind, much resistance is felt and little “lift” is generated, for there is a turbulent region behind the hand. The ratio of lift to drag is low. When the hand is held parallel to the wind, there is far less drag and a moderate amount of lift is generated, the turbulence smooths out, and there is a better ratio of lift to drag. However, if the hand is turned slightly so that its forward edge is raised to a higher angle of attack, the generation of lift will increase. This favourable increase in the lift-to-drag ratio will create a tendency for the hand to “fly” up and over. The greater the speed, the greater the lift and drag will be. Thus, total lift is related to the shape of the airfoil, the angle of attack, and the speed with which the wing passes through the air.

      Weight is a force that acts opposite to lift. Designers thus attempt to make the aircraft as light as possible. Because all aircraft designs have a tendency to increase in weight during the development process, modern aerospace engineering staffs have specialists in the field controlling weight from the beginning of the design. In addition, pilots must control the total weight that an aircraft is permitted to carry (in passengers, fuel, and freight) both in amount and in location. The distribution of weight (i.e., the control of the centre of gravity of the aircraft) is as important aerodynamically as the amount of weight being carried.

      Thrust, the forward-acting force, is opposed to drag as lift is opposed to weight. Thrust is obtained by accelerating a mass of ambient air to a velocity greater than the speed of the aircraft; the equal and opposite reaction is for the aircraft to move forward. In reciprocating or turboprop-powered aircraft, thrust derives from the propulsive force caused by the rotation of the propeller, with residual thrust provided by the exhaust. In a jet engine, thrust derives from the propulsive force of the rotating blades of a turbine compressing air, which is then expanded by the combustion of introduced fuel and exhausted from the engine. In a rocket-powered aircraft, the thrust is derived from the equal and opposite reaction to the burning of the rocket propellant. In a sailplane, height attained by mechanical, orographic, or thermal techniques is translated into speed by means of gravity.

      Acting in continual opposition to thrust is drag, which has two elements. Parasitic drag is that caused by form resistance (due to shape), skin friction, interference, and all other elements that are not contributing to lift; induced drag is that created as a result of the generation of lift.

      Parasitic drag rises as airspeed increases. For most flights it is desirable to have all drag reduced to a minimum, and for this reason considerable attention is given to streamlining the form of the aircraft by eliminating as much drag-inducing structure as possible (e.g., enclosing the cockpit with a canopy, retracting the landing gear, using flush riveting, and painting and polishing surfaces). Some less obvious elements of drag include the relative disposition and area of fuselage and wing, engine, and empennage surfaces; the intersection of wings and tail surfaces; the unintentional leakage of air through the structure; the use of excess air for cooling; and the use of individual shapes that cause local airflow separation.

      Induced drag is caused by that element of the air deflected downward which is not vertical to the flight path but is tilted slightly rearward from it. As the angle of attack increases, so does drag; at a critical point, the angle of attack can become so great that the airflow is broken over the upper surface of the wing, and lift is lost while drag increases. This critical condition is termed the stall.

      Lift, drag, and stall are all variously affected by the shape of the wing planform. An elliptical wing like that used on the Supermarine Spitfire fighter of World War II, for example, while ideal aerodynamically in a subsonic aircraft, has a more undesirable stall pattern than a simple rectangular wing.

      The aerodynamics of supersonic flight are complex. Air is compressible, and, as speeds and altitudes increase, the speed of the air flowing over the aircraft begins to exceed the speed of the aircraft through the air. The speed at which this compressibility affects an aircraft is expressed as a ratio of the speed of the aircraft to the speed of sound, called the Mach number, in honour of the Austrian physicist Ernst Mach. The critical Mach number for an aircraft has been defined as that at which on some point of the aircraft the airflow has reached the speed of sound.

      At Mach numbers in excess of the critical Mach number (that is, speeds at which the airflow exceeds the speed of sound at local points on the airframe), there are significant changes in forces, pressures, and moments acting on the wing and fuselage caused by the formation of shock waves. One of the most important effects is a very large increase in drag as well as a reduction in lift. Initially designers sought to reach higher critical Mach numbers by designing aircraft with very thin airfoil sections for the wing and horizontal surfaces and by ensuring that the fineness ratio (length to diameter) of the fuselage was as high as possible. Wing thickness ratios (the thickness of the wing divided by its width) were about 14 to 18 percent on typical aircraft of the 1940–45 period; in later jets the ratio was reduced to less than 5 percent. These techniques delayed the local airflow reaching Mach 1.0, permitting slightly higher critical Mach numbers for the aircraft. Independent studies in Germany and the United States showed that reaching the critical Mach could be delayed further by sweeping the wings back. Wing sweep was extremely important to the development of the German World War II Messerschmitt Me 262, the first operational jet fighter, and to postwar fighters such as the North American F-86 Sabre and the Soviet MiG-15. These fighters operated at high subsonic speeds, but the competitive pressures of development required aircraft that could operate at transonic and supersonic speeds. The power of jet engines with afterburners made these speeds technically possible, but designers were still handicapped by the huge rise in drag in the transonic area. The solution involved adding volume to the fuselage ahead of and behind the wing and reducing it near the wing and tail, to create a cross-sectional area that more nearly approximated the ideal area to limit transonic drag. Early applications of this rule resulted in a “wasp-waist” appearance, such as that of the Convair F-102. In later jets application of this rule is not as apparent in the aircraft's planform.

Devices for aerodynamic control
      In some flight conditions—descent, preparing to land, landing, and after landing—it is desirable to be able to increase drag to decelerate the aircraft. A number of devices have been designed to accomplish this. These include speed brakes, which are large flat-plate areas that can be deployed by the pilot to increase drag dramatically and are most often found on military aircraft, and spoilers, which are surfaces that can be extended on the wing or fuselage to disrupt the air flow and create drag or to act in the same manner as ailerons. Drag can also be provided by extension of the landing gear or, at the appropriate airspeeds, deployment of the flaps and other lift devices. Lift and drag are roughly proportional to the wing area of an aircraft; if all other factors remain the same and the wing area is doubled, both lift and drag will be doubled. Designers therefore attempt to minimize drag by keeping the wing area as small as possible, while enhancing lift with certain types of trailing-edge flaps and leading-edge slats, which have the ability to increase wing area mechanically. (These devices also alter the camber of the wing, increasing both lift and drag.) A passenger in an aft window seat of a modern airliner can observe the remarkable way in which the wing quite literally transforms itself from a smooth, slim, streamlined surface into almost a half-circle of surfaces by the deployment of a formidable array of lift- and drag-inducing devices.

      Flaps are extensions of the trailing edge of the wing and can be deflected downward as much as 45°. Many flaps effectively increase wing area, adding to lift and to drag. The angle to which the flaps are deployed determines the relative amount of additional lift or drag obtained. At smaller angles, lift is typically increased over drag, while at greater angles, drag is dramatically increased over lift. Flaps come in a wide variety of types, including the simple split flap, in which a hinged section of the undersurface of the trailing edge of the wing can be extended; the Fowler flap, which extends the wing area by deploying on tracks, creating a slotted effect; and the Kreuger flap, which is a leading-edge flap often used in combination with Fowler or other trailing-edge flaps.

      Various modern proprietary systems of multiple slotted flaps are used in conjunction with leading-edge slats and flaps, all specially designed to suit the flight characteristics of the particular airplane. Leading-edge flaps alter the camber of the wing and provide additional lift; leading-edge slats are small cambered airfoil surfaces arranged near the leading edge of the wing to form a slot. Air flows through the slot and over the main wing, smoothing out the airflow over the wing and delaying the onset of the stall. Leading-edge slots, which can be either fixed or deployable, are spanwise apertures that permit air to flow through a point behind the leading edge and, like the slat, are designed to smooth out the airflow over the wing at higher angles of attack.

      The deployment of these devices can be varied to suit the desired flight regime. For takeoff and in the approach to landing, their deployment is generally to provide greater lift than drag. In flight or after touchdown, if rapid deceleration is desired, they can be deployed in a manner to greatly increase drag.

Primary flight controls
      All four forces—lift, thrust, drag, and weight—interact continuously in flight and are in turn affected by such things as the torque effect of the propeller, centrifugal force in turns, and other elements, but all are made subject to the pilot by means of the controls.

Elevator, aileron, and rudder controls
      The pilot controls the forces of flight and the aircraft's direction and attitude by means of flight controls. Conventional flight controls consist of a stick or wheel control column and rudder pedals, which control the movement of the elevator and ailerons (aileron) and the rudder, respectively, through a system of cables or rods. In very sophisticated modern aircraft, there is no direct mechanical linkage between the pilot's controls and the control surfaces; instead they are actuated by electric motors. The catch phrase for this arrangement is “fly-by-wire.” In addition, in some large and fast aircraft, controls are boosted by hydraulically or electrically actuated systems. In both the fly-by-wire and boosted controls, the feel of the control reaction is fed back to the pilot by simulated means.

      In the conventional arrangement the elevator, attached to the horizontal stabilizer, controls movement around the lateral axis and in effect controls the angle of attack. Forward movement of the control column lowers the elevator, depressing the nose and raising the tail; backward pressure raises the elevator, raising the nose and lowering the tail. Many modern aircraft combine the elevator and stabilizer into a single control surface called the stabilator, which moves as an entity to control inputs.

      The ailerons are movable surfaces hinged to the trailing edge of each wing, which move in the opposite direction to control movement around the aircraft's longitudinal axis. If the pilot applies left pressure to the control column (stick or wheel), the right aileron deflects downward and the left aileron deflects upward. The force of the airflow is altered by these control changes, causing the left wing to lower (because of decreased lift) and the right wing to rise (because of increased lift). This differential in lift causes the aircraft to turn to the left.

