Category FLIGHT and M ОТІOIM

It is easy to tell if a plane is flying faster than sound because a supersonic plane makes a telltale boom as it flies past. The sound of an aircraft travels away from it in all directions through the air. Sound
travels faster than a subsonic plane, so a plane such as a jumbo jet can be heard coming before it passes overhead. A supersonic plane flies faster than its own sound, so it passes silently overhead before its sound arrives.

Supersonic aircraft compress, or squash, the air ahead of them, producing a high-pressure wave called a shock wave. Several shock waves form on var­ious parts of the aircraft. As a plane nears the speed of sound, a shock wave forms in the middle of each wing. The shock wave forms over the wing first because the air flowing over the top of the wing speeds up. The speed of the wing relative to the air is therefore high­er than the speed of other parts of the
aircraft relative to the air. So even when a plane is flying just below the speed of sound, the tops of its wings may be supersonic relative to the air, and that is where the first shock waves appear.

The biggest shock wave forms on an aircraft’s nose. It makes a cone shape, with the aircraft’s nose at the cone’s tip. This shock wave spreads out into the surrounding air and travels all the way down to the ground. As it sweeps across the ground, the sudden change in air pressure it causes sounds like a boom to anyone below the shock wave.

The intensity of this sonic boom depends on an aircraft’s altitude. The higher it is, the more time the shock wave has to spread out into the sur­rounding air, reducing its intensity. A supersonic plane flying at an altitude of more than 35,000 feet (10,700 meters) makes a loud, thunderlike sonic boom. The same plane flying at the same speed, but at an altitude of less than 10,000 feet (3,050 meters), produces a sonic boom powerful enough to shatter windows. For this reason, Concorde was only allowed to fly at supersonic speeds when at high altitude and over water.

When something falls through the air toward the ground, the force of gravity makes it accelerate. As it accelerates, the drag (air resistance) it experiences increases. If it falls for long enough, its weight is exactly balanced by drag, and it stops accelerating. This velocity is called its terminal velocity.

A large but very light object, such as a feather, reaches its terminal velocity very quickly. A heavier object of the same size has a higher terminal velocity, because a higher speed is needed to cre­ate enough drag to balance its weight.

In the usual skydiving position­falling with arms and legs held out-a skydiver’s terminal velocity is about 120 miles per hour (195 kilometers per hour). With arms and legs pulled in, however, there is less drag, and the skydiver accelerates. Terminal velocity increases to about 200 miles per hour (320 kilo­meters per hour) before weight and drag are once again balanced.

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SEE ALSO:

• Gravity • Laws of Motion

• Relativity, Theory of • Skydiving

• Speed

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Awing, or airfoil, is a surface that produces a lifting force when it moves through air. A flat surface creates lift if it moves through air at the correct angle, like a kite. A curved sur­face produces more lift and less drag.

Wing Shapes

Most aircraft wings are curved on top and flatter underneath. Fixed-wing air­craft generally have one of four types of wing: straight, swept, delta, or swing. Small, slow airplanes usually have wings that stick straight out from the sides of the plane’s body, or fuselage. Straight wings are not suitable for high­speed flight, because they create too much drag. Wings that are angled back­ward, called swept wings, are better for high-speed aircraft, such as jet airliners. A delta wing is a triangular wing that
is very efficient for supersonic flight. It produces little lift at low speeds, how­ever, so delta-wing aircraft have to take off and land faster than other aircraft, and they are not very maneuverable. One solution is to fit the plane’s nose with small, movable wings called canards. The canards create more lift and swiveling them makes the plane react faster during maneuvers.

A few planes have swing wings, which are straight at low speeds and swept back at high speeds. Their wings actually move, swinging backward as a plane accelerates. This is called variable geometry, or swing-wing technology. These planes are rarely built, however, because the swing-wing mechanism is heavy and complicated.

In September 1940, the German Luftwaffe switched its attention to bombing British cities, beginning the Blitz. The British retaliated by bombing the German capital of Berlin. These offensives were the start of a strategic bombing war.

The Germans did not have a heavy bomber. Lacking a first-class bombsight to help them bomb from high altitudes, German designers had been told to give all bombers a dive-bombing capability so they could dive low over their targets for accuracy.

Dive bombers attacked in a steep dive; the pilot released his bomb above the target and zoomed away to avoid the explosion. Dive bombers were good at attacking ground targets, such as airfields, and enemy ships; the German Ju-87 Stuka was widely used for this.

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This bomber’s weakness was its relative­ly slow speed, which made it vulnerable to fighters.

The British and the Americans were building four-engine heavy bombers, such as the Lancaster, Halifax, B-17, and B-24. From the time the United States joined the war (after the attack on Pearl Harbor in December 1941), these bombers played an important role in Allied offensive strategies.

The worst skyjacking in history hap­pened in the United States in 2001. On the morning of September 11, nineteen terrorists hijacked four U. S. airliners: American Airlines Flights 11 and 77 and United Airlines Flights 93 and 175. Flights 11 and 175 had taken off from Boston, Massachusetts; Flight 77 left from Dulles Airport in Washington, D. C.; and Flight 93 departed from Newark, New Jersey. The first three airplanes were on early-morning flights to Los Angeles, California; the fourth was heading for San Francisco, California.

Two of the hijacked planes (Flight 11 and Flight 175) were deliberately flown into the twin towers of the World Trade Center in New York City. Flight 11 hit the North Tower just before 8:45 a. m. Flight 175 hit the South Tower at 9:03 a. m. Both 110-story structures became fiery infernos, pouring black smoke into a blue sky, before collapsing to the ground. At 9:40 a. m., Flight 77 was flown into the side of the Pentagon in Washington, D. C.

