Friday, October 19, 2012


Airplane, engine-driven vehicle that can fly through the air supported by the action of air against its wings. Airplanes are heavier than air, in contrast to vehicles such as balloons and airships, which are lighter than air. Airplanes also differ from other heavier-than-air craft, such as helicopters, because they have rigid wings; control surfaces, such as movable parts of the wings and tail, which make it possible to guide their flight; and power plants, or special engines that permit level or climbing flight.
Modern airplanes range from ultralight aircraft weighing no more than 46 kg (100 lb) and meant to carry a single pilot, to great jumbo jets, capable of carrying several hundred people or several hundred tons of cargo. The largest commercial passenger airplanes weigh nearly 560 metric tons and the largest cargo jets up to 640 metric tons.
Airplanes are adapted to specialized uses. Today there are land planes (aircraft that take off from and land on the ground), seaplanes (aircraft that take off from and land on water), amphibians (aircraft that can operate on both land and sea), and airplanes that can leave the ground using the jet thrust of their engines or rotors (rotating wings) and then switch to wing-borne flight.
An airplane flies because its wings create lift, the upward force on the plane, as they interact with the flow of air around them. The wings alter the direction of the flow of air as it passes. The exact shape of the surface of a wing is critical to its ability to generate lift. The speed of the airflow and the angle at which the wing meets the oncoming airstream also contribute to the amount of lift generated.
An airplane’s wings push down on the air flowing past them, and in reaction, the air pushes up on the wings. When an airplane is level or rising, the front edges of its wings ride higher than the rear edges. The angle the wings make with the horizontal is called the angle of attack. As the wings move through the air, this angle causes them to push air flowing under them downward. Air flowing over the top of the wing is also deflected downward as it follows the specially designed shape of the wing. A steeper angle of attack will cause the wings to push more air downward. The third law of motion formulated by English physicist Isaac Newton states that every action produces an equal and opposite reaction (see Mechanics: The Third Law). In this case, the wings pushing air downward is the action, and the air pushing the wings upward is the reaction. This causes lift, the upward force on the plane.
Lift is also often explained using Bernoulli’s principle, which states that, under certain circumstances, a faster moving fluid (such as air) will have a lower pressure than a slower moving fluid. The air on the top of an airplane wing moves faster and is at a lower pressure than the air underneath the wing, and the lift generated by the wing can be modeled using equations derived from Bernoulli’s principle.
Lift is one of the four primary forces acting upon an airplane. The others are weight, thrust, and drag. Weight is the force that offsets lift, because it acts in the opposite direction. The weight of the airplane must be overcome by the lift produced by the wings. If an airplane weighs 4.5 metric tons, then the lift produced by its wings must be greater than 4.5 metric tons in order for the airplane to leave the ground. Designing a wing that is powerful enough to lift an airplane off the ground, and yet efficient enough to fly at high speeds over extremely long distances, is one of the marvels of modern aircraft technology.
Thrust is the force that propels an airplane forward through the air. Thrust is provided by the airplane’s propulsion system: either a propeller or jet engine or combination of the two.
A fourth force acting on all airplanes is drag. Drag is created because any object moving through a fluid, such as an airplane through air, produces friction as it interacts with that fluid and because it must move the fluid out of its way to do its work. A high-lift wing surface, for example, may create a great deal of lift for an airplane, but because of its large size, it is also creating a significant amount of drag. That is why high-speed fighters and missiles have such thin wings—they need to minimize drag created by lift. Conversely, a crop duster, which flies at relatively slow speeds, may have a big, thick wing because high lift is more important than the amount of drag associated with it. Drag is also minimized by designing sleek, aerodynamic airplanes, with shapes that slip easily through the air.
Managing the balance between these four forces is the challenge of flight. When thrust is greater than drag, an airplane will accelerate. When lift is greater than weight, it will climb. Using various control surfaces and propulsion systems, a pilot can manipulate the balance of the four forces to change the direction or speed. A pilot can reduce thrust in order to slow down or descend. The pilot can lower the landing gear into the airstream and deploy the landing flaps on the wings to increase drag, which has the same effect as reducing thrust. The pilot can add thrust either to speed up or climb. Or, by retracting the landing gear and flaps, and thereby reducing drag, the pilot can accelerate or climb.
In addition to balancing lift, weight, thrust, and drag, modern airplanes have to contend with another phenomenon. The sound barrier is not a physical barrier but a speed at which the behavior of the airflow around an airplane changes dramatically. Fighter pilots in World War II (1939-1945) first ran up against this so-called barrier in high-speed dives during air combat. In some cases, pilots lost control of the aircraft as shock waves built up on control surfaces, effectively locking the controls and leaving the crews helpless. After World War II, designers tackled the realm of supersonic flight, primarily for military airplanes, but with commercial applications as well.
Supersonic flight is defined as flight at a speed greater than that of the local speed of sound. At sea level, sound travels through air at approximately 1,220 km/h (760 mph). At the speed of sound, a shock wave consisting of highly compressed air forms at the nose of the plane. This shock wave moves back at a sharp angle as the speed increases.