      The rudder is a vertical surface, and it controls movement around the aircraft's vertical axis. It does not cause the aircraft to turn; instead, it counteracts the adverse yaw (rotation around the vertical axis) produced by the ailerons. The lowered wing has both decreased lift and decreased drag; the raised wing has both increased lift and increased drag. The added drag of the raised wing tries to pull the nose of the aircraft toward it (i.e., away from the direction of the turn). Pressure on the rudder is used to counter this adverse yaw. Because the turn results in a net decrease in lift, application of elevator pressure is necessary. Thus, a turn is the result of the combined inputs of the ailerons, rudder, and elevator.

      Trim tabs are used by the pilot to relieve the requirement of maintaining continuous pressure on the controls. These are smaller surfaces inset into the rudder, elevator, and ailerons, which can be positioned by mechanical or electrical means and which, when positioned, move the control surface to the desired trimmed position. Trimming the aircraft is a continual process, with adjustments necessary for changes to the flight or power controls that result in changes in speed or attitude.

Thrust controls
      The pilot controls thrust by adjustment of the control levers for the engine. In an aircraft with a reciprocating engine these can consist of a throttle, mixture control (to control the ratio of fuel and air going to the engine), and propeller control as well as secondary devices such as supercharger controls or water-alcohol injection. In a turbojet engine, the principal control is the throttle, with auxiliary devices such as water injection and afterburners (afterburner). With water injection, a water-alcohol mixture is injected into the combustion area to cool it, which allows more fuel to be burned. With afterburners, fuel is injected behind the combustion section and ignited to increase thrust greatly at the expense of high fuel consumption. The power delivered by reciprocating and jet engines (jet engine) is variously affected by airspeed and ambient air density (temperature, humidity, and pressure), which must be taken into consideration when establishing power settings. In a turboprop engine, power is typically set by first adjusting the propeller speed with a propeller lever and then adjusting fuel flow to obtain the desired torque (power) setting with the power lever.

Propellers (propeller)
      Propellers are basically rotating airfoils, and they vary in type, including two-blade fixed pitch, four-blade controllable (variable) pitch, and eight-blade contrarotating pitch. The blade angle on fixed-pitch propellers is set for only one flight regime, and this restriction limits their performance. Some fixed-pitch propellers can be adjusted on the ground to improve performance in one part of the flight regime. Variable-pitch propellers permit the pilot to adjust the pitch to suit the flight condition, using a low pitch for takeoff and a high pitch for cruising flight. Most modern aircraft have an automatic variable-pitch propeller, which can be set to operate continuously in the most efficient mode for the flight regime. If an engine fails, most modern propellers can be feathered (mechanically adjusted) so that they present the blade edgewise to the line of flight, thereby reducing drag. In large piston engine aircraft, some propellers can be reversed after landing to shorten the landing run. (Jet engines have thrust reversers, usually incorporating a noise-suppression system, to accomplish the same task.)

      The pilot also has an array of instruments by which to check the condition of flight, the engine, and other systems and equipment. In small private aircraft, the instrumentation is simple and may consist only of an altimeter to register height, an airspeed indicator, and a compass. The most modern commercial air transports, in contrast, have fully automated (automation) “glass cockpits” in which a tremendous array of information is continually presented on cathode-ray tube displays of the aircraft's height, attitude, heading, speed, cabin pressure and temperature, route, fuel quantity and consumption, and the condition of the engines and the hydraulic, electrical, and electronic systems. These displays also provide readouts for both routine and emergency checklists. Aircraft are also provided with inertial guidance systems for automatic navigation from point to point, with continuous updating for changing weather conditions, beneficial winds, or other situations. Cockpits have become so automated that training emphasis is focused on “resource management” to assure that the crew members keep alert and do not become complacent as their aircraft flies automatically from one point to the next.

      This array of instrumentation is supplemented by vastly improved meteorological forecasts, which reduce the hazard from weather, including such difficult to predict elements as wind shear and microburst. In addition, the availability of precise positioning from Earth-orbiting satellites makes navigation a far more exact science. Sophisticated defogging and anti-icing systems complement instrumentation for operation in adverse weather.

Flight simulators (flight simulator)
      There are three factors that force the increased use of flight simulators in training: the complexity of larger aircraft, the expense of their operation, and the increased complexity of the air-traffic control environment in which they operate. Modern simulators duplicate aircraft exactly in terms of cockpit size, layout, and equipment. They also duplicate the external environment and create a realistic sense of flying by means of the three-axis motion platform on which they are placed. Perhaps the most important use of flight simulators is to train crews in emergency situations, so that they can experience firsthand situations that could not safely be demonstrated in actual flight training. However, the simulator is also far less expensive than using actual aircraft for routine transition and proficiency training. So realistic is simulator training that airline crews are sometimes qualified on a new aircraft in a simulator prior to ever flying the aircraft itself.

Types of aircraft
      There are a number of ways to identify aircraft by type. The primary distinction is between those that are lighter than air and those that are heavier than air.

      Aircraft such as balloons (balloon), nonrigid airships (airship) (blimps (blimp)), and dirigibles are designed to contain within their structure a sufficient volume that, when filled with a gas lighter than air (heated air, hydrogen, or helium), displaces the surrounding ambient air and floats, just as a cork does on the water. Balloons are not steerable and drift with the wind. Nonrigid airships, which have enjoyed a rebirth of use and interest, do not have a rigid structure but have a defined aerodynamic shape, which contains cells filled with the lifting agent. They have a source of propulsion and can be controlled in all three axes of flight. Dirigibles are no longer in use, but they were lighter-than-air craft with a rigid internal structure, which was usually very large, and they were capable of relatively high speeds. It proved impossible to construct dirigibles of sufficient strength to withstand routine operation under all weather conditions, and most suffered disaster, either breaking up in a storm, as with the U.S. craft Shenandoah, Akron, and Macon, or through ignition of the hydrogen, as with the German Hindenburg in 1937.

      This type of aircraft must have a power source to provide the thrust necessary to obtain lift. Simple heavier-than-air craft include kites (kite). These are usually a flat-surfaced structure, often with a stabilizing “tail,” attached by a bridle to a string that is held in place on the ground. Lift is provided by the reaction of the string-restrained surface to the wind.

      Another type of unmanned aircraft is the remotely piloted vehicle. Sometimes called drones or RPVs, these aircraft are radio-controlled from the air or the ground and are used for scientific and military purposes.

      Unpowered manned heavier-than-air vehicles must be launched to obtain lift. These include hang gliders, gliders, and sailplanes.

      Hang gliders are aircraft of various configurations in which the pilot is suspended beneath the (usually fabric) wing to provide stability and control. They are normally launched from a high point. In the hands of an experienced pilot, hang gliders (glider) are capable of soaring (using rising air columns to obtain upward gliding movement).

 Gliders are usually used for flight training and have the capability to fly reasonable distances when they are catapulted or towed into the air (see photograph—>), but they lack the dynamic sophistication of sailplanes. These sophisticated unpowered craft have wings of unusually high aspect ratio (that is, a long wing span in proportion to wing width). Most sailplanes are towed to launch altitude, although some employ small, retractable auxiliary engines. They are able to use thermals (currents more buoyant than the surrounding air, usually caused by higher temperature) and orographic lift to climb to higher altitude and to glide for great distances. Orographic lift results from the mechanical effect of wind blowing against a terrain feature such as a cliff. The force of the wind is deflected upward by the face of the terrain, resulting in a rising current of air.

      Ultralights, which were originally merely hang gliders adapted for power by the installation of small engines similar to those used in chain saws, have matured into specially designed aircraft of very low weight and power but with flying qualities similar to conventional light aircraft. They are intended primarily for pleasure flying, although advanced models are now used for training, police patrol, and other work, including a proposed use in combat.

      Experimental craft have been designed to make use of human and solar power. These are very lightweight, sophisticated aircraft, designed with heavy reliance on computers and using the most modern materials. Paul MacCready (MacCready, Paul Beattie) of Pasadena, Calif., U.S., was the leading exponent of the discipline; he first achieved fame with the human-powered Gossamer Condor, which navigated a short course in 1977. Two of his later designs, the human-powered Gossamer Albatross and the solar-powered Solar Challenger, successfully crossed the English Channel. Others in the field have carried on MacCready's work, and a human-powered helicopter has been flown. Solar-powered aircraft are similar to human-powered types, except that they use solar panels to convert the Sun's energy directly to power an electric motor.

Civil aircraft
      All nonmilitary planes are civil aircraft. These include private and business planes and commercial airliners.

      Private aircraft are personal planes used for pleasure flying, often single-engine monoplanes with nonretractable landing gear. They can be very sophisticated, however, and may include such variants as: “warbirds,” ex-military planes flown for reasons of nostalgia, ranging from primary trainers to large bombers; “homebuilts,” aircraft built from scratch or from kits by the owner and ranging from simple adaptations of Piper Cubs to high-speed, streamlined four-passenger transports; antiques and classics, restored older aircraft flown, like the warbirds, for reasons of affection and nostalgia; and aerobatic planes, designed to be highly maneuverable and to perform in air shows.

