Category FLIGHT and M ОТІOIM

Sound Wave

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ound is a form of energy that travels through the air as a series of pressure waves. The waves’ effect on the ear produces the sensation of hearing.

Anything that vibrates produces sound waves that spread out through the air. When something vibrates, it pushes against the air next to it and squashes the air. This pressure creates a pulse

MEASURING SOUND

The frequency of a sound is meas­ured in hertz (abbreviated as Hz).

One hertz is equal to one compres­sion per second. The human ear can hear sounds ranging in frequency from about 20 hertz to 20,000 hertz. Sounds below 20 hertz are called infrasound, and sounds above 20,000 hertz are called ultrasound.

Sound intensity is measured in units called decibels (abbreviated as dB). The quietest sound that can be heard has an intensity of 0 decibels.

A sound ten times more intense is 10 decibels. A sound 100 times more intense is 20 decibels. A sound 1,000 times more intense is 30 decibels. A whisper is about 20 decibels, while a jet engine 100 feet (30 meters) away is about 150 decibels.

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that travels away through the air. After pushing against the air, the object pulls back again before the next push. When it pulls back, it creates a region of low pressure in the air. These high – pressure pulses are called compressions, and the low-pressure regions between them are known as rarefactions. The sound waves they create are called longitudinal waves.

The distance between one compression and the next is a sound wavelength. The number of compressions that pass any point in a second is the sound’s frequency. A high-pitched whistling sound has a high frequency, or a lot of pulses per second. A deep rumbling sound has a low frequency. Frequency and wavelength are linked. As frequency increases, wavelengths get smaller.

Sound needs something to travel through. It can travel through liquids and solid materials as well as air and other gases, but it cannot travel through space without air. The speed of sound in air depends on how fast vibrations spread from molecule to molecule, and this depends on the temperature of the air. Sound travels faster in warm air than in cold air. On an average day, with an air temperature of about 59°F (15°C), the speed of sound in air is about 760 miles per hour (1,220 kilometers per hour).

Vibrations produce sound, but sound itself also can make things vibrate. We hear because sound waves enter our ears and make our eardrums vibrate. Every machine, and every part of a machine, has a resonant frequency at which it

Tympanic membrane (eardrum)

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in liquid

Подпись: о This diagram shows how sound waves enter a human ear and reverberate inside the eardrum so we can hear sounds. о The Space Shuttle has to be protected from the roar of its own engines when it is launched. to prevent damage to the spacecraft. Just before the engines fire, 300,000 gallons (1,135,500 liters) of water start pouring onto the launch pad, as shown in this photo taken during a system test. The water absorbs the sound and stops it from bouncing back to the spacecraft.

vibrates very strongly. Engineers try to ensure that aircraft and their engines do not vibrate at their resonant frequency. This is because strong vibrations can cause damage, shake things loose, and make an aircraft very noisy.

Vibrating rotors and propellers can cause a lot of noise inside helicopters and propeller planes. The noise makes it difficult for pilots to hear messages in their radio headsets. To counteract this, pilots can wear headsets that remove unwanted sound by making more sound. The system listens to the background noise and instantly makes a copy of it with the compressions and rarefactions reversed. The compressions of one sound fill up the rarefactions of the other sound, and thus the two sounds cancel each other out.

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SEE ALSO:

• Air and Atmosphere • Communi­cation • Energy • Pressure

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Soviets Take the Lead

Sputnik 1 weighed 183 pounds (83 kilograms), compared with the tiny U. S. satellite weighing 3.5 pounds (1.5 kilo­grams) that still awaited launch by a U. S. Navy Vanguard rocket. Soviet leader Nikita Khrushchev promised an even bigger surprise. This came on November 3, 1957, when the Russians

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sent up Sputnik 2, carrying a dog named Laika. In January 1958, the United States responded by launching its first satellite, Explorer 1.

The success of the Soviet space pro­gram caused a reaction in the United States. To counter the Soviet lead in space exploration and rocketry, the U. S. government established the National Aeronautics and Space Administration (NASA) in October 1958.