Supersonic flight was achieved in 1947 for the first time by the Bell X-1 rocket plane, flown by Air Force test pilot Chuck Yeager. Speeds at or near supersonic flight are measured in units called Mach numbers, which represent the ratio of the speed of the airplane to the speed of sound as it moves air. An airplane traveling at less than Mach 1 is traveling below the speed of sound (subsonic); at Mach 1, an airplane is traveling at the speed of sound (transonic); at Mach 2, an airplane is traveling at twice the speed of sound (supersonic flight). Speeds of Mach 1 to 5 are referred to as supersonic; speeds of Mach 5 and above are called hypersonic. Designers in Europe and the United States developed succeeding generations of military aircraft, culminating in the 1960s and 1970s with Mach 3+ speedsters such as the Soviet MiG-25 Foxbat interceptor, the XB-70 Valkyrie bomber, and the SR-71 spy plane. In 2004 the experimental X-43 plane smashed previous airplane speed records by flying at nearly Mach 10. The unpiloted craft was constructed by the National Aeronautics and Space Administration (NASA).
The shock wave created by an airplane moving at supersonic and hypersonic speeds represents a rather abrupt change in air pressure and is perceived on the ground as a sonic boom, the exact nature of which varies depending upon how far away the aircraft is and the distance of the observer from the flight path. Sonic booms at low altitudes over populated areas are generally considered a significant problem and have prevented most supersonic airplanes from efficiently utilizing overland routes. For example, the Anglo-French Concorde, a commercial supersonic aircraft, was generally limited to over-water routes, or to those over sparsely populated regions of the world. This limitation impacted the commercial viability of the Concorde, which ended its regular passenger service in October 2003. Designers today believe they can help lessen the impact of sonic booms created by supersonic airliners but probably cannot eliminate them.
One of the most difficult practical barriers to supersonic flight is the fact that high-speed flight produces heat through friction. At such high speeds, enormous temperatures are reached at the surface of the craft. For example, the Concorde was forced to fly a flight profile dictated by temperature requirements; if the aircraft moved too fast, then the temperature rose above safe limits for the aluminum structure of the airplane. Titanium and other relatively exotic, and expensive, metals are more heat-resistant, but harder to manufacture and maintain. Airplane designers have concluded that a speed of Mach 2.7 is about the limit for conventional, relatively inexpensive materials and fuels. Above that speed, an airplane needs to be constructed of more temperature-resistant materials.
Airplanes generally share the same basic configuration—each usually has a fuselage, wings, tail, landing gear, and a set of specialized control surfaces mounted on the wings and tail.
The materials that airplanes are made from have evolved as technology has advanced. The earliest airplanes were built mainly from wood and fabric with some metal parts. By the end of the 1920s many airplanes had metal frames and were covered with riveted metal sheets. Lightweight aluminum became the metal most commonly used in manufacturing airplanes for most of the 20th century. In the 1980s composite materials such as carbon fiber-reinforced plastic (CFRP) began to be incorporated into aircraft, helping to make them lighter and more fuel efficient.
The fuselage is the main cabin, or body of the airplane. Generally the fuselage has a cockpit section at the front end, where the pilot controls the airplane, and a cabin section. The cabin section may be designed to carry passengers, cargo, or both. In a military fighter plane, the fuselage may house the engines, fuel, electronics, and some weapons. In some of the sleekest of gliders and ultralight airplanes, the fuselage may be nothing more than a minimal structure connecting the wings, tail, cockpit, and engines.
All airplanes, by definition, have wings. Some are nearly all wing with a very small cockpit. Others have minimal wings, or wings that seem to be merely extensions of a blended, aerodynamic fuselage, such as the space shuttle.
Before the 20th century, wings were made of wooden ribs and spars (or beams), covered with fabric that was sewn tightly and varnished to be extremely stiff. A conventional wing has one or more spars that run from one end of the wing to the other. Perpendicular to the spar are a series of ribs, which run from the front, or leading edge, to the rear, or trailing edge, of the wing. These are carefully constructed to shape the wing in a manner that determines its lifting properties. Wood and fabric wings often used spruce for the structure, because of that material’s relatively light weight and high strength, and linen for the cloth covering.
Early airplanes were usually biplanes—craft with two wings on each side of the fuselage, usually one mounted about 1.5 m (about 5 to 6 ft) above the other. Aircraft pioneers found they could build such wings relatively easily and brace them together using wires to connect the upper and lower wing to create a strong structure with substantial lift. In pushing the many cables, wood, and fabric through the air, these designs created a great deal of drag, so aircraft engineers eventually pursued the monoplane, or single-wing airplane. A monoplane’s single wing gives it great advantages in speed, simplicity, and visibility for the pilot.
After World War I (1914-1918), designers began moving toward wings made of steel and aluminum, and, combined with new construction techniques, these materials enabled the development of modern all-metal wings capable not only of developing lift but of housing landing gear, weapons, and fuel.
Over the years, many airplane designers have postulated that the ideal airplane would, in fact, be nothing but wing. Flying wings, as they are called, were first developed in the 1930s and 1940s. American aerospace manufacturer Northrop Grumman Corporation’s flying wing, the B-2 bomber, or stealth bomber, developed in the 1980s, has been a great success as a flying machine, benefiting from modern computer-aided design (CAD), advanced materials, and computerized flight controls. Popular magazines routinely show artists’ concepts of flying-wing airliners, but airline and airport managers have been unable to integrate these unusual shapes into conventional airline and airport facilities.