      Business aircraft are used to generate revenues for their owners and include everything from small single-engine aircraft used for pilot training or to transport small packages over short distances to four-engine executive jets that can span continents and oceans. Business planes are used by salespeople, prospectors, farmers, doctors, missionaries, and many others. Their primary purpose is to make the best use of top executives' time by freeing them from airline schedules and airport operations. They also serve as an executive perquisite and as a sophisticated inducement for potential customers. Other business aircraft include those used for agricultural operations, traffic reporting, forest-fire fighting, medical evacuation, pipeline surveillance, freight hauling, and many other applications. One unfortunate but rapidly expanding segment of the business aircraft population is that which employs aircraft illegally for transporting narcotics and other illicit drugs. A wide variety of similar aircraft are used for specialized purposes, like the investigation of thunderstorms, hurricane tracking, aerodynamic research and development, engine testing, high-altitude surveillance, advertising, and police work.

      Commercial airliners are used to haul passengers and freight on a scheduled basis between selected airports. They range in size from single-engine freight carriers to the Boeing 747 and in speed from below 200 miles per hour to supersonic, in the case of the Anglo-French Concorde, which was in service from 1976 to 2003.

Aircraft configurations

wing types
      Aircraft can also be categorized by their configurations. One measure is the number of wings, and the styles include monoplanes (monoplane), with a single wing (that is, on either side of the fuselage); biplanes (biplane), with two wings, one atop the other; and even, though rarely, triplanes and quadplanes. A tandem-wing craft has two wings, one placed forward of the other.

      The wing planform is the shape it forms when seen from above. Delta wings are formed in the shape of the Greek letter delta (Δ); they are triangular wings lying at roughly a right angle to the fuselage. The supersonic Concorde featured delta wings.

      Swept wings are angled, usually to the rear and often at an angle of about 35°. Forward swept wings also are used on some research craft.

      Some aircraft have wings that may be adjusted in flight to attach at various angles to the fuselage; these are called variable incidence wings. Variable geometry (swing) wings can vary the sweep (i.e., the angle of a wing with respect to the plane perpendicular to the longitudinal axis of the craft) of their wings in flight. These two types have primarily military applications, as does the oblique wing, in which the wing is attached at an angle of about 60° as an alternative to the standard symmetrical wing sweep.

      Another configuration limited to military craft is the so-called flying wing, a tailless craft having all its elements encompassed within the wing structure (as in the Northrop B-2 bomber). Unlike the flying wing, the lifting-body aircraft (such as the U.S. space shuttle) generates lift in part or totally by the shape of the fuselage rather than the wing, which is severely reduced in size or altogether absent.

Takeoff and landing gear
      Another means of categorizing aircraft is by the type of gear used for takeoff and landing. In a conventional aircraft the gear consists of two primary wheels under the forward part of the fuselage and a tailwheel. The opposite configuration is called a tricycle gear, with a single nose wheel and two main wheels farther back. An aircraft with two main undercarriage assemblies in the fuselage and wing tip protector wheels is said to have bicycle gear.

      Large aircraft, such as the Boeing 747, incorporate multiple bogies (several wheels arranged in a variety of configurations) in their landing gear to spread out the weight of the aircraft and to facilitate stowage after retraction in flight.

      A few aircraft use skis or other structures to allow takeoff from or landing in water. These include floatplanes, which are fitted with pontoons for operation on water; flying boats, in which the fuselage also serves as a hull for water travel; and amphibians, which are equipped to land on and take off from both land and water.

      The demands placed on naval planes used on aircraft carriers require a heavier structure to withstand the stresses of catapult launches and landings abruptly terminated by arresting gear. Landing-gear mechanisms are also reinforced, and a tail hook is installed to engage the arresting gear, a system that is also used for land-based heavy military aircraft.

      The mode of takeoff and landing also differs among aircraft. Conventional craft gather speed (to provide lift) on an airfield prior to liftoff and land on a similar flat surface. A variety of means have been used in the design of aircraft intended to accomplish short takeoffs and landings (STOL vehicles (STOL airplane)). These range from optimized design of the wing, fuselage, and landing gear as in the World War II Fieseler Storch (which featured Handley Page automatic slots, extendable flaps, and a long-stroke undercarriage) to the combination of generous wing area, large flap area, and the use of large propellers to direct airflow over the wing as in the prewar Crouch-Bolas, or even such specialized innovations as large U-shaped channels in the wings as with the Custer Channel Wing aircraft. Vertical-takeoff-and-landing (VTOL (VTOL airplane)) vehicles include the helicopter, tilt rotors, and “jump jets,” which lift off from the ground in a vertical motion. Single-stage-to-orbit (SSTO) aircraft can take off and land on conventional runways but can also be flown into an orbital flight path.

Propulsion systems
      The engines used to provide thrust may be of several types.

Reciprocating engines
      Often an internal-combustion piston engine is used, especially for smaller planes. They are of various types, based on the arrangement of the cylinders. Horizontally opposed engines employ four to six cylinders lying flat and arrayed two or three on each side. In a radial engine the cylinders (ranging from 5 to as many as 28, depending on engine size) are mounted in a circle around the crankshaft, sometimes in banks of two or more. Once the dominant piston-engine type, radials are now in only limited production; most new requirements are met by remanufacturing existing stock.

      Four to eight cylinders may be aligned one behind the other in an in-line engine; the cylinders may be upright or inverted, the inverted having the crankshaft above the cylinders. V-type in-line engines, with the cylinders arranged in banks of three, four, or six, also are used.

      An early type of engine in which the propeller is affixed to the body of the cylinders, which rotate around a stationary crankshaft, is the rotary engine. Modern rotary engines are patterned after the Wankel principle of internal-combustion engines.

      Automobile and other small engines are modified for use in homebuilt and ultralight aircraft. These include two-stroke, rotary, and small versions of the conventional horizontally opposed type.

      Early in aviation history, most aircraft engines were liquid-cooled, first by water, then by a mixture of water and ethylene glycol, the air-cooled rotaries being an exception. After Charles Lindbergh's epic transatlantic flight in 1927, a trend began toward radial air-cooled engines for reasons of reliability, simplicity, and weight reduction, especially after streamlined cowlings (covers surrounding aircraft engines) were developed to smooth out air flow and aid cooling. Designers continued to use liquid-cooled engines when low frontal drag was an important consideration. Because of advances in engine cooling technology, there has emerged a minor trend to return to liquid-cooled engines for higher efficiency.

Jet engines (jet engine)
      The gas turbine engine has almost completely replaced the reciprocating engine for aircraft propulsion. Jet engines derive thrust by ejecting the products of combustion in a jet at high speed. A turbine engine that passes all the air through the combustion chamber is called a turbojet. Because its basic design employs rotating rather than reciprocating parts, a turbojet is far simpler than a reciprocating engine of equivalent power, weighs less, is more reliable, requires less maintenance, and has a far greater potential for generating power. It consumes fuel at a faster rate, but the fuel is less expensive. In simplest terms, a jet engine ingests air, heats it, and ejects it at high speed. Thus in a turbojet, ambient air is taken in at the engine inlet (induction), compressed about 10 to 15 times in a compressor consisting of rotor and stator blades (compression), and introduced into a combustion chamber where igniters ignite the injected fuel (combustion). The resulting combustion produces high temperatures (on the order of 1,400° to 1,900° F, or 760° to 1,040° C). The expanding hot gases pass through a multistage turbine, which turns the air compressor through a coaxial shaft, and then into a discharge nozzle, thereby producing thrust from the high-velocity stream of gases being ejected to the rear (exhaust).

      A turbofan is a turbine engine having a large low-pressure fan ahead of the compressor section; the low-pressure air is allowed to bypass the compressor and turbine, to mix with the jet stream, increasing the mass of accelerated air. This system of moving large volumes of air at a slower speed raises efficiency and cuts both fuel consumption and noise.

      A turboprop is a turbine engine connected by a reduction gearbox to a propeller. Turboprop engines are typically smaller and lighter than a piston engine, produce more power, and burn more but cheaper fuel.

      Propfans, unducted fan jet engines, obtain ultrahigh bypass airflow using wide chord propellers driven by the jet engine. Rockets are purely reactive engines, which usually use a fuel and an oxidizing agent in combination. They are used primarily for research aircraft and for launching the space shuttle vehicles and satellites.

      A ramjet is an air-breathing engine that, after being accelerated to high speeds, acts like a turbojet without the need for a compressor or turbine. A scramjet (supersonic combustion ramjet) is an engine designed for speeds beyond Mach 6, which mixes fuel into air flowing through it at supersonic speeds; it is intended for hypersonic aircraft.

Engine placement
      Aircraft types can also be characterized by the placement of their power plants. An aircraft with the engine and propeller facing with the line of flight is called a tractor type; if the engine and the propeller face opposite the line of flight, it is a pusher type. (Both pusher propellers and canard surfaces were used on the Wright Flyer; these have now come back into vogue on a number of aircraft. Canards are forward control surfaces and serve to delay the onset of the stall. Some aircraft also have forward wings, which provide lift and delay the stall, but these are not control surfaces and hence not canards.)

      Jet engines are variously disposed, but the most common arrangement is to have them placed underneath the wing in nacelles suspended on pylons or placed on stub fixtures at the rear of the fuselage. Supersonic and hypersonic aircraft are usually designed with the engine as an integral part of the undersurface of the fuselage, while in some special military stealth applications, the engine is entirely submerged within the wing or fuselage structure.

Materials and construction

Early technology
      For reasons of availability, low weight, and prior manufacturing experience, most early aircraft were of wood and fabric construction. At the lower speeds then obtainable, streamlining was not a primary consideration, and many wires, struts, braces, and other devices were used to provide the necessary structural strength. Preferred woods were relatively light and strong (e.g., spruce), and fabrics were normally linen or something similarly close-weaved, not canvas as is often stated.