The Soviets, however, continued to chalk up firsts. In 1959 their Luna 2 probe made a crash landing on the Moon. Then another Luna probe pho­tographed the far side of the Moon for the first time. Soviet cosmonauts and American astronauts were already in training for manned missions. Soviet plans remained shrouded in secrecy, however, while NASA introduced its seven Mercury astronauts to the media.

In 1961, the Soviet Union made more headlines when Yuri Gagarin became the first person in space. His Vostok space­craft was three times heavier than the U. S. Mercury capsule, in which John Glenn became the first American to orbit Earth in 1962.

More Soviet Successes

Sputnik 1 was crude by modern stan­dards, but the Soviets launched the much larger Sputnik 2 on November 3, 1957. Inside that spacecraft was the first animal to orbit Earth, a dog named Laika. Sputnik 2 was not designed to reenter the atmosphere and land, so Laika died in space.

By the end of 1957 the Soviet Union was clearly the winner of the first lap of the space race. Sputnik was a political triumph, trumpeted by the Soviet Union and its supporters as a brilliant example of Communist achievement.

SPUTNIK REPLICAS

A model of Sputnik 1 was presented to the United Nations and is on dis­play at its building in New York City. There is also a replica at the National Aerospace Museum in Washington,

D. C. In 1997, to mark the fortieth anniversary of the launch of Sputnik 1, a scale model of the satellite – made by students at one-third of the original size-was launched from the Mir space station.

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Altogether there were ten Sputnik missions. Sputnik 3 (May 1958) was the Soviets’ original large science satellite. On August 19, 1960, Sputnik 5 carried into orbit two dogs (named Belka and Strelka), forty mice, two rats, and an assortment of plants. The following day, the spacecraft landed successfully with its animal and plant passengers alive and well. Sputnik 10 was the last Sputnik, in March 1961, and it also carried a dog, Zvezdochka. This final mission was flown in preparation for the first manned spaceflight by Yuri Gagarin in April 1961.

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SEE ALSO:

• Gagarin, Yuri • NASA • Satellite

• Space Race

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. The Future of Supersonic Flight

There are plans to build new supersonic passenger aircraft, but most of them are for small business jets. Any future supersonic transport will have to fly passengers farther, less noisily, and more efficiently than earlier aircraft.

Designers believe that they already know how to deal with the problems of engine efficiency, engine noise, and sonic booms. There are designs for more efficient engines that behave like ordi­nary, quieter jet engines for takeoff and climbing. At high altitude, these engines fly supersonically. This type of engine is called a variable cycle engine.

In 2003, an experimental aircraft car­ried out test flights called the Shaped Sonic Boom Demonstration. Using a Northrop F-5E fighter, scientists showed
that it is possible to make a sonic boom quieter by changing the shape of an air­craft’s nose. In the Shaped Sonic Boom Demonstration the intensity of the sonic boom was reduced by about half. Researchers are working toward build­ing an experimental aircraft incorporat­ing every known technique for reducing the intensity of sonic booms to see how quiet a supersonic aircraft can be.

SEE ALSO:

• Bell X-1 • Concorde • Future of

Aviation • Shock Wave

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VERTICAL TAKEOFF IN BIRDS AND INSECTS

The masters of vertical flight are the common housefly and the humming­bird. Both of them have remarkable flight control and are able to take off vertically and hover motionless in the air.

Most insects have two pairs of wings, but the housefly has only one pair—its hind wings have withered away to form two little stumps called halteres. The housefly flaps its wings about 200 times every second. Instead of flapping up and down, they make a figure-eight shape in the air. This directs air downward for takeoff. As the insect flies, the halteres vibrate. If the fly changes direction, the halteres’ vibration is disturbed, instantly giving the fly information about how it is moving. This helps the fly to control its flight with great precision.

Hummingbirds are the helicopters of the natural world. Most birds’ wings generate all of their lift on the down – stroke, but a hummingbird’s wings gen­erate lift on the upstroke, too. The bird’s body is tipped up so that its wings beat back and forth parallel to the ground. About three-fourths of the lift comes from the forward downstroke and the rest from the backward upstroke. To generate enough lift, the wings flap at up to eighty times a second.