Tail Assembly
Most airplanes, except for flying wings, have a tail assembly attached to the rear of the fuselage, consisting of vertical and horizontal stabilizers, which look like small wings; a rudder; and elevators. The components of the tail assembly are collectively referred to as the empennage.
The stabilizers serve to help keep the airplane stable while in flight. The rudder is at the trailing edge of the vertical stabilizer and is used by the airplane to help control turns. An airplane actually turns by banking, or moving, its wings laterally, but the rudder helps keep the turn coordinated by serving much like a boat’s rudder to move the nose of the airplane left or right. Moving an airplane’s nose left or right is known as a yaw motion. Rudder motion is usually controlled by two pedals on the floor of the cockpit, which are pushed by the pilot.
Elevators are control surfaces at the trailing edge of horizontal stabilizers. The elevators control the up-and-down motion, or pitch, of the airplane’s nose. Moving the elevators up into the airstream will cause the tail to go down and the nose to pitch up. A pilot controls pitch by moving a control column or stick.
Landing Gear
All airplanes must have some type of landing gear. Modern aircraft employ brakes, wheels, and tires designed specifically for the demands of flight. Tires must be capable of going from a standstill to nearly 322 km/h (200 mph) at landing, as well as carrying nearly 454 metric tons. Brakes, often incorporating special heat-resistant materials, must be able to handle emergencies, such as a 400-metric-ton airliner aborting a takeoff at the last possible moment. Antiskid braking systems, common on automobiles today, were originally developed for aircraft and are used to gain maximum possible braking power on wet or icy runways.
Larger and more complex aircraft typically have retractable landing gear—so called because they can be pulled up into the wing or fuselage after takeoff. Having retractable gear greatly reduces the drag generated by the wheel structures that would otherwise hang out in the airstream.
Control Components
An airplane is capable of three types of motion that revolve around three separate axes. The plane may fly steadily in one direction and at one altitude—or it may turn, climb, or descend. An airplane may roll, banking its wings either left or right, about the longitudinal axis, which runs the length of the craft. The airplane may yaw its nose either left or right about the vertical axis, which runs straight down through the middle of the airplane. Finally, a plane may pitch its nose up or down, moving about its lateral axis, which may be thought of as a straight line running from wingtip to wingtip.
An airplane relies on the movement of air across its wings for lift, and it makes use of this same airflow to move in any way about the three axes. To do so, the pilot will manipulate controls in the cockpit that direct control surfaces on the wings and tail to move into the airstream. The airplane will yaw, pitch, or roll, depending on which control surfaces or combination of surfaces are moved, or deflected, by the pilot.
In order to bank and begin a turn, a conventional airplane will deflect control surfaces on the trailing edge of the wings known as ailerons. In order to bank left, the left aileron is lifted up into the airstream over the left wing, creating a small amount of drag and decreasing the lift produced by that wing. At the same time, the right aileron is pushed down into the airstream, thereby increasing slightly the lift produced by the right wing. The right wing then comes up, the left wing goes down, and the airplane banks to the left. To bank to the right, the ailerons are moved in exactly the opposite fashion.
In order to yaw, or turn the airplane’s nose left or right, the pilot must press upon rudder pedals on the floor of the cockpit. Push down on the left pedal, and the rudder at the trailing edge of the vertical stabilizer moves to the left. As in a boat, the left rudder moves the nose of the plane to the left. A push on the right pedal causes the airplane to yaw to the right.
In order to pitch the nose up or down, the pilot usually pulls or pushes on a control wheel or stick, thereby moving the elevators at the trailing edge of the horizontal stabilizer. Pulling back on the wheel deflects the elevators upward into the airstream, pushing the tail down and the nose up. Pushing forward on the wheel causes the elevators to drop down, lifting the tail and forcing the nose down.
Airplanes that are more complex also have a set of secondary control surfaces that may include devices such as flaps, slats, trim tabs, spoilers, and speed brakes. Flaps and slats are generally used during takeoff and landing to increase the amount of lift produced by the wing at low speeds. Flaps usually droop down from the trailing edge of the wing, although some jets have leading-edge flaps as well. On some airplanes, they also can be extended back beyond the normal trailing edge of the wing to increase the surface area of the wing as well as change its shape. Leading-edge slats usually extend from the front of the wing at low speeds to change the way the air flows over the wing, thereby increasing lift. Flaps also often serve to increase drag and slow the approach of a landing airplane.
Trim tabs are miniature control surfaces incorporated into larger control surfaces. For example, an aileron tab acts like a miniature aileron within the larger aileron. These kinds of controls are used to adjust more precisely the flight path of an airplane that may be slightly out of balance or alignment. Elevator trim tabs are usually used to help set the pitch attitude (the angle of the airplane in relation to the Earth) for a given speed through the air. On some airplanes, the entire horizontal stabilizer moves in small increments to serve the same function as a trim tab.
Airplane pilots rely on a set of instruments in the cockpit to monitor airplane systems, to control the flight of the aircraft, and to navigate. By the end of the 20th century traditional instrument displays using analog dials and indicators began to be replaced with computer-controlled electronic displays in new designs of aircraft.
