Category FLIGHT and M ОТІOIM

The United States entered World War I in April 1917 with hardly any airplanes. U. S. pilots, some of whom trained in Europe, flew mostly French planes, although DH-4s were built in the United States. Flying schools were set up to train American pilots, and in February 1918 the 95 th Pursuit Squadron was the first U. S. Army fighter squadron to arrive in France. The 94th Pursuit Squadron scored the Americans’ first victories, in April 1918, when two of its pilots flying French Nieuport 28 fighters shot down two German planes. The 96 th Aero Squadron, formed in France in May 1918, was the first U. S. bomber squadron.

Air commanders such as Britain’s Hugh Trenchard wanted to use air power independently of the armies and navies, but they were restricted by lack of air­planes and by orders to support Allied army offensives. Trenchard got his wish in April 1918 with the formation of the British Royal Air Force (RAF). By the end of the war, the RAF had 22,000 air­craft and had destroyed more than 8,000 enemy airplanes and airships.

Even at this late stage of the war, most air battles were small. Just two British 0/400 bombers, for example, flew to attack the Badische industrial plant in Germany in August 1918. On a rare occasion, large numbers of airplanes were used together. In September 1918, U. S. Brigadier General Billy Mitchell massed 1,500 Allied aircraft for an attack on German positions during the Battle of Saint Mihiel in France.

When the space age began in the 1950s, scientists were eager to expand their knowledge of the worlds beyond Earth, previously seen only through tele­scopes. After the launch of the first satellites by the United States and Soviet Union in 1957 and 1958, the world waited expectantly for the first

rocket shot at the Moon. This came in January 1959, when the Soviet probe Luna 1 flew within 3,700 miles (5,920 kilometers) of the Moon. Two months later, the United States sent its probe Pioneer 4 to fly by the Moon. In September 1959, the Soviet Luna 2 probe crashed onto the Moon. Luna 3 flew around the Moon in October 1959 and took photographs of the far side, never before seen from Earth.

peed is the rate of an object’s motion usually expressed as the distance traveled per unit of time, such as miles per hour or meters per sec­ond. Aircraft fly at a wide range of speeds, from an airship gliding slowly through the air to a fighter plane streak­ing across the sky faster than the speed of sound. Spacecraft travel even faster.

Regimes of Flight

Aircraft speeds are divided into bands. From the slowest to the fastest, the bands are: low speed, medium speed, high speed, supersonic, and hypersonic. These speed bands also are called regimes of flight.

The low-speed regime includes light­weight craft, such as hang gliders and

airships, that fly up to 100 miles per hour (160 kilometers per hour). The medium-speed regime includes propeller planes flying up to about 350 miles per hour (560 kilometers per hour). High­speed aircraft fly up to about 700 miles per hour (1,100 kilometers per hour)— these aircraft are mostly jet airliners. Supersonic planes, such as the F-22 Raptor fighter, fly between Mach 1 (the speed of sound) and Mach 5 (five times the speed of sound). Hypersonic craft fly faster than five times the speed of sound. The Space Shuttle is an example of a hypersonic craft.

Stealth airplanes rely almost entirely on being nearly invisible to radar. This fact brings with it certain limitations that can put stealth aircraft at a disadvan­tage. First, neither the F-117 nor the B-2 carries defensive armament, so they cannot defend themselves. Second, most high-performance jet airplanes rely on engine afterburners to boost their speed in combat. Afterburning produces high­ly visible emissions of exhaust gases, however, which make the airplane more detectable. Stealth aircraft, therefore, lack afterburners and so do not have supersonic performance. This makes them vulnerable to faster fighters once they are detected. Third, the design of stealth aircraft may protect them from enemy missiles that use radar, but it offers no protection from other weapons. Fourth, the unorthodox shape and slow speed of stealth planes make them inferior to conventional fighters when engaged in aerial dogfights.

The stealth aircraft’s sophistication also can be a disadvantage. The elec­tronic fly-by-wire system required to keep a stealth plane like the F-117 flying adds to both the cost and weight of the aircraft. Advanced computers on the airplane are also a potential risk since electromagnetic equipment gives off radiation that can be detected by sensors on the ground, revealing the presence of the plane. All stealth airplanes need meticulous maintenance, since the air­craft skin must be kept flawless to pre­serve its radar anonymity. Even a scratch from a pebble thrown up during a landing, or weather damage to the paint, may increase the radar signature.

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SEE ALSO:

• Aerodynamics • Aircraft, Experimental • Aircraft Design

• Control System • Radar

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A pilot lands an airplane by first reduc­ing power so that the plane begins to slow down and lose altitude. Closer to the runway, the engines are throttled back to reduce the airspeed even more. To make up for the loss of lift, the plane’s nose is raised to increase the angle of attack of the wings. The flaps and slats used for takeoff are deployed again, and the landing gear is lowered.