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VERTICAL TAKEOFF IN BIRDS AND INSECTS

О The Convair XFY-1 Pogo was a tail-sitter, a type of VTOL developed in the 1950s. At touch­down, the landing gear compressed several feet, like a pogo stick, to cushion the landing. The Pogo made successful flights, but the program for its development ended in 1955.

Spoilers and Ailerons

Hinged panels called spoilers on top of a wing serve the opposite purpose from flaps and slats. When a spoiler is raised, it spoils the wing’s aerodynamic shape, increases drag, and cuts the amount of lift. Spoilers are used to cut lift after an airplane has landed. They may be used to help a big, heavy plane turn. The spoilers on one wing are raised to help the ailerons.

Ailerons are panels in the trailing edge of an airplane’s wings that are used for turning the plane. One aileron tilts up, and the other tilts down. One wing produces more lift than the other and rises. The wing that produces less lift falls, and the plane rolls into a turn.

Big airliners often have two sets of ailerons. The ailerons closest to the wingtips have more leverage for rolling the plane at low speeds. At high speed, the ailerons closest to the fuselage have enough leverage to turn the plane.

The War at Sea

At sea, the aircraft carrier proved a deci­sive weapon. In the Pacific, both sides deployed naval strike forces headed by aircraft carriers flying four main types of combat airplanes: fighters, dive – bombers, high-level bombers, and torpe­do bombers.

The first sea battle to be won by naval aircraft rather than battleships was the Battle of the Coral Sea (May 1942). At the Battle of Midway (June 1942), dive-bombers from U. S. Navy carriers destroyed four of the Japanese

О Naval airplanes played a key part in the war in the Pacific. This photograph shows a B-25 leaving the deck of a U. S. aircraft carrier to take part in the first bombing raid on Japan in 1942.

The War at Sea

Navy’s carriers. The biggest naval air battle of the war was Leyte Gulf, yet another U. S. victory in October 1944.

During the island-hopping Pacific battles, U. S. Navy and U. S. Marine Corps pilots flew hundreds of miles across the ocean to attack Japanese ships and island bases, using the speed and power of strike planes such as the P-38 and F6F to great effect. At sea, flying boats such as the Catalina proved effective. As well as attacking enemy ships and sub­marines, they located and picked up Allied pilots shot down in the water and tracked enemy battle fleets. Equally valuable were land-based reconnais­sance and submarine-hunting aircraft, such as the B-24.

The Development of Jets

Until 1944, all combat planes had piston engines driving propellers. The Germans had flown the world’s first jet airplane, the He-178, in 1939. The Me-262, the work of designer Willy Messerschmitt (1898-1978), became the first jet fighter in 1944. Two other German jets, the Heinkel He-162 and the Arado Ar-234 jet bomber, were hurried into service before German’s surrender in May 1945.

No Japanese jet plane fought in the war, although several were under devel­opment when peace came in August 1945. The U. S. Bell Airacomet and Lockheed XP-80 Shooting Star jets did not see combat, but the British Meteor jet flew its first operational missions in the summer of 1944, chasing German V-1 flying bombs.

Spaceflight

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paceflight means traveling in space. Manned spacecraft of dif­ferent kinds, from the space cap­sules of the 1960s and 1970s to the Space Shuttle, have carried people on spaceflights. Spacecraft called satellites are in flight as they orbit Earth. Space probes have visited planets such as Mars, Venus, and Jupiter, and they have even flown beyond the solar system.

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WHERE DOES SPACE START?

There are several definitions for where space begins. The atmosphere, a blanket of air around Earth, gradu­ally thins as altitude (distance from Earth’s surface) increases. Earth’s atmosphere fades away almost completely at the top of the thermo­sphere, which is about 400 miles (640 kilometers) from Earth’s surface. In terms of spaceflight, however, people define space as beginning much closer to home, either 50 miles (80 kilometers) or 62 miles (100 kilo­meters) above Earth’s surface. Space scientists and engineers refer to "entry interface" as being just over 75 miles (120 kilometers) above Earth’s surface. This is the point at which the air becomes thick enough to begin heating up a spacecraft as it returns from space.

Spaceflight

О The invention of powerful rockets, along with other significant developments in technology, has enabled human beings to launch themselves into space.