Systems instruments will tell a pilot about the condition of the airplane’s engines and electrical, hydraulic, and fuel systems. Piston-engine instruments monitor engine and exhaust-gas temperatures, and oil pressures and temperatures. Jet-engine instruments measure the rotational speeds of the rotating blades in the turbines, as well as gas temperatures and fuel flow.
Flight instruments are those used to tell a pilot the course, speed, altitude, and attitude of the airplane. They may include an airspeed indicator, an artificial horizon, an altimeter, and a compass. These instruments have many variations, depending on the complexity and performance of the airplane. For example, high-speed jet aircraft have airspeed indicators that may indicate speeds both in nautical miles per hour (slightly faster than miles per hour used with ground vehicles) and in Mach number. The artificial horizon indicates whether the airplane is banking, climbing, or diving, in relation to the Earth. An airplane with its nose up may or may not be climbing, depending on its airspeed and momentum.
General-aviation (private aircraft), military, and commercial airplanes also have instruments that aid in navigation. The compass is the simplest of these, but many airplanes now employ satellite navigation systems and computers to navigate from any point on the globe to another without any help from the ground. The Global Positioning System (GPS), developed for the United States military but now used by many civilian pilots, provides an airplane with its position to within a few meters. Many airplanes still employ radio receivers that tune to a ground-based radio-beacon system in order to navigate cross-country. Specially equipped airplanes can use ultraprecise radio beacons and receivers, known as Instrument Landing Systems (ILS) and Microwave Landing Systems (MLS), combined with special cockpit displays, to land during conditions of poor visibility.
Airplanes use either piston or turbine (rotating blades) engines to provide propulsion. In smaller airplanes, a conventional gas-powered piston engine turns a propeller, which either pulls or pushes an airplane through the air. In larger airplanes, a turbine engine either turns a propeller through a gearbox, or uses its jet thrust directly to move an airplane through the air. In either case, the engine must provide enough power to move the weight of the airplane forward through the airstream.
The earliest powered airplanes relied on crude steam or gas engines. These piston engines are examples of internal-combustion engines. Aircraft designers throughout the 20th century pushed their engineering colleagues constantly for engines with more power, lighter weight, and greater reliability. Piston engines, however, are still relatively complicated pieces of machinery, with many precision-machined parts moving through large ranges and in complex motions. Although enormously improved over the past 90 years of flight and still suitable for many smaller general aviation aircraft, they fall short of the higher performance possible with modern jet propulsion and required for commercial and military aviation.
The turbine or jet engine operates on the principle of Newton’s third law of motion, which states that for every action, there is an opposite but equal reaction. A jet sucks air into the front, squeezes the air by pulling it through a series of spinning compressors, mixes it with fuel and ignites the mixture, which then explodes with great force rearward through the exhaust nozzle. The rearward force is balanced with an equal force that pushes forward the jet engine and the airplane attached to it. A rocket engine operates on the same principle, except that, in order to operate in the airless vacuum of space, the rocket must carry along its own air, in the form of solid propellant or liquid oxidizer, for combustion.
There are several different types of jet engines. The simplest is the ramjet, which takes advantage of high speed to ram or force the air into the engine, eliminating the need for the spinning compressor section. This elegant simplicity is offset by the need to boost a ramjet to several hundred miles an hour before ram-air compression is sufficient to operate the engine. Scramjets are ramjets that operate at supersonic speeds.
The turbojet is based on the jet-propulsion system of the ramjet, but with the addition of a compressor section, a combustion chamber, a turbine to take some power out of the exhaust and spin the compressor, and an exhaust nozzle. In a turbojet, all of the air taken into the compressor at the front of the engine is sent through the core of the engine, burned, and released. Thrust from the engine is derived purely from the acceleration of the released exhaust gases out the rear.
A modern derivative known as the turbofan, or fan-jet, adds a large fan in front of the compressor section. This fan pulls an enormous amount of air into the engine case, only a relatively small fraction of which is sent through the core for combustion. The rest runs along the outside of the core case and inside the engine casing. This fan flow is mixed with the hot jet exhaust at the rear of the engine, where it cools and quiets the exhaust noise. In addition, this high-volume mass of air, accelerated rearward by the fan, produces a great deal of thrust by itself, even though it is never burned, acting much like a propeller.
In fact, some smaller jet engines are used to turn propellers. Known as turboprops, these engines produce most of their thrust through the propeller, which is usually driven by the jet engine through a set of gears. As a power source for a propeller, a turbine engine is extremely efficient, and many smaller airliners in the 19- to 70-passenger-capacity range use turboprops. They are particularly efficient at lower altitudes and medium speeds up to 640 km/h (400 mph).
There are a wide variety of types of airplanes. Land planes, carrier-based airplanes, seaplanes, amphibians, vertical takeoff and landing (VTOL), short takeoff and landing (STOL), and space shuttles all take advantage of the same basic technology, but their capabilities and uses make them seem only distantly related.
Land Planes
Land planes are designed to operate from a hard surface, typically a paved runway. Some land planes are specially equipped to operate from grass or other unfinished surfaces. A land plane usually has wheels to taxi, take off, and land, although some specialized aircraft operating in the Arctic or Antarctic regions have skis in place of wheels. The wheels are sometimes referred to as the undercarriage, although they are often called, together with the associated brakes, the landing gear. Landing gear may be fixed, as in some general-aviation airplanes, or retractable, usually into the fuselage or wings, as in more-sophisticated airplanes in general and commercial aviation.