      As speeds advanced, so did structural requirements, and designers analyzed individual aircraft parts for both strength and wind resistance. Bracing wires were given a streamlined shape, and some manufacturers began to make laminated wood fuselages of monocoque construction (stresses carried by the skin) for greater strength, better streamlining, and lighter weight. The 1912 record-setting French Deperdussin racers, the German Albatros fighters of World War I, and the later American Lockheed Vega were among the aircraft that used this type of construction.

      Aircraft made of wood and fabric were difficult to maintain and subject to rapid deterioration when left out in the elements. This, plus the need for greater strength, led to the use of metal in aircraft. The first general use was in World War I, when the Fokker aircraft company used welded steel tube fuselages, and the Junkers company made all-metal aircraft of dual tubing and aluminum covering.

      During the period from 1919 through 1934, there was a gradual trend to all-metal construction, with some aircraft having all-metal (almost always of aluminum or aluminum alloy) structures with fabric-covered surfaces, and others using an all-metal monocoque construction. Metal is stronger and more durable than fabric and wood, and, as the necessary manufacturing skills were developed, its use enabled airplanes to be both lighter and easier to build. On the negative side, metal structures were subject to corrosion and metal fatigue, and new procedures were developed to protect against these hazards. A wide variety of aluminum alloys were developed, and exotic metals like molybdenum and titanium were brought into use, especially in vehicles where extreme strength or extraordinary thermal resistance was a requirement. As aircraft were designed to operate at Mach 3 (three times the speed of sound) and beyond, a variety of techniques to avoid the effects of aerodynamic heating were introduced. These include the use of fuel in the tanks as a “heat sink” (to absorb and dissipate the generated heat), as well as the employment of exotic materials such as the advanced carbon-carbon composites, silicon carbide ceramic coatings, titanium-aluminum alloys, and titanium alloys reinforced with ceramic fibres. Additionally, some designs call for the circulation of very cold hydrogen gas through critical areas of aerodynamic heating.

Current trends in aircraft design and construction
      While the basic principles of flight that the Wright brothers applied still pertain, there have been enormous changes over the years to the means by which those principles are understood and applied. The most pervasive and influential of these changes is the broad variety of applications of computer technology in all aspects of aviation (aerospace industry). A second factor has been the widespread development of the use of composite materials in aircraft structures. While these two elements are the results of advances in engineering (industrial design), they are also indirectly the product of changing social and legal considerations.

      The social issues are manifold and include the increasing global interdependence of business, the unprecedented political revolutions in every part of the world, and the universal human desire for travel. All these come at a time when diminishing fossil-fuel resources have caused large increases in fuel prices. As a result, both computers and composite materials are necessary to create lighter, stronger, safer, more fuel-efficient aircraft.

      The legal issues are equally complex, but for the purposes of this section revolve around two elements. The first of these is that the design, test, and certification of an aircraft has become such an extraordinarily costly project that only the most well-funded companies can undertake the development of even relatively small aircraft. For larger aircraft it is now common practice for several manufacturers, often from different countries, to ally themselves to underwrite a new design. This international cooperation was done most successfully first with the Anglo-French Concorde supersonic transport and has since been evident in a number of aircraft. A component of this process is the allocation of the production of certain elements of the aircraft in certain countries, as a quid pro quo for those countries not developing indigenous aircraft of a similar type.

      The second legal element is that the potential of very large damages being awarded as a result of liability in the event of a crash has forced most aircraft companies to cease the manufacture of the smaller types of personal aircraft. The reason for this is that the exposure to damages from a large number of small single-engine planes is greater than the exposure from the equivalent market value of a few larger planes, because the larger planes generally have better maintenance programs and more highly trained pilots. The practical effect of this has been an enormous growth in the home-built aircraft industry, where, ironically, the use of computers and composites have effected a revolution that has carried over to the commercial aircraft industry.

Use of computers (computer)
      Since the mid-1960s, computer technology has been continually developed to the point at which aircraft and engine designs can be simulated and tested in myriad variations under a full spectrum of environmental conditions prior to construction. As a result, practical consideration may be given to a series of aircraft configurations, which, while occasionally and usually unsuccessfully attempted in the past, can now be used in production aircraft. These include forward swept wings, canard surfaces, blended body and wings, and the refinement of specialized airfoils (wing, propeller, and turbine blade). With this goes a far more comprehensive understanding of structural requirements, so that adequate strength can be maintained even as reductions are made in weight.

      Complementing and enhancing the results of the use of computers in design is the pervasive use of computers on board the aircraft itself. Computers are used to test and calibrate the aircraft's equipment, so that, both before and during flight, potential problems can be anticipated and corrected. Whereas the first autopilots (automatic pilot) were devices that simply maintained an aircraft in straight and level flight, modern computers permit an autopilot system to guide an aircraft from takeoff to landing, incorporating continuous adjustment for wind and weather conditions and ensuring that fuel consumption is minimized. In the most advanced instances, the role of the pilot has been changed from that of an individual who continuously controlled the aircraft in every phase of flight to a systems manager who oversees and directs the human and mechanical resources in the cockpit.

      The use of computers for design and in-flight control is synergistic, for more radical designs can be created when there are on-board computers to continuously adapt the controls to flight conditions. The degree of inherent stability formerly desired in an aircraft design called for the wing, fuselage, and empennage (tail assembly) of what came to be conventional size and configurations, with their inherent weight and drag penalties. By using computers that can sense changes in flight conditions and make corrections hundreds and even thousands of times a second—far faster and more accurately than any pilot's capability—aircraft can be deliberately designed to be unstable. Wings can, if desired, be given a forward sweep, and tail surfaces can be reduced in size to an absolute minimum (or, in a flying wing layout, eliminated completely). Airfoils can be customized not only for a particular aircraft's wing or propeller but also for particular points on those components.

Use of composite materials (composite material)
      The use of composite materials, similarly assisted in both design and application by the use of computers, has grown from the occasional application for a nonstructural part (e.g., a baggage compartment door) to the construction of complete airframes. These materials have the additional advantage in military technology of having a low observable (stealth) quality to radar.

      Some aircraft of composite materials began to appear in the late 1930s and '40s; normally these were plastic-impregnated wood materials, the most famous (and largest) example of which is the Duramold construction of the eight-engine Hughes flying boat. A few production aircraft also used the Duramold construction materials and methods.

      During the late 1940s, interest developed in fibreglass materials, essentially fabrics made up of glass fibres. By the 1960s, enough materials and techniques had been developed to make more extensive use possible. The term “composite” for this method of construction indicates the use of different materials that provide strengths, light weight, or other functional benefits when used in combination that they cannot provide when used separately. They usually consist of a fibre-reinforced resin matrix. The resin can be a vinyl ester, epoxy, or polyester, while the reinforcement might be any one of a variety of fibres, ranging from glass through carbon, boron, and a number of proprietary types.

      To these basic elements, strength is sometimes added by the addition of a core material, making in effect a structural sandwich. A core can be made up of a number of plastic foams (polystyrene, polyurethane, or others), wood, honeycombs (multicellular structures) of paper, plastic, fabric or metal, and other materials.

      The desired final shape, in terms of both external appearance and the internal structure required for adequate strength, of a component made of composite materials can be arrived at by a variety of means. The simplest is the laying up of fibreglass sheets, much as is done in building a canoe, impregnating the sheets with a resin, and letting the resin cure. More sophisticated techniques involve fashioning the material into specific shapes by elaborate machinery. Some techniques require the use of male or female molds or both, while others employ vacuum bags that allow the pressure of the atmosphere to press the parts into the desired shape.

      The use of composite materials opened up whole new methods of construction and enabled engineers to create less expensive, lighter, and stronger parts of more streamlined shapes than had previously been feasible with wood or metal. Like the computer, the use of composites has spread rapidly throughout the industry and will be developed even further in the future.

      The coincident arrival of the new technology in computers and composite materials influenced commercial air transportation, where aircraft larger than the Boeing 747 and faster than the Concorde are not only possible but inevitable. In the field of business aircraft, the new technologies have resulted in a host of executive aircraft with the most modern characteristics. These include the uniquely configured Beech Starship, which is made almost entirely of composite materials, and the Piaggio Avanti, which also has a radical configuration and employs primarily metal construction but includes a significant amount of composite material. Commercial air transports are using composite materials in increasing amounts and may ultimately follow the pattern of the military services, where large aircraft like the Northrop B-2 are made almost entirely of advanced composite materials.

      The previously mentioned legal considerations, combined with the advances in computers and composites, has completely revised the role of the homebuilt aircraft. While the homebuilt aircraft has always been a part of the aviation scene (the Wright Flyer was in fact a “homebuilt”), the designs were for years typically quite conventional, often using components from existing aircraft. Since the emergence of the Experimental Aircraft Association (founded 1953) in the United States, the homebuilt movement has operated in advance of the aviation industry, pioneering the use of computers and composites and, especially, radical configurations. While there are many practitioners in the field, one man, the American designer Burt Rutan (Rutan, Burt), epitomized this transition of the homebuilt movement from backyard to leading-edge status. Rutan, of Mojave, Calif., had a long series of successful designs, which reached the highest degree of recognition with the Voyager aircraft, in which his brother Dick Rutan and Jeana Yeager made a memorable nonstop, nonrefueled flight around the world in 1986.

      Three other areas of civil aviation have benefited enormously from these advances in technology. The first of these are vertical-takeoff-and-landing aircraft, including helicopters. The second are sailplanes, which have reached new levels in structural and aerodynamic refinement. The third are the wide variety of hang gliders and ultralight aircraft, as well as the smaller but more sophisticated aircraft that depend on human or solar power. Each of these has been vastly improved by contemporary advances in design and construction, and each holds great promise for the future.