On Flight 93, the hijackers-who had smuggled knives onboard-had locked themselves in the cockpit and headed the plane toward Washington, D. C. Flight 93’s passengers learned from cell

О Terrorists flew an airplane into the Pentagon building in Washington, D. C., as part of their skyjacking operation on September 11, 2001.

phone calls to friends and relatives what had happened to the other three planes. They decided to attack the hijackers. The plane went out of control and crashed near Shanksville, Pennsylvania.

Every person onboard the four hijacked airplanes, including the hijack­ers, was killed. Furthermore, many more people were killed on the ground in New York City and Washington, D. C. The total death count was 2,752 at the World Trade Center, 189 at the Pentagon, and 44 in Pennsylvania.

he space race was a period of rivalry between the United States and the Soviet Union during which the two nations competed in the exploration of space. The race was at its height in the 1950s and 1960s.

The Cold War

The space race had its origins in the Cold War, a time of mutual suspicion between the United States and the Soviet Union that began after World War II (1939-1945). The hostility resulted from the deep divide between two political systems: Soviet communism and U. S. capitalism, or free enterprise. The two nations became the world’s military super-powers, each building up huge stocks of weapons, including rockets.

In 1945, after Germany’s defeat in World War II, both the United States and the Soviet Union acquired German rock­et technology. The German V-2 rocket could fly faster than sound and reach a height of 60 miles (96 kilometers). German scientists were taken to the Soviet Union and the United States, where they worked on more advanced rockets for their adopted countries.

On October 4, 1957, the rocket carrying “Elementary Satellite 1,” or Sputnik 1, took off from the desert near Tyura-Tam, in what is now the Kazakh Republic. All went well, and the first radio signals from space told the waiting Soviet sci­entists that Sputnik 1 was in orbit. They announced their success to the world, and almost every newspaper, radio, and television network carried the story. Sputnik 1 was headline news. Astronomers trained radio telescopes on the tiny satellite. Amateur radio enthu­siasts in many countries picked up the satellite’s “beep beep” radio signals as it passed overhead.

Sputnik 1 had a brief working life, but its historic flight proved that a human-made craft could survive launch and fly in orbit. The scientific data gath­ered from the flight was limited, but Sputnik 1 did help to identify the density of the layers in the upper atmos­phere and provided useful information on how radio signals from space were received on Earth.

Scientists had feared that space dust or meteoroid impact might damage the spacecraft. Even a minute hole in the satellite would have caused a detectable drop in pressure and temperature. However, Sputnik 1 continued to orbit Earth undamaged. The spacecraft’s radio batteries ran out before the end of October, after which no more signals were received. The satellite stayed in orbit until January 1958, when it burned up as it reentered Earth’s atmosphere, having traveled about 37 million miles (60 million kilometers).

Shock waves explain why World War II pilots lost control of their aircraft in high-speed dives. As shock waves devel­op over the wing, they disturb the air so violently that the smooth flow breaks up into a swirling turbulent flow. The air­craft’s control surfaces are trapped inside this chaotic air. Robbed of the smooth airflow they need, they cease to work. The turbulence caused by shock waves is the reason for the violent shak­ing that World War II pilots experienced.

Modern supersonic aircraft do not suffer from these problems because of their shape. Their wings are thinner and more swept back, so they cut cleanly through the air and delay the formation of shock waves. When the aircraft goes faster than the speed of sound and the shock wave cone forms around it, the swept-back shape of the wings means that the whole aircraft fits neatly inside the shock cone and flies more smoothly.

he capability of a fixed-wing air­craft to take off and land vertical­ly is known as vertical takeoff and landing, or VTOL. Some aircraft have the ability to take off and land on a very short runway-these are V/STOL aircraft. V/STOL stands for vertical or short takeoff and landing. Still others are STOVL aircraft-they are capable of a short takeoff and vertical landing. The terms are used to describe a small group of fixed-wing aircraft. They do not include helicopters, airships, or rockets, all of which also have the ability of vertical takeoff.

Propeller Planes

Fixed-wing aircraft normally need a long takeoff run to get airborne. They cannot take off until their wings are moving through the air fast enough to create enough lift to overcome the plane’s weight. If a fixed-wing aircraft is to take off vertically, it needs to direct its engine power downward with enough force to overcome its weight.

Propeller planes can do this by tilting their engines and propellers so they work like helicopter rotors. The V-22 Osprey, a V/STOL aircraft, swivels its propeller engines up for takeoff and landing and angles them forward for regular flight. Its propellers work like helicopter rotors for vertical flight and like propellers for forward flight, so they are called prop-rotors. They are made bigger and stronger than normal pro­pellers because they must support the entire weight of the aircraft for takeoff and landing. Aircraft such as the V-22 Osprey are called tilt-rotors.

When a wing slices through air, two things happen to the air. First, the air

О The new Eurofighter Typhoon has delta wings and uses canards for lift and maneuverability.

flowing over the curved top of the wing speeds up. As Bernoulli’s principle states, when air speeds up, its pres­sure falls. Low air pressure above a wing and higher pressure below it produce an upward force. Second, a wing deflects air downward. According to Newton’s third law of motion, forcing air downward produces an equal and opposite force that pushes the wing upward. The total upward force on a wing is lift.

If a wing is tilted up at the front, it deflects air downward even more power­fully and produces more lift and more drag. The angle between a wing and the airflow is called the wing’s angle of attack. If the angle of attack is increased too much, the wing will stall. When a wing stalls, the smooth flow of air over it breaks up, and lift sud­denly disappears.