Carrier-Based Aircraft
Carrier-based airplanes are a specially modified type of land plane designed for takeoff from and landing aboard naval aircraft carriers. Carrier airplanes have a strengthened structure, including their landing gear, to handle the stresses of catapult-assisted takeoff, in which the craft is launched by a steam-driven catapult; and arrested landings, made by using a hook attached to the underside of the aircraft’s tail to catch one of four wires strung across the flight deck of the carrier.
Seaplanes, sometimes called floatplanes or pontoon planes, are often ordinary land planes modified with floats instead of wheels so they can operate from water. A number of seaplanes have been designed from scratch to operate only from water bases. Such seaplanes have fuselages that resemble and perform like ship hulls. Known as flying boats, they may have small floats attached to their outer wing panels to help steady them at low speeds on the water, but the weight of the airplane is borne by the floating hull.
Amphibians, like their animal namesakes, operate from both water and land bases. In many cases, an amphibian is a true seaplane, with a boat hull and the addition of specially designed landing gear that can be extended to allow the airplane to taxi right out of the water onto land. Historically, some flying boats were fitted with so-called beaching gear, a system of cradles on wheels positioned under the floating aircraft, which then allowed the aircraft to be rolled onto land.
Vertical Takeoff and Landing Airplanes
Vertical Takeoff and Landing (VTOL) airplanes typically use the jet thrust from their engines, pointed down at the Earth, to take off and land straight up and down. After taking off, a VTOL airplane usually transitions to wing-borne flight in order to cover a longer distance or carry a significant load. A helicopter is a type of VTOL aircraft, but there are very few VTOL airplanes. One unique type of VTOL aircraft is the tilt-rotor, which has large, propeller-like rotating wings or rotors driven by jet engines at the wingtips. For takeoff and landing, the engines and rotors are positioned vertically, much like a helicopter. After takeoff, however, the engine/rotor combination tilts forward, and the wing takes on the load of the craft.
The most prominent example of a true VTOL airplane flying today is the AV-8B Harrier II, a military attack plane that uses rotating nozzles attached to its jet engine to direct the engine exhaust in the appropriate direction. Flown in the United States by the Marine Corps, as well as in Spain, Italy, India, and United Kingdom, where it was originally developed, the Harrier can take off vertically from smaller ships, or it can be flown to operating areas near the ground troops it supports in its ground-attack role.
Short Takeoff and Landing Airplanes
Short Takeoff and Landing (STOL) airplanes are designed to be able to function on relatively short runways. Their designs usually employ wings and high-lift devices on the wings optimized for best performance during takeoff and landing, as distinguished from an airplane that has a wing optimized for high-speed cruise at high altitude. STOL airplanes are usually cargo airplanes, although some serve in a passenger-carrying capacity as well.
Space Shuttle
The space shuttle, flown by NASA, is an aircraft unlike any other because it flies as a fixed-wing airplane within the atmosphere and as a spacecraft outside Earth’s atmosphere. When the space shuttle takes off, it flies like a rocket with wings, relying on the 3,175 metric tons of thrust generated by its solid-fuel rocket boosters and liquid-fueled main engines to power its way up, through, and out of the atmosphere. During landing, the shuttle becomes the world’s most sophisticated glider, landing without propulsion.
Airplanes can be grouped into a handful of major classes, such as commercial, military, and general-aviation airplanes, all of which fall under different government-mandated certification and operating rules.
Commercial Airplanes
Commercial aircraft are those used for profit making, usually by carrying cargo or passengers for hire (see Air Transport Industry). They are strictly regulated—in the United States, by the Federal Aviation Administration (FAA); in Canada, by Transport Canada; and in other countries, by other national aviation authorities.
Modern large commercial-airplane manufacturers—such The Boeing Company in the United States and Airbus in Europe—offer a wide variety of aircraft with different capabilities. Today’s jet airliners carry anywhere from 100 passengers to more than 500 over short and long distances.
Beginning in 1976 the British-French Concorde supersonic transport (SST) carried passengers at twice the speed of sound. The Concorde flew for British Airways and Air France, flag carriers of the two nations that funded its development during the late 1960s and 1970s. The United States had an SST program, but it was ended because of budget and environmental concerns in 1971. The Concorde ended its regular passenger service in October 2003 due to its lack of profitability. Declining ticket sales for the high-priced service, which cost about $9,000 and up for a round-trip fare, combined with higher costs led to the Concorde’s demise. A fatal air crash in 2000 grounded the Concorde for a full year. It returned to service only to witness a sharp decline in airline travel following the September 11 terrorist attacks.
Military Airplanes
Military aircraft are usually grouped into four categories: combat, cargo, training, and observation (see Military Aviation). Combat airplanes are generally either fighters or bombers, although some airplanes have both capabilities. Fighters are designed to engage in air combat with other airplanes, in either defensive or offensive situations. Since the 1950s many fighters have been capable of Mach 2+ flight (a Mach number represents the ratio of the speed of an airplane to the speed of sound as it travels through air). Some fighters have a ground-attack role as well and are designed to carry both air-to-air weapons, such as missiles, and air-to-ground weapons, such as bombs. Fighters include aircraft such as the Panavia Tornado, the Boeing F-15 Eagle, the Lockheed-Martin F-16 Falcon, the MiG-29 Fulcrum, and the Su-27 Flanker.