Walter James Boyne

History of flight (flight, history of)
      Before recorded history humans knew of flight because they observed the birds, and in Greek mythology they sought to copy it, with grim consequences for Icarus. But experiments continued. In 1781 Karl Friedrich Meerwein, an architect to the prince of Baden, apparently succeeded in flying in an ornithopter (a flapping-wing machine, essentially a glider) at Giessen, Ger. This was one of the two main approaches to flying followed for a century and a quarter before directed human flight can be said to have been accomplished. The other approach was also observable in nature: in some conditions, such as that seen in the bubbles formed at the edge of waves breaking on a beach, enclosures of gas within a thin membrane would float off the Earth's surface, seeming to defy gravity. In time it was appreciated that different gases had different weights and that a lighter gas contained within a cell separated from a heavier general atmosphere formed the floating bubble buoyed upon the heavier gas. The gas-supported cell became a balloon, and as a source of flight it is a “lighter-than-air” craft, whereas the much refined successor of the ornithopter, which must do work to keep aloft, is a “heavier-than-air” craft.

      Within a three-year period in the 1780s the two types had their first successful trials—fully documented in history for the balloon and more questionably so for the ornithopter. That flying machine, first “successfully” flown at Giessen, was a highly specialized form of glider, and only by using strong updrafts of air was it lifted off the surface. For most of the time until the Wright brothers' flight in 1903 the bubble was very much ahead in the competition for flight.

Lighter-than-air craft
 In 1783 just south of St. Étienne at Annonay in southwestern France, two brothers, Joseph and Étienne Montgolfier, normally papermakers, experimented with a large cell contrived of paper in which they could collect heated air. When a sufficient quantity had been collected, the paper balloon ascended and could be so maintained as long as it contained air lighter than that of the atmosphere. As the air in the balloon cooled, the vehicle sank back to earth. On Sept. 19, 1783, the Montgolfiers sent aloft a balloon with a rooster, a duck, and a sheep, and on November 21 the first manned flight was made by Jean-François Pilâtre de Rozier and François Laurent, Marquis d'Arlandes (see photograph—>), a flight from the Chateau de la Muette across the Bois de Boulogne on the edge of Paris. French aeronautics advanced rapidly, adding hydrogen balloons (because hydrogen was a lighter gas than hot air, it could rise higher and also did not so directly depend on temperature differences).

      In the 19th century the balloon was an important specialized vehicle used in warfare (for spying behind the enemy's front lines, as did the French in the Battle of Maubeuge in 1793) and for peacetime operations (used to take the earliest aerial photographs). Balloons gained importance as their flights increased into hundreds of miles, but they were essentially unsteerable.

The dirigible (airship)
      During the American Civil War, a volunteer officer in the Union army, the former German cavalryman Count Ferdinand von Zeppelin (Zeppelin, Ferdinand, Graf (count) von), observed a free balloon ascent in St. Paul, Minn. He became so fascinated that he spent much of the remainder of his life working with balloons, particularly on the steering problem.

      As the experimentation on dirigibles continued, hydrogen and illuminating gas were substituted for hot air, and a motor was mounted on a gas bag fitted with propellers and rudders. Small steam engines were tried, but as progress took place first electric motors and, in Germany after 1888, gasoline engines (gasoline engine) were used. The problem remained how to maintain the shape of the gas bags. Fully filled with gas under the right pressure, a cigar shape could be maintained and steered; but a partially deflated bag was almost impossible to direct. It was Zeppelin who first saw clearly that maintaining a steerable shape was essential, so he created a rigid but very light frame. This solved many of the steering problems, but how to give the frame sufficient strength to deal with torque introduced by air currents in storms continued to be a severe challenge.

 At the turn of the century Alberto Santos-Dumont (Santos-Dumont, Alberto) began experimenting with steerability (see photograph—>). Adopting the gasoline engine, he was able to gain enough power in 1901 for a flight of more than three miles from St. Cloud near Paris to and around the Eiffel Tower within half an hour. Santos-Dumont recognized that he was an “aerostatic sportsman” and that his dirigibles probably had limited practical applications. He began to turn his attention to a machine-powered heavier-than-air craft.

      The most lasting work on the dirigible was that carried out by Zeppelin, who on July 2, 1900, near Friedrichshafen, Ger., on Lake Constance, undertook the first experimental flight of what he called an airship (Luftschiff); the LZ-l flew for 17 minutes before sinking to the surface of the lake and impaling itself on a buoy that punctured the gas bag. After years of cautious changes in design he was ready in 1908 with the LZ-4, 446 feet long and carrying more than half a million cubic feet of hydrogen. On July 1 he achieved 12 hours of sustained flight at a speed of 40 mile/h over central Switzerland.

      With the LZ-5, the dirigible became a potentially practical air transport. A German company, Deutsche-Luftschiffahrts AG (Delag), was organized in 1910, becoming the first well-financed air transportation company. In the five-year period up to the outbreak of World War I Delag made 1,588 flights, safely carrying 34,228 passengers, covering a total of some 170,000 miles. During the war 88 zeppelins (as they came to be known) were constructed for military purposes, among which was the introduction of the first sustained distant aerial warfare (air warfare) (which included the bombing of London and a flight from Yambol, Bulg., of 2,800 miles toward German East Africa).

      It was clear that zeppelins could fly at 45 to 50 mile/h over thousands of miles without having to land. Because the lofting of the craft depended on the lift of the gas bags, fuel loadings were relatively modest. When Germany was permitted to return to civilian flying in the mid-1920s, the Zeppelin Company began planning a transatlantic passenger voyage. Soon thereafter the company sent a new airship, the Graf Zeppelin, on an around-the-world flight. The circumnavigation was carried out in 21 days, 5 hours, and 54 minutes (of which only 47 hours had been spent on the ground, yielding an average speed of 70.7 mile/h).

      The Graf Zeppelin in the late 1920s and '30s successfully and safely flew more than one million miles in commercial service. When Hitler (Hitler, Adolf) came to power in Germany in 1933, interest turned to making a larger airship to demonstrate the surpassing ability of the Third Reich. The LZ-129 was to cruise at 78 mile/h, to be lofted by more than 7 million cubic feet of hydrogen, and to be able to carry about 50 passengers. Named the Hindenburg, for the German president at the time of Hitler's rise to power, the LZ-129 made its inaugural flight in 1936. Service was resumed in the spring of 1937, after a gap for the stormy winter months; all went well until the docking procedure at Lakehurst, N.J., on May 6, 1937, when the dirigible burst into flames and exploded with a loss of 36 lives. That afternoon the dirigible ceased to be effective competition for the airplane, which commenced transatlantic civil air service only two years later.

Heavier-than-air craft: early history
Early experiments
      The ornithopter in the 1780s had demonstrated that by applying a considerable amount of power to a machine of very light weight it should be possible to take off and fly above the Earth's surface in a heavier-than-air craft. This was accomplished by the “superlight” aircraft flights of the 1980s, including the successful crossing of the English Channel in a craft powered only by a single man's muscles.

      Two problems arose: to find a favourable ratio between the weight of the vehicle and the power applied and to find a mechanical means to apply that power to lifting off the ground and achieving steerable forward motion. In 1799 the English physicist George Cayley (Cayley, Sir George, 6th Baronet) worked out most of the aerodynamic theory (aerodynamics). After Cayley's writing the ornithopter experiments were largely abandoned and replaced by trials of gliders (glider), including Cayley's own in 1852–53. By the end of the 19th century the conditions were nearly ready for heavier-than-air flight. The development of the internal-combustion engine and of petroleum-based fuels (naphtha and gasoline) that were powerful in relation to weight meant that the problem of securing lift had essentially been solved. What remained were additional problems of applying that power to the vehicle. It is not without reason that the successful inventors of the airplane were two bicycle manufacturers from Dayton, Ohio: many of the problems of developing a rider-powered bicycle were reflected in shaping a self-powered heavier-than-air plane.

The Wright brothers
      Wilbur and Orville Wright in the course of their experiments came increasingly to consider Cayley's diagram of how a wing works, particularly the role played by the speed of the wind passing over the top of the wing. This led them to seek a site with a strong and persistent ambient wind (the Vogels Mountain where the 1781 ornithopter may have flown has just such a high ambient wind, as do the hills near Elmira, N.Y., and Fremont, Calif., classic gliding courses). From the U.S. Weather Bureau the Wrights secured a list of windy sites in the United States, from which they chose the Outer Banks of North Carolina, specifically Kitty Hawk (Wright flyer of 1903). On Kill Devil Hill there on Dec. 17, 1903, Orville Wright became the first man ever to fly in an aeroplane (airplane) (as they were at first known), initially using as a frame a biplane of 40-foot 4-inch wingspan and equipped with the 12-horsepower engine. He lifted off the ground in a 20–27-mile/h wind and flew a distance of 120 feet in 12 seconds. Having a strong wind certainly aided in that accomplishment, but the brothers soon demonstrated that such a wind was not absolutely essential.

      After further experiments at Kitty Hawk they returned to Dayton to build a second plane, Flyer No. 2. Neither the balloons and dirigibles nor the earlier ornithopter and glider experiments had produced flight: what they had done was to harness the dynamics of the atmosphere to lift a craft off the ground, using what power (if any) they supplied primarily to steer. The Wrights initially used atmospheric dynamics to help in lifting the plane, but they subsequently demonstrated that they were able to lift a plane off the ground in still air.

      In the long run their most significant invention was a way to steer the plane. After carefully watching a great number of birds (bird), they became convinced that birds directed their flight by internally warping their wings, distorting them in one fashion or another. To do this in their plane, the Wrights constructed a ridged but distorted wing that might, through the use of wires fixed to the edge of the wing, be flexed to pass through the air in changing directions. This distortable wing was relatively misunderstood by other early plane experimenters.