Lights and radio beacons on the ground show the way to the runway. The plane also receives two radio signals from the runway: one is the localizer and the other is the glide slope. The localizer is transmitted along the run­way. It tells the crew whether the plane is flying to the left or right of the run­way or is flying down the centerline. The glide slope shows whether the plane is descending at the correct rate. Just after touchdown, spoilers spring up from the wings to reduce lift and keep the plane on the ground. Jet planes often use reverse thrust to help them slow down.

Helicopters

Helicopters do not need runways to land and take off. Like rockets and airships, they are capable of vertical takeoff and landing. This gives helicopters the abili­ty to land in and take off from small spaces or remote locations where there are no airports.

Helicopters and VTOL aircraft (verti­cal takeoff and landing aircraft other than helicopters) take off vertically by directing their engine power downward to thrust them upward. The helicopter pilot uses the collective pitch lever and the engine throttle to get the aircraft off the ground. The collective pitch control tilts the rotor blades, increasing their angle of attack and producing more lift. The throttle increases the engine speed, which makes the rotor spin faster. To land, these controls are used to reduce lift.

Airships

An airship is balanced so that it is neu­trally buoyant on the ground. Its weight is balanced by the lift produced by the lighter-than-air gas inside it. When it is ready for takeoff, an airship’s propellers and the elevators in its tail swivel up to drive the airship upward. Modern air­ships are filled with helium, and they

also have air-filled bags called ballonets inside them. As an airship climbs, the outside air pressure falls, and the gas inside the airship expands. Valves pop open and let air escape from the ballonets so the helium can expand safely.

To land, the propellers and elevators tilt down to drive the airship down. The outside air pressure rises and compresses the gas inside the airship. The airship’s engines force extra air into the ballonets to keep the airship fully inflated as it descends.

The first wind tunnel was designed by the British engineer Francis Herbert Wenham and it was built by John Browning at Greenwich, England, in 1871. The wind tunnel was 12 feet (about 4 meters) long and 18 inches (45 centimeters) high and wide. Air could be blown through it at up to 40 miles per hour (64 kilometers per hour).

The swirling, unsteady airflow inside the first wind tunnels made it very diffi­cult to obtain reliable measurements. The first wind tunnel to provide useful aerodynamic data for designing an actu­al aircraft was built by Orville and Wilbur Wright in 1901.

The brothers were disappointed with the performance of the early gliders they had built. They needed a way of testing different shapes and angles of wings instead of relying on data provided by other people. The Wrights tried testing small model wings fixed to a bicycle, but they really wanted to test each wing in a controlled environment with exactly the same airflow. So Orville Wright built a

О While many wind tunnels use models to test new designs, this wind tunnel in the 1950s could hold full-scale aircraft.

TECH^TALK

PARTS OF A WIND TUNNEL

A wind tunnel has five main parts:

• The drive section pushes air through the tunnel.

• The settling chamber straightens the airflow.

• The contraction zone speeds up the air.

• The test section contains the object to be studied.

• The diffuser slows the air down.

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wind tunnel from an old box. It was only 18 inches (46 centimeters) long. A fan driven by their workshop engine blew air through the box, and a glass window on top showed what was hap­pening inside.

In World War I, the main role of the warplane was tactical. Generals want­ed airplanes to shoot at troops on the ground, help artillery guns locate tar­gets, and provide intelligence about enemy movements. Strategists soon realized, however, that airplanes could have a greater effect on a battle, by attacking enemy transportation routes, for example. Aircraft also could dis­rupt industry by bombing factories and cities far behind the frontline.

Most warplanes in World War I were land planes, flying from grass airfields. There also were seaplanes able to land on water, and naval avia­tion progressed rapidly. Ships were hurriedly adapted to carry seaplanes, and spotter planes were used in naval battles. Deck landing trials in 1917 led to the first aircraft carriers.

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SEE ALSO:

• Aircraft, Military • Aircraft

Carrier • Airship • Bomber

• Fighter Plane • Mitchell, Billy

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WORLD WAR I AVIATION ADVANCES

The war taught aviators many lessons: about fighter tactics, bombing, and the different roles that aircraft could per­form in war. World War I saw rapid progress in airplane design. When the war began in 1914, planes were made of wood and fabric, carried no guns or bombs, and flew slowly, at low heights.

By the end of the war in 1918, most planes were still biplanes, but they were much improved. By 1918, planes could fly as high as 24,000 feet (7,300 meters). Pilots carried radios to talk to the ground. Trainee pilots learned to fly on dual-control trainers. Interrupter gear had transformed the experience of air-to-air combat. Fast, single-seat fighters flew at about 120 miles per hour (190 kilometers per hour) and fired two machine guns.

Fighter planes were the aircraft with the most impact in World War I. There also were specialized airplanes, including ground-attack strike planes and naval airplanes able to land on ships or water. The development of large, multi-engine bombers was a significant step toward future warfare.