Bombers are designed to carry large air-to-ground-weapons loads and either penetrate or avoid enemy air defenses in order to deliver those weapons. Some well-known bombers include the Boeing B-52, the Boeing B-1, and the Northrop-Grumman B-2 stealth bomber.
Today’s military cargo airplanes are capable of carrying enormous tanks, armored personnel carriers, artillery pieces, and even smaller aircraft. Cargo planes such as the giant Lockheed C-5B and Boeing C-17 were designed expressly for such roles. Some cargo planes can serve a dual role as aerial gas stations, refueling different types of military airplanes while in flight. Such tankers include the Boeing KC-135 and KC-10.
All military pilots go through rigorous training and education programs using military training airplanes to prepare them to fly the high-performance aircraft of the armed forces. They typically begin the flight training in relatively simple, propeller airplanes and move into basic jets before specializing in a career path involving fighters, bombers, or transports. Some military trainers include the T-34 Mentor, the T-37 and T-38, and the Boeing T-45 Goshawk.
A final category of military airplane is the observation, or reconnaissance, aircraft. With the advent of the Lockheed U-2 spy plane in the 1950s, observation airplanes were developed solely for highly specialized missions. Lockheed’s SR-71, a two-seat airplane, uses specialized engines and fuel to reach altitudes greater than 25,000 m (80,000 ft) and speeds well over Mach 3.
Unmanned aerial vehicles (UAVs) also were developed for reconnaissance in situations considered too dangerous for piloted aircraft or in instances where pilot fatigue would be a factor. UAVs include the Predator drone, made by General Atomics Aeronautical Systems, Inc., based in San Diego, California. These unpiloted aircraft are flown by software programs containing navigational instructions and operated from the ground. They relay video and infrared images in real time to military commanders, providing instantaneous views of battlegrounds during the day or at night. Some UAVs, known as Unmanned Combat Aerial Vehicles (UCAVs), also carry weapons that can be fired by ground operators using the aircraft’s video and infrared cameras to locate their targets.
General-Aviation Aircraft
General-aviation aircraft are certified for and intended primarily for noncommercial or private operations.
Pleasure aircraft range from simple single-seat, ultralight airplanes to sleek twin turboprops capable of carrying eight people. Business aircraft transport business executives to appointments. Most business airplanes require more reliable performance and more range and all-weather capability.
Another class of general-aviation airplanes is used in agriculture. Large farms require efficient ways to spread fertilizer and insecticides over a large area. A very specialized type of airplane, crop dusters are rugged, highly maneuverable, and capable of hauling several hundred pounds of chemicals. They can be seen swooping low over farm fields. Not intended for serious cross-country navigation, crop dusters lack sophisticated navigation aids and complex systems.
Before the end of the 18th century, few people had applied themselves to the study of flight. One was the Italian Renaissance artist Leonardo da Vinci, during the 15th century. Leonardo was preoccupied chiefly with bird flight and with flapping-wing machines, called ornithopters. His aeronautical work lay unknown until late in the 19th century, when it could furnish little of technical value to experimenters but was a source of inspiration to aspiring engineers. Apart from Leonardo’s efforts, three devices important to aviation had been invented in Europe in the Middle Ages and had reached a high stage of development by Leonardo’s time—the windmill, an early propeller; the kite, an early airplane wing; and the model helicopter.
The First Airplanes
Between 1799 and 1809 English baronet Sir George Cayley created the concept of the modern airplane. Cayley abandoned the ornithopter tradition, in which both lift and thrust are provided by the wings, and designed airplanes with rigid wings to provide lift, and with separate propelling devices to provide thrust. Through his published works, Cayley laid the foundations of aerodynamics. He demonstrated, both with models and with full-size gliders, the use of the inclined plane to provide lift, pitch, and roll stability; flight control by means of a single rudder-elevator unit mounted on a universal joint; streamlining; and other devices and practices. In 1853, in his third full-size machine, Cayley sent his unwilling coachman on the first gliding flight in history.
In 1843 British inventor William Samuel Henson published his patented design for an Aerial Steam Carriage. Henson’s design did more than any other to establish the form of the modern airplane—a fixed-wing monoplane with propellers, fuselage, and wheeled landing gear, and with flight control by means of rear elevator and rudder. Steam-powered models made by Henson in 1847 were promising but unsuccessful.
In 1890 French engineer Clément Ader built a steam-powered airplane and made the first actual flight of a piloted, heavier-than-air craft. However, the flight was not sustained, and the airplane brushed the ground over a distance of 50 m (160 ft). Inventors continued to pursue the dream of sustained flight. Between 1891 and 1896 German aeronautical engineer Otto Lilienthal made thousands of successful flights in hang gliders of his own design. Lilienthal hung in a frame between the wings and controlled his gliders entirely by swinging his torso and legs in the direction he wished to go. While successful as gliders, his designs lacked a control system and a reliable method for powering the craft. He was killed in a gliding accident in 1896.