      During the summer of 1904 the Wrights made 105 takeoffs and managed to fly on a circular course up to 2.75 miles for a sustained flight that lasted 5 minutes 4 seconds. Because they took a proprietary view of their invention, publicity about their work was minimal. After further trials in 1905 they stopped their experiments, using the time to obtain patents on their contribution. Only in 1908 did they break their secrecy when Wilbur Wright went to France to promote their latest plane.

Developments between the wars
      There were significant further developments from the Wrights' plane. Glenn Curtiss (Curtiss, Glenn Hammond), another bicycle builder, developed an airplane that came to be known as the “1909 type” (it won the Reims air race of that year). At Hammondsport in upstate New York Curtiss built planes noted for their powerful engines. Since then, American (United States) plane manufacture has been notable for engine strength. By 1914 Curtiss was building a twin-engined seaplane that he intended to fly across the Atlantic. World War I interrupted this effort, but flying service in Florida across the 22 miles of Tampa Bay between Tampa and St. Petersburg that year became the first commercial airplane service in the world.

      Although World War I interrupted commercial developments, it led to rapid technical improvements in aircraft. In 1919 a Curtiss NC-4 flying boat accomplished the first aerial crossing of the Atlantic—between Newfoundland and Lisbon, with a stop in the Azores—under the command of Lieutenant Commander A.C. Read. Only a month later, in June 1919, a nonstop flight from Newfoundland to Galway in Ireland was accomplished by British Captain John Alcock (Alcock, Sir John William) and Lieutenant Arthur Whitten Brown (Brown, Sir Arthur Whitten) in 16 hours and 27 minutes, making an average speed of 118.5 mile/h in a converted Vickers Vimy bomber. These tests used military aircraft, but after the war the airplane industry designed avowedly commercial planes. The French aeronaut Louis Blériot (Blériot, Louis) had begun the work in 1907 by building his Number VII as a monoplane, followed two years later by an improved machine in which he accomplished the first flight across the English Channel.

      After the war Anthony H.G. Fokker (Fokker, Anthony Herman Gerard) in Holland pursued the high-wing monoplane with a stressed wooden skin, while Hugo Junkers (Junkers, Hugo) in Germany used a stressed metal skin and a low wing that reduced weight. The designer John Northrop (Northrop, John Knudsen) and the Lockheed Aircraft Company (Lockheed Martin Corporation) in the United States produced what in many ways became the model for modern commercial aircraft in the Vega of 1927. As was the American practice, the Vega was well-powered, with radial engines of either 220- or 425-horsepower, which allowed a pilot and six passengers to be flown at between 110 and 135 mile/h at a range between 500 and 900 miles. The use of a stressed wooden skin allowed about a 35 percent savings in weight over a stressed metal skin.

Aviation goes commercial
Formation of airlines
      With practical planes in hand in 1918 the organization of an airline to operate these craft on a scheduled basis over a consistent route was attempted. The first airline was formed in Germany; the Deutsche Luftreederie began service from Berlin to Leipzig and Weimar on Feb. 5, 1919, followed only three days later by the French Farman Company on the trans-channel crossing from Paris to London using a converted Goliath bomber. In August 1919, the first daily service was established on this route from Le Bourget to Hounslow. The oldest surviving airline, KLM, was organized in The Netherlands in 1919 and jointly with a British company began flying the Amsterdam-London route the following year. Outside Europe, the Queensland and Northern Territories Aerial Services, Ltd. (Qantas (Qantas Airways Limited)) was founded in 1920; it eventually became the Australian national airline.

      Most of the airlines founded in the 1920s and '30s were created at least in part to encourage the purchase of aircraft of domestic manufacture; but the privately owned Swissair (Swiss International Air Lines) was the first European airline to purchase American aircraft. The intertwining of domestic aircraft manufacture and national airline operation was widely advocated as critical to national defense. In the United States airline pioneers were private operators, as were the aircraft builders, and there was no national policy concerning either operation. Throughout the 1920s there were no adequately financed airlines, and most lasted for only short periods before failing or merging. Given the large area of the United States, an airline with routes of national or even regional coverage was the exception. And it was only in the late 1920s that any thought was given to the question of encouraging a domestic aircraft industry or the promotion of domestic airline companies.

      A second factor, especially in Europe, was the colonial airline. Britain, France, The Netherlands, and Germany all developed colonial airlines, with Belgium, Italy, and the United States joining the operation less extensively. Routes for national airlines were limited to destinations within a country or its possessions, except by agreement. The extensive colonial empires still in existence in the 1920s and '30s became natural sites for extended airlines. Britain, for example, created Imperial Airways by first using bilateral agreements with other European countries to reach the Mediterranean and, once there, to project a continuation based on British colonies and protectorates in Malta, Cyprus, Palestine, Trans-Jordan, the Iraq and Persian Gulf protectorates, India, Burma, the Malay Protectorate, Australia, and New Zealand. China, Central Africa, and South Africa could be reached by other routes. Only the North Atlantic and the northern Pacific resisted a “British” national airline. France shaped a colonial airline from Provence across the Mediterranean to Algeria, the French Sahara, French Equatorial Africa, and Madagascar. Working out landing rights between Belgium and France provided a route to the Belgian Congo. The Netherlands, again through trades with Britain, shaped a colonial route for KLM to the Dutch East Indies.

      In the 1930s these colonial routes were the main long-distance air routes available not only because a far-flung empire simplified the problem of securing landing rights but also because the operating “stage”—that is, the maximum distance that might be flown without stopping to refuel—was then only about 500 miles. The Pacific and the Atlantic were the major “water jumps” that remained unconquered by civil aircraft in 1930. The American air routes showed the way to the solution. Pan American Airlines (Pan American World Airways, Inc.) was first organized to fly from Miami to Key West in Florida and to Havana and by the 1930s from Brownsville, Texas, to Mexico City and Panama. Pan American founder Juan Trippe (Trippe, Juan T.) advocated the concept of the “chosen instrument”—international connections for the United States should be provided by a single American company flying only outside the country. The American “empire” in this sense was Latin America (Latin America, history of), where American investment was extensive but political control was only indirect. Germany, which after World War I lost its empire, similarly turned to South America, particularly Colombia, to shape an extensive system of air routes. In the American case, Pan American's ultimately extensive route structure in the Caribbean, on the east coast of South America, and in Central America provided experience in operating a long-distance international airline.

      By the early 1930s three airlines in particular were seeking to develop world-scale route patterns—Pan American, Imperial Airways, and KLM. Such a development called for a set of aircraft that were entirely new in concept from those that had been derived from the planes of World War I. Specifically, what was needed were seaplanes (seaplane), which offered some of the advantages that the Zeppelin company, Delag, had obtained with their dirigibles. They could fly stages of considerably greater length than could be flown with standard land planes because the sea-based plane enjoyed an almost infinite takeoff runway, that of a long stretch of water in a sheltered embayment. Several miles might be used at a time when a 1,000-foot airport runway was the norm. Long runways, either on land or on water, meant that planes could be quite large, use multiple engines, have large enough fuel tanks to fly an extended stage, and require less strength in the undercarriage.

      The tradition of high-powered planes introduced between 1907 and 1909 by Glen Curtiss continued. In addition to the Curtiss company, Martin (Martin, Glenn L) and Sikorsky (Sikorsky, Igor) each produced large four-engine seaplanes with the potential for stages of more than 500 miles. Because of its size, the United States showed a concern for lengthening the stage even of land-based planes. When Pan American adopted the seaplane in the early 1930s, the Sikorsky S-42 flying boat had four engines that permitted it to fly to Buenos Aires, Arg., by making a series of water crossings between Puerto Rico and the Río de la Plata.

      After World War I, another factor contributed to airline development: the desire for an air service to speed up the mails (mail). Unlike Europe, where the nationalized airlines carried the mail, in the United States the Army Air Corps was assigned the job, with generally dreary results. The problems of flying in a country the size of the United States were considerable. Particularly in the East, with the broad band of the Appalachians lying athwart the main routes, bad flying conditions were endemic and crashes were frequent. The introduction of aircraft beacons helped, but the low altitudes at which most contemporary planes could operate continued to plague service. Commercial flying began in earnest in 1925 when, under the Kelly Act, the United States Post Office Department established contracts for carrying mail over assigned routes. Payments were made in return for the weight of mail carried. This practice often gave earnings that made the difference between marginal operation and flying at outright losses. Later, the method of airmail payments was revised; instead of paying for the weight of mail carried, the Post Office paid instead for the space reserved for airmail were it to be offered to the airline company to transport. The result was an incentive to the companies to increase the size of the planes they normally flew.

Growth of the aviation industry
      Competition for the airmail routes led to the formation of several large American aviation companies. William Boeing, who during World War I as a lumber producer in Seattle had built planes from Sitka spruce (a wood with fibres of great tensile strength), bid on what came to be called the “Columbia Route” (New York City to California's San Francisco Bay area), winning the western segment from Chicago to Oakland. Henry Ford (Ford, Henry), who for several years had been building a trimotor plane (rather similar to the Fokker Trimotor), secured the Cleveland-to-Chicago route. To serve the western section Boeing experimented with new and larger planes built by the Boeing Aircraft Company (Boeing Company), which in the following 60 years became the world's largest and most comprehensive civilian aircraft manufacturer. United Aircraft and Transport joined with National Air Transport (which later became United Airlines) and others to create a second aviation company that secured the contract for the eastern segment of the Columbia Route (from Chicago to New York City) and for the north-south route on the west coast from Vancouver, B.C., to Los Angeles. A further recipient of an airmail contract was the Aviation Corporation (North American and Curtiss aircraft builders), which became American Airlines. The General Motors Corporation held major ownership in Transcontinental Air Transport (T.A.T.) as well as Eastern Transport on the north-south airmail route on the east coast. With Pan American, which was assigned several foreign routes, these aviation companies constituted the “Big Five” airlines, which survived as the dominant U.S. carriers until the 1990s.