NASA Advances

The Soviet successes pushed the U. S. government into funding the $20 billion Apollo program that aimed to land

THE PRESIDENT’S CHALLENGE

"I believe this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth. No single space project. . . will be more exciting, or more impressive to mankind. . . and none will be so difficult or expensive to accomplish."

President John F. Kennedy addressing Congress, May 25, 1961

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NASA Advances

О The United States launched its first satellite, Explorer 1, on a Jupiter rocket at Cape Canaveral, Florida, in January 1958. The Soviets had launched their first satellite a few months before.

astronauts on the Moon before the end of the 1960s. NASA successes and failures, broadcast live on television and radio, were very public. Soviet flights were revealed only after a successful launch. The Soviets made more headlines with the first two-person spacecraft, the first woman astronaut, and the first space­walk. With the U. S. Gemini program, however, NASA demonstrated essential space techniques, such as docking. U. S. computer and ground-tracking systems also were far ahead of Soviet electronics at the time.

THE HUMAN COST OF THE SPACE RACE

There were human casualties of the space race. Three U. S. astronauts were killed in a fire in January 1967 when they were trapped in their capsule as it caught fire during testing. The men were Apollo 1 crewmen Virgil Grissom, Roger Chaffee, and Edward White. Soviet cosmonaut Vladimir Komarov was killed in Soyuz 1 in April 1967. Three other Soviet cosmonauts (Georgi Dobrovolski, Viktor Patsayev, Vladislav Volkov) died in Soyuz 11, in 1971.

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By the mid-1960s, manned space­flights attracted huge public interest. The promise of Apollo overshadowed scientific missions by robot probes to the planets Mars and Venus, which were launched by both the Soviet Union and the United States.

By 1969, the Americans had a rocket to match the Soviet boosters: the mighty Saturn V, built to send Apollo to the Moon. Each Apollo test flight aroused high public excitement. The world was watching when, on July 20, 1969, the Apollo 11 lunar lander touched down on the Moon, and Neil Armstrong radioed a message to Earth: “Houston, Tranquility Base here. The Eagle has landed.”

Stability and Control

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n airplane’s stability is its ability to correct itself when it is upset by turbulence or a gust of wind. The stability of an aircraft affects its performance, handling, and control in the air. Most airplanes are designed to be stable in all three dimensions: longi­tudinal stability (stability in pitch), later­al stability (stability in roll), and finally directional stability (stability in yaw).

Longitudinal and Lateral Stability

Longitudinal stability helps an airplane to stay level from nose to tail. This form of stability is provided by the aircraft’s horizontal stabilizers, or tailplanes. If a gust of wind pushes a plane’s nose up, the horizontal stabilizers produce more lift and raise the tail as well, keeping it level with the nose. If a gust pushes the nose down, the tail also dips.

Lateral stability helps to stop a plane from rolling, or banking, unless the pilot

wants it to roll. The wings provide the lateral stability. Most airplane wings tilt up at an angle from the fuselage to the wingtips, making a shallow V shape. This is called the wing dihedral. If the plane rolls to one side, one wingtip rises, and the other falls. The plane also begins to slide sideways toward the lower wing. As it starts to slide, air pushes back against it. The air resistance lifts the lower wing and levels the aircraft.

An airplane’s wings usually are con­nected to the plane at the bottom of the fuselage, but a few planes have wings that connect at the top of the fuselage. These wings do not have a dihedral shape. Instead, they are level or droop down a little. The drooping wing shape is the opposite of dihedral, and it is called negative dihedral or anhedral.

Подпись: vertica stabilizer

Stability and Control Stability and Control Подпись: (Directional stability)

Stability and ControlО This diagram shows the dihedral angle of the wing used in most airplanes. The angle provides lateral stability and keeps the aircraft from rolling during flight. The stabilizers in the tail provide longitudinal and directional stability.

Stability and Control

Подпись: О Early winged aircraft were highly unstable. This plane crashed near El Paso, Texas, in 1911.

This type of airplane gets its lateral stability from the weight of its fuselage. If the plane is pushed over into a roll by a gust of wind, the fuselage works like the heavy weight at the bottom of a pen­dulum and swings the plane back to an upright position. Hang gliders and microlight aircraft are stable for the same reason-the pilot’s weight hanging below the wing provides stability.