American inventor Samuel Pierpont Langley had been working for several years on flying machines. Langley began experimenting in 1892 with a steam-powered, unpiloted aircraft, and in 1896 made the first sustained flight of any mechanically propelled heavier-than-air craft. Launched by catapult from a houseboat on the Potomac River near Quantico, Virginia, the unpiloted Aerodrome, as Langley called it, suffered from design faults. The Aerodrome never successfully carried a person, and thus prevented Langley from earning the place in history claimed by the Wright brothers.
The First Airplane Flight
American aviators Orville Wright and Wilbur Wright of Dayton, Ohio, are considered the fathers of the first successful piloted heavier-than-air flying machine. Through the disciplines of sound scientific research and engineering, the Wright brothers put together the combination of critical characteristics that other designs of the day lacked—a relatively lightweight (337 kg/750 lb), powerful engine; a reliable transmission and efficient propellers; an effective system for controlling the aircraft; and a wing and structure that were both strong and lightweight.
At Kitty Hawk, North Carolina, on December 17, 1903, Orville Wright made the first successful flight of a piloted, heavier-than-air, self-propelled craft, called the Flyer. That first flight traveled a distance of about 37 m (120 ft). The distance was less than the wingspan of many modern airliners, but it represented the beginning of a new age in technology and human achievement. Their fourth and final flight of the day lasted 59 seconds and covered only 260 m (852 ft). The third Flyer, which the Wrights constructed in 1905, was the world’s first fully practical airplane. It could bank, turn, circle, make figure eights, and remain in the air for as long as the fuel lasted, up to half an hour on occasion.
Early Military and Public Interest
The airplane, like many other milestone inventions throughout history, was not immediately recognized for its potential. During the very early 1900s, prior to World War I (1914-1918), the airplane was relegated mostly to the county-fair circuit, where daredevil pilots drew large crowds but few investors. One exception was the United States War Department, which had long been using balloons to observe the battlefield and expressed an interest in heavier-than-air craft as early as 1898. In 1908 the Wrights demonstrated their airplane to the U.S. Army’s Signal Corps at Fort Myer, Virginia. In September of that year, while circling the field at Fort Myer, Orville crashed while carrying an army observer, Lieutenant Thomas Selfridge. Selfridge died from his injuries and became the first fatality from the crash of a powered airplane.
On July 25, 1909, French engineer Louis Blériot crossed the English channel in a Blériot XI, a monoplane of his own design. Blériot’s channel crossing made clear to the world the airplane’s wartime potential, and this potential was further demonstrated in 1910 and 1911, when American pilot Eugene Ely took off from and landed on warships. In 1911 the U.S. Army used a Wright brothers’ biplane to make the first live bomb test from an airplane. That same year, the airplane was used in its first wartime operation when an Italian captain flew over and observed Turkish positions during the Italo-Turkish War of 1911 to 1912. Also in 1911, American inventor and aviator Glenn Curtiss introduced the first practical seaplane. This was a biplane with a large float beneath the center of the lower wing and two smaller floats beneath the tips of the lower wing.
The year 1913 became known as the “glorious year of flying.” Aerobatics, or acrobatic flying, was introduced, and upside-down flying, loops, and other stunts proved the maneuverability of airplanes. Long-distance flights made in 1913 included a 4,000-km (2,500-mi) flight from France to Egypt, with many stops, and the first nonstop flight across the Mediterranean Sea, from France to Tunisia. In Britain, a modified Farnborough B.E. 2 proved itself to be the first naturally stable airplane in the world. The B.E. 2c version of this airplane was so successful that nearly 2,000 were subsequently built.
Planes of World War I
During World War I, the development of the airplane accelerated dramatically. European designers such as Louis Blériot and Dutch-American engineer Anthony Herman Fokker exploited basic concepts created by the Wrights and developed ever faster, more capable, and deadlier combat airplanes. Fokker’s biplanes, such as the D-VII and D-VIII flown by German pilots, were considered superior to their Allied competition. In 1915 Fokker mounted a machine gun with a timing gear so that the gun could fire between the rotating propellers. The resulting Fokker Eindecker monoplane fighter was, for a time, the most successful fighter in the skies.
The concentrated research and development made necessary by wartime pressures produced great progress in airplane design and construction. During World War I, outstanding early British fighters included the Sopwith Pup (1916) and the Sopwith Camel (1917), which flew as high as 5,800 m (19,000 ft) and had a top speed of 190 km/h (120 mph). Notable French fighters included the Spad (1916) and the Nieuport 28 (1918). By the end of World War I in 1918, both warring sides had fighters that could fly at altitudes of 7,600 m (25,000 ft) and speeds up to 250 km/h (155 mph).
Development of Commercial Aviation
Commercial aviation began in January 1914, just 10 years after the Wrights pioneered the skies. The first regularly scheduled passenger line in the world operated between Saint Petersburg and Tampa, Florida. Commercial aviation developed slowly during the next 30 years, driven by the two world wars and service demands of the U.S. Post Office for airmail.
In the early 1920s the air-cooled engine was perfected, along with its streamlined cowling, or engine casing. Light and powerful, these engines gave strong competition to the older, liquid-cooled engines. In the mid-1920s light airplanes were produced in great numbers, and club and private pleasure flying became popular. The inexpensive DeHavilland Moth biplane, introduced in 1925, put flying within the financial reach of many enthusiasts. The Moth could travel at 145 km/h (90 mph) and was light, strong, and easy to handle.