Improvements in aircraft operation
      In the late 1920s airlines were stymied by two problems: night flying and high-altitude flying. Both were too dangerous for passenger transportation. In the United States, crossing the Appalachians was possible, as the operating ceiling of the planes exceeded the necessary 3,000 to 4,000 feet. In the Rockies and the western Coast Ranges, however, there were 8,000- to 10,000-foot passes. And continuous flight over a major part of the United States could not be accomplished during daylight hours.

      In 1929 Transcontinental Air Transport and the Pennsylvania Railroad (Pennsylvania Railroad Company) joined forces to solve, at least in part, these altitude and darkness problems. They organized a rail-plane route between New York City and Los Angeles. The “Airway Limited” departed New York's Pennsylvania Station at 6:05 PM, using a Pullman sleeper to reach Port Columbus, Ohio, a new landing field outside the Ohio capital. There passengers boarded a Ford Trimotor at 8:15 AM, which carried 10 passengers to Waynoka, Okla., by 6:24 PM, in time to board a second Pullman sleeper on the Santa Fe Railway at 11:00 PM. This was to arrive in Clovis, N.M., at 8:10 AM, when the passengers boarded a second plane to fly to Los Angeles, and, for through passengers, on to San Francisco by 7:45 PM. The route avoided most night flying and any mountains over about 5,000 feet.

      Such an arrangement demonstrated the need for planes better than the Ford Trimotor, the workhorse of American carriers in the late 1920s. By 1928 Ford had improved speed on his plane from 100 mile/h on the 1926 model to 120 mile/h on the 1928 model through the introduction of stronger radial engines that were coming into use in the United States, such as that found on Charles Lindbergh (Lindbergh, Charles A.)'s Ryan monoplane, which made the first solo flight across the Atlantic in 1927. By 1929 the United States was building 5,500 aircraft, up from only 60 five years earlier. The Vega of 1927 had increased cruising speed up to 150 mile/h.

      In 1930, Boeing (Boeing Company)'s Monomail demonstrated the virtues of all-metal planes with the installation of retractable landing gear. Most experts view the Boeing-247 of 1933 as the first modern commercial aircraft. It showed that twin-engined planes were safer than trimotors because they could be maneuvered more easily and might be flown on a single engine. So many of the planes were ordered that when Transcontinental and Western Airlines (TWA (Trans World Airlines, Inc.), formerly T.A.T.) sought to order some, Boeing declined. TWA turned to a smaller builder, the Douglas Company, and commissioned a similar plane as a trial. The prototype was the DXCX-l; in its developed form as the DC-2/3, it proved to be the most significant commercial plane ever built.

      The plane was first introduced as a prototype (the DC-1) in 1933 and put into production as the DC-2 (and in an evolved form as the DC-3 in 1936). The first DC-2 was put in service on the Newark-Pittsburgh-Chicago run, after only 11 months' development time. In an era when American engine builders were introducing new and more powerful engines at a regular and rapid rate, the Wright Engine Company had been able to substitute an improved and more economical engine by the time quantity production began. American Airlines asked for a slight enlargement of the DC-2 (which thus became the DST, a sleeper transport built to allow space for berths for use on the circuitous transcontinental route flown by American). When fitted out with seats this enlargement held 21 passengers and was called a DC-3. As such, it was the first airliner to operate at a profit with a reasonable load factor. The DC-3 had a ceiling of above 5,000 feet, could fly on only one engine, and with a stressed aluminum sheathing was a strong plane with a retractable landing gear. In the 10 years it was in production, the DC-3 became the unrivaled master airliner, carrying the majority of American traffic. It was found on most of the world's airlines, was used for military cargo (as the C-47 in the United States and the Dakota in Britain), and was constructed in a run of more than 13,000 planes. Even 60 years after its introduction, the DC-3 is still seen in out-of-the-way places and for certain purposes. Undoubtedly its greatest contribution was that it showed with great clarity that flying could be safe, reliable, affordable, and profitable for the operator. Flying was a curiosity when the DC-3 was first built but was standard transportation when it was last manufactured.

      Between 1927 and the end of the 1930s the smaller aircraft engine rapidly advanced in its technology. Before World War I the Russian aeronautic engineer Igor Sikorsky had constructed a 12-engine flying boat. In the progression from DC-1 through DC-3 knowledge secured from earlier expressions of a basic design was then used to enlarge that design so as to gain size, speed, and economy. Certain general qualities were standardized. The typical DC plane had a squarely rounded fuselage, a low wing, a particular way of carrying engine pods, and other features that had become standard. For example, if enlarging the passenger load was sought, the fuselage would be lengthened rather than widened (which tended to change the aerodynamic qualities of the plane). A longer plane required no other changes than enlarging the engines. Engines could be made more powerful by turbocharging them (supercharging (supercharger) them using centrifugal blowers driven by exhaust gas turbines), enlarging the cylinders, and making other mechanical elaborations. American aircraft builders became very adept at securing more power to go faster, farther, or cheaper.

Modern aviation era
The four-engine plane
      Eventually the four-engine plane was planned. Sikorsky (Sikorsky, Igor) had built the first four-engine plane, the Bolsche of 1913. As long as a single aircraft engine could not generate much more than 1,000 horsepower, multiple engines became the only way to gain the total amount of power necessary to lift the large loads of fuel needed for long journeys. When Pan American (Pan American World Airways, Inc.) sought to open a service from Alameda (Oakland), Calif., to Manila and China, it faced a 2,400-mile maximum stage between the San Francisco Bay area and Honolulu. Only a four-engine plane could lift enough fuel to make such a “jump.” A further constraint entered the planning: such large planes and the fuel load they would carry could not lift off the ground on the landing strips then available. Only landing on the surface of sheltered waters would provide the thousands of feet required. The Germans in attempting to establish a transatlantic airmail route experimented with artificially calmed stretches of ocean, but the operation was far too risky ever to be used in passenger service. Only through the use of insular stepping-stones properly spaced, such as the Americans controlled west of Honolulu, could an ocean crossing be obtained. In 1932 Pan American signed a contract with Sikorsky to build a four-engine flying boat capable of carrying mail and passengers across the Pacific and a second contract that same year for an even larger flying boat, weighing 26 tons, to be built by Glenn Martin (Martin, Glenn L). On Nov. 22, 1935, the first airmail flight left Alameda for Manila using the Martin M-130 (the China Clipper), with a wingspan of 130 feet (equal to the Boeing 727 of a generation later). Passengers were added to the service in 1936, when the first long transoceanic flight began.

      The success of these huge flying boats greatly whetted the appetite of American airline operators because it demonstrated the advantages that might be hoped for from four-engine planes, particularly in raising the ceiling on normal commercial flight so that airlines might “fly above the weather.” To do so, it was necessary to artificially pressurize plane cabins above 6,000 to 8,000 feet. Half the weight of the atmosphere (atmospheric pressure) is normally found in the column below 18,000 feet, and most of the turbulence is located there. Early experimental flights had shown that as an aircraft rises in the atmosphere it tends to encounter less stormy conditions; most of the “weather” is found below 4,000 feet. If planes could operate at such higher altitudes, flights would be more comfortable and there would be less resistance to forward movement, allowing the same input of power to move the plane at a greater speed. The first hurdle came in securing an airtight cabin, but success in this operation had to be accompanied by better engines, as was done in the Boeing Stratoliner introduced in 1940. Capable of flying at 14,000 feet and at a speed of 200 mile/h, the Stratoliner had just begun service when war in Europe broke out; development of this pioneering four-engine plane was taken over by the government for the duration of the war. It was the only commercial aircraft to be able to fly directly from Newfoundland to Northern Ireland during World War II. With its powerful supercharged engines the Stratoliner could navigate not only above weather but over rather than around mountains. Thus routes could be chosen because they formed parts of great circles on the Earth's surface and were thereby the shortest possible distances between two points.

      A second four-engine plane was designed just before World War II when the general configuration of the DC-3 was transformed into a four-engine size. Unlike the Stratoliner, this was not a pressurized plane, so it represented the last phase of one line of advance more than the beginning of a postwar design. The enlarged DC-4 was flown throughout the war, becoming the main transatlantic aircraft, in the form of the United States Army's C-54 troop transport.

Postwar developments
      Near the end of World War II, the nature of the postwar airline industry began to concern the Western Allies. At the Chicago Convention on International Civil Aviation held in November–December, 1944, the United States advocated an “open skies” policy. Strongly opposed was Britain, which argued that freedom of the skies actually had five expressions, of which the last was the most important. They were (1) the right of transit—that is, to pass through the airspace of a country without landing there, (2) the right to make a technical stop in a country, to pick up fuel or to make repairs, (3) the right to discharge passengers at an airport in the country involved, (4) the right to pick up passengers in that country to return them to the country of origin of the airline, and (5) the right to discharge passengers in that foreign country and then pick up passengers originating there and carry them to a third country. Of these purported rights the first four were already in effect. It was what came to be known as “the fifth freedom” that caused heat at the Chicago Conference.