By the war’s end, all the warring nations had air forces of some form. These air forces that evolved during World War I would play a much larger role in the future.

eronautics is the science of building and flying aircraft. The term covers scientific study, design, and technology. It also includes the manufacture and operation of all types of aircraft, both lighter than air (such as airships) and heavier than air (such as airplanes).

Aeronautics involves a great variety of scientific and engineering disciplines. Aerodynamics and propulsion are important in aeronautics. So are materi­als, structures, control systems, and computing.

Energy from the Sun passes through the atmosphere’s layers to warm Earth’s sur­face. The surface then warms the atmos­phere. The warm air rises and draws in more air underneath to replace it. These air currents caused by the Sun and also by the spinning motion of Earth are responsible for the weather.

Some gases in the atmosphere are very good at soaking up warmth instead of letting it escape into space. These gases make the atmosphere warmer overall. The rise in temperature caused by these gases trapping heat is called the greenhouse effect, and the gases that trap the heat are known as greenhouse gases. Water vapor, carbon dioxide, and methane trap the most heat.

Without the greenhouse effect, the world would be about 60°F (33°C) cold­er than it is now. The world is gradually warming up, however, in a process called global warming. Many scientists believe this is happening because of the increase in greenhouse gases produced
by human activities. When fossil fuels such as coal and oil are burned, they release carbon dioxide, which traps heat.

Depending on how much the atmos­phere warms up, global warming could cause droughts in some parts of the world, floods in other parts, and more

BLUE SKIES AND RED SUNSETS

Air is colorless, so why is a clear sky blue? Sunlight contains all the colors of the rainbow mixed together. As sunlight streams through the atmosphere, it hits air molecules. The molecules scatter the light in all directions, but the blue part of the light is scattered more than the other colors, because its wavelengths are shorter. The sky, therefore, looks blue in every direction.

Brilliant red sunsets are also caused by light scattering. When the Sun is low in the sky just before it sets, sunlight travels more than thirty times farther through the atmosphere to reach the eyes of an observer than it does when the Sun is high in the sky. Most of the blue part of the sunlight is scattered out, leaving the red and orange parts of the light (with their longer wavelengths) to give the sunset those colors.

violent storms everywhere. If a warmer atmosphere melts the polar ice caps, the sea level could rise enough to flood coastal cities. Crops could fail in some places as the climate changes. Many people believe the effects of global warming are already being seen.

The introduction of jet planes brought about a revolution in passenger flying. The De Havilland Comet first flew in 1949 and went into service in 1952. It was followed by the Boeing 707, which could fly at 580 miles per hour (933 kilometers per hour) at 40,000 feet (12,190 meters). When the 707 entered service in 1958, critics argued that no airline would be able to fill its 130 seats. By 1969, however, the Boeing 747 was offering seats for 350 passengers.

Turboprop airliners, such as the Vickers Viscount and Lockheed Electra, proved briefly popular. Turboprop planes have gas turbine engines that turn propellers. They were slower than jets, but they were quiet and efficient. These aircraft were soon replaced, how­ever, by new medium-size jets.

The commercial aerospace industry of the late twentieth century had to

PRESSURIZED CABINS

The air is very thin at high altitudes, and early pilots flying above 10,000 feet (3,050 meters) would have needed oxy­gen tanks to breathe. A better system, introduced during World War II, was the pressurized cabin. Pressurized cabins were first used commercially in 1940, in the Boeing 307. After the war, pressur­ized cabins became standard on aircraft carrying passengers.

Commercial aircraft fly between 30,000 and 40,000 feet (9,150 meters to 12,200 meters). Whatever the temper­ature and altitude may be outside, the compressed air inside a pressurized cabin allows people to function as they would at lower altitudes. The pressurized air gives passengers enough oxygen to breathe comfortably. The introduced air system also maintains a comfortable temperature.

The pressurized air, which comes from compressors in the aircraft engines, flows through the wings to air condi­tioning units under the floor of the cabin.

It is mixed with filtered air already in the cabin (the filters trap any microbes) and is circulated in a continuous flow that dilutes odors and regulates temperature.

The air in the cabin changes every two to three minutes. In spite of the general belief that airplane air is full of recycled germs, airline passengers breathe cleaner air than most office workers.

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anticipate public demand for the future. Did passengers want cheaper fares in bigger aircraft, like the Boeing 747 and Airbus 380? Or would they pay more for the high-speed flights offered by the supersonic Concorde? The 747 won the commercial battle easily. Boeing 747s are still being built, while Concorde was retired in 2003. Only sixteen Concorde planes were ever flown commercially.

By 2000 Boeing aircraft had come to dominate the global market in com­mercial aviation. Boeing’s main rival is the European consortium Airbus Industries, a group of aerospace compa­nies that makes the Airbus family of commercial jets.