Instrument flying became practical in 1929, when the American inventor Elmer Sperry perfected the artificial horizon and directional gyro. On September 24, 1929, James Doolittle, an American pilot and army officer, proved the value of Sperry’s instruments by taking off, flying over a predetermined course, and landing, all without visual reference to the Earth.
Introduced in 1933, Boeing’s Model 247 was considered the first truly modern airliner. It was an all-metal, low-wing monoplane, with retractable landing gear, an insulated cabin, and room for ten passengers. An order from United Air Lines for 60 planes of this type tied up Boeing’s production line and led indirectly to the development of perhaps the most successful propeller airliner in history, the Douglas DC-3. Trans World Airlines, not willing to wait for Boeing to finish the order from United, approached airplane manufacturer Donald Douglas in Long Beach, California, for an alternative, which became, in quick succession, the DC-1, the DC-2, and the DC-3.
The DC-3 carried 21 passengers, used powerful, 1,000-horsepower engines, and could travel across the country in less than 24 hours of travel time, although it had to stop many times for fuel. The DC-3 quickly came to dominate commercial aviation in the late 1930s, and some DC-3s are still in service today.
Boeing provided the next major breakthrough with its Model 307 Stratoliner, a pressurized derivative of the famous B-17 bomber, entering service in 1940. With its regulated cabin air pressure, the Stratoliner could carry 33 passengers at altitudes up to 6,000 m (20,000 ft) and at speeds of 320 km/h (200 mph).
Aircraft Developments of World War II
It was not until after World War II (1939-1945), when comfortable, pressurized air transports became available in large numbers, that the airline industry really prospered. When the United States entered World War II in 1941, there were fewer than 300 planes in airline service. Airplane production concentrated mainly on fighters and bombers, and reached a rate of nearly 50,000 a year by the end of the war. A large number of sophisticated new transports, used in wartime for troop and cargo carriage, became available to commercial operators after the war ended. Pressurized propeller planes such as the Douglas DC-6 and Lockheed Constellation, early versions of which carried troops and VIPs during the war, now carried paying passengers on transcontinental and transatlantic flights.
Wartime technology efforts also brought to aviation critical new developments, such as the jet engine. Jet transportation in the commercial-aviation arena arrived in 1952 with Britain’s DeHavilland Comet, an 885-km/h (550-mph), four-engine jet. The Comet quickly suffered two fatal crashes due to structural problems and was grounded. This complication gave American manufacturers Boeing and Douglas time to bring the 707 and DC-8 to the market. Pan American World Airways inaugurated Boeing 707 jet service in October of 1958, and air travel changed dramatically almost overnight. Transatlantic jet service enabled travelers to fly from New York City to London, England, in less than eight hours, half the propeller-airplane time. Boeing’s new 707 carried 112 passengers at high speed and quickly brought an end to the propeller era for large commercial airplanes.
After the big, four-engine 707s and DC-8s had established themselves, airlines clamored for smaller, shorter-range jets, and Boeing and Douglas delivered. Douglas produced the DC-9 and Boeing both the 737 and the trijet 727.
The Jumbo Jet Era
The next frontier, pioneered in the late 1960s, was the age of the jumbo jet. Boeing, McDonnell Douglas, and Lockheed all produced wide-body airliners, sometimes called jumbo jets. Boeing developed and still builds the 747. McDonnell Douglas built a somewhat smaller, three-engine jet called the DC-10, produced later in an updated version known as the MD-11. Lockheed built the L-1011 Tristar, a trijet that competed with the DC-10. The L-1011 is no longer in production, and Lockheed-Martin no longer builds commercial airliners.
In the 1980s McDonnell Douglas introduced the twin-engine MD-80 family, and Boeing brought online the narrow-body 757 and wide-body 767 twin jets. Airbus had developed the A300 wide-body twin during the 1970s. During the 1980s and 1990s Airbus expanded its family of aircraft by introducing the slightly smaller A310 twin jet and the narrow-body A320 twin, a unique, so-called fly-by-wire aircraft with sidestick controllers for the pilots rather than conventional control columns and wheels. Airbus also introduced the larger A330 twin and the A340, a four-engine airplane for longer routes, on which passenger loads are somewhat lighter. In 2000 the company launched production of the A380, a superjumbo jet that seats 555 passengers on two decks, both of which extend the entire length of the fuselage. Because of its size, the A380 may require modifications to some airports, including wider runways and special passenger boarding facilities. The world’s largest passenger jet, the A380 is scheduled to enter commercial service in late 2007.
Boeing introduced the 777, a wide-body jumbo jet that can hold up to 400 passengers, in 1995. In 1997 Boeing acquired longtime rival McDonnell Douglas, and a year later the company announced its intention to halt production of the passenger workhorses MD-11, MD-80, and MD-90. The company ceded the superjumbo jet market to Airbus and instead focused its efforts on developing a fuel-efficient midsize passenger airplane called the 787 Dreamliner. Designed to permit long-range nonstop flights, the 787 can carry from 210 to 330 passengers and is the first commercial aircraft with a primary structure made largely of composite material (mostly carbon fiber-reinforced plastic) rather than traditional aluminum. The 787 is scheduled to enter commercial service in 2008.

No comments:

Post a Comment