      Today the main restriction on flying appears under two headings: exception of the fifth freedom from certain specific bilateral agreements and general enforcement of the law of cabotage. This law has operated since the Middle Ages, reserving the trade within a country to that country. Thus, though a Dutch plane might land in New York City on an around-the-world flight and land again in Los Angeles, it would not be permitted to carry passengers or goods between those two cities. It was not foreseen at the time of the signing of the Chicago Convention that the stage of planes would become long enough to cross, for example, the United States.

      After World War II air transportation was quickly restored to civilian life. The Stratoliner and the DC-4 began immediate service on the longer routes, even across the Atlantic and the Pacific. Even more important was the introduction of a plane that for a decade became the prime competitor of the DC-4, the Lockheed (Lockheed Martin Corporation) Constellation. The rapid growth in the power produced by American aircraft engines encouraged TWA to turn to the Lockheed company in search of a plane that would add more than 100 miles/h to the speed of the DC-3 (175 mile/h) rather than the marginal 25 mile/h increase of the DC-4. In addition, TWA engineers sought to lengthen the stage of planes so that a single-stop transcontinental flight was possible in either direction. When put into service, the Constellation had an 80 mile/h speed advantage over the DC-4. When the Super-Constellation went into service in 1957, it weighed twice as much as its precursor, was considerably faster, and carried a much increased payload.

      The very rapid growth of air traffic in the 10 years after 1945 called forth a number of different planes to deal with extended routes and enlarging markets. In large part this expansion could take place because there was a market for used aircraft. As airlines strove to fly faster and with lengthened stages, more people switched from trains or ships to planes. By 1953 the DC-7 was put in service with a stage of up to 3,000 miles and a speed reaching 300 mile/h. By 1957 the number of passengers crossing the Atlantic by air was greater than by sea. Once jet planes came into service at the end of the 1950s, flying the Atlantic accelerated to the point that little more than a decade of steamship service remained before the end of the Atlantic Ferry.

The jet era
      During that critical decade great technical changes were made in passenger flying. During the first eight years after the war the DC-4 and the Constellation competed grimly to dominate long-distance flying. The DC-6 replaced the DC-4 on the most prestigious runs as the Super-Constellation took over from its more modest predecessor. In the final stage in this drive for the ultimate piston-engine plane, the DC-7 and the Super-Constellation were built, but they held the lead only briefly. The piston engine had reached its ultimate perfection.

      The search then shifted to the British aircraft industry, which had tried throughout the postwar years to gain an important role in civil aviation. British hopes for success turned in the direction of the jet turbine engine. In the 1950s, when British competitors of the Douglas and Lockheed planes failed to find an extensive market, they advanced the theory of the turbine-engined jet plane, first proposed by Frank Whittle (Whittle, Sir Frank) when he was a Royal Air Force cadet in 1927–28. In 1929 he settled on the pure gas turbine as the engine best suited to increasing the speed of flight. In the 1930s Hans von Ohain (Ohain, Hans Joachim Pabst von) at Göttingen, Ger., and at the Heinkel Aircraft Works in Warnemünde, also worked on the jet engine. In that same period Wernher von Braun (Braun, Wernher von) in Germany and Robert Goddard (Goddard, Robert Hutchings) at Clark University in Worcester, Mass., U.S., were experimenting with the rocket motor to accomplish the same end. By 1937 Whittle had an operating engine with all the basic features of a turbojet, and by August 1939, the German aircraft designers Ernst Heinrich Heinkel (Heinkel, Ernst Heinrich) and Ohain had built the first turbine. These jet engines demonstrated the ability to operate at high speeds when there seemed not to be airframes strong enough for the task. The experiments had shown that the planes could operate effectively at high speeds but not at what might be termed intermediate speeds of 300 to 350 mile/h. The DC-7 flew at 300 mile/h using the giant piston engines built for it.

      Even before the ceiling on speed of the piston plane was reached in the DC-7 in the mid-1950s, the Vickers company in Britain had flown an adaptation of the turbine that used the favourable power-to-weight ratio of the jet engine harnessed by gears to a propeller and placed in an airframe that could operate as a turboprop plane at 40 or 50 mile/h faster than the fastest piston engines similarly geared. Although British, French, and American aircraft builders ultimately constructed specifically turboprop planes, most builders simply put turboprop engines in the latest models of their planes. European airlines took up the turboprop plane more enthusiastically than did American airlines. In the United States the relatively short stage of these planes and the high fuel consumption in comparison with the best piston planes never made them exceptionally popular. The Vickers Viscount was adopted for its newness and its successor the Vanguard for its large windows. Finally in 1957 the Bristol company in Britain built the Britannia, a turboprop that operated at a reasonable cost and with a longer stage than others. Unfortunately it was only a year later that the eminently successful pure-jet Boeing (Boeing Company) 707 was put in service; the British turboprop continued for some years to do yeoman service on nonscheduled charter flights and other supporting rather than starring roles.

      The turboprop rather quickly disappeared when it was discovered that jet engines could be placed in planes of varying size and purpose. It was anticipated that the jet would revolutionize the speed of air travel: what was rather unexpected was that it would sharply reduce its cost when provided by a jetliner large enough to carry an economical load. The Boeing 707 was so economical when it was placed in service, by Pan American (Pan American World Airways, Inc.), on Oct. 26, 1958, that it played the role for commercial jets that the DC-3s had for piston planes. When the fan jet was substituted for the simple jet engine, the family of Boeing jets earned a reputation for economical working just as the DC-6 had in the last generation of piston planes. Within a few years Boeing had developed specialized jets for nearly the full range of commercial flying. The Boeing 727 became an intermediate-range jet carrying more than 100 passengers, rivaling in size the largest piston planes. Later, the Boeing 737 became the workhorse of North American airlines. When it was discovered that the cost of operating jets was considerably less per passenger mile than the cost of operating even the best piston-engine planes, flying grew rapidly and became quite common over considerably greater distances. The Boeing Company began planning what came to be known as a “jumbo jet,” the 747. When placed in service in 1970, the 747 was capable of carrying up to about 500 passengers, but most models were fitted out for about 400, with substantial space allocated for baggage, mail, and freight.

      The longevity of jet planes was also not fully anticipated. The upkeep on jet engines is simpler and more long-lasting, so considerably less time is taken up by maintenance. This is reflected in geographic patterns of operation. The longer air route tends to be operated with larger planes operating at a lesser frequency. Transatlantic and transpacific air service tends toward a single flight by a company per day connecting each pair of cities it serves. Exceptions occur mostly for London, New York City, Los Angeles, San Francisco, and Tokyo, where there may be two flights between a pair. Owing to the speed of flying and the progression of time around the world during the day, virtually all westbound flights, from Europe to North America and North America to eastern Asia, take place during the daylight hours, whereas eastbound flights from East Asia to western North America and eastern North America to Europe operate during hours of darkness. A single plane operating on one of the world's longer runs, for example, the Paris–Los Angeles route, can leave Paris in late afternoon, arrive in Los Angeles in the evening, and there reload for Paris, where it returns in midafternoon, thus flying about 10,000 miles in a 24-hour period. Unlike the early days of shorter stages using multiple aircraft and frequent landings, only one plane and two airports are involved, but a transfer between the west coast of North America and the west coast of Europe of nearly 1,000 people per day may take place.

James E. Vance, Jr.

Additional Reading
The basic elements of flying are treated in Pilot's Handbook of Aeronautical Knowledge, rev. ed. (1986), prepared by the U.S. Federal Aviation Administration; Richard L. Taylor, Understanding Flying (1977, reissued 1987), covering both human and mechanical aspects of the process; and Richard von Mises, Theory of Flight (1945, reissued 1959; originally published in German, 4th ed., 1936), rather technical but comprehensive in coverage. Walter J. Boyne, The Smithsonian Book of Flight (1987), is a historical treatment of a wide variety of aviation topics for the general reader. George Geoffrey Smith, Gas Turbines and Jet Propulsion, 6th ed., rev. and enlarged by F.C. Sheffield (1955), explains the functions of the turbine jet engine; Bill Gunston, World Encyclopaedia of Aero Engines, 2nd ed. (1989), discusses a wide range of engines in historical context; L.J.K. Setright, The Power to Fly: The Development of the Piston Engine in Aviation (1971), is a history of this particular type of engine; and W.H. Deckert and J.A. Franklin, Powered-Lift Aircraft Technology (1989), is a short overview, prepared by the National Aeronautics and Space Administration, of aircraft that have the capability to vary in flight the direction of the force of the propulsive system.Darrol Stinton, The Design of the Aeroplane: Which Describes Common-Sense Mechanics of Design as They Affect the Flying Qualities of Aeroplanes Needing Only One Pilot (1983), is useful for understanding the broader aspects of aircraft design; and Edward H. Heinemann, Rosario Rausa, and K.E. Van Every, Aircraft Design (1985), surveys the more sophisticated elements of design with a minimum of mathematics. An insightful look into the incremental steps in the refinement of aircraft design over the years is offered in Laurence K. Loftin, Jr., Quest for Performance: The Evolution of Modern Aircraft (1985). Descriptions, illustrations, and specifications of aircraft of a number of countries are provided by Jane's All the World's Aircraft (annual).For the history of aviation, see C.H. Gibbs-Smith, Flight Through the Ages: A Complete Illustrated Chronology from the Dreams of Early History to the Age of Space Exploration (1974); L.T.C. Rolt, The Aeronauts: A History of Ballooning, 1783–1903 (1966, reissued 1985); Carl Solberg, Conquest of the Skies: A History of Commercial Aviation in America (1979); R.E.G. Davies, A History of the World's Airlines (1964); and John Toland, Ships in the Sky: The Story of the Great Dirigibles (1957).Walter James Boyne James E. Vance, Jr.

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Universalium. 2010.

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