Category FLIGHT and M ОТІOIM

Big drops in air pressure are even more serious. Pilots and passengers of high­flying aircraft need protection from the low air pressure outside of the plane. Early airliners did not fy higher than about 10,000 feet (3,050 meters)-above that height, some passengers began to feel faint. To fy higher safely, an aircraft has to be pressurized. Extra air is pumped inside a modern airliner to raise the pressure. The air inside a pressurized airliner is not at sea level pressure. It is the same as the pressure at an altitude of about 8,000 feet (2,500 meters), which is about 11 psi, or 75.8 kilopascals.

Fighter pilots sit in a pressurized cockpit, but they also wear an oxygen mask in case the canopy shatters and the cockpit loses pressure.

If an airliner suffers a sudden loss of pressure at its cruising altitude of about

35,0 feet (10,600 meters), oxygen masks drop down automatically from the ceiling. The crew and passengers have about 30 seconds to put them on before losing consciousness.

The Space Shuttle and International Space Station (ISS) are both pressurized to sea level pressure. The spacesuits worn by astronauts, however, are pres­surized to only 4.3 psi (about 30 kilopas – cals) to prevent them from blowing up like a balloon. If an astronaut breathed air at such a low pressure, there would not be enough oxygen, so the suit is supplied with pure oxygen.

Because the pressure in a spacesuit is much lower than the pressure inside the spacecraft, the air pressure around astronauts preparing to leave their spacecraft is lowered in stages so that they can adjust safely. The day before a spacewalk, the air pressure inside the Space Shuttle is lowered to 10.2 psi (70 kilopascals). Space Station astro­nauts spend several hours inside an air­lock where the air pressure is even lower, to prepare for the lower pressure inside their spacesuits.

HIGH RISKS

Early balloonists were the first avia­tors to discover the hazards of high – altitude flight. On September 5, 1862, Henry Coxwell and James Glaisher made a balloon ascent to more than 30,000 feet (9,100 meters). No manned balloon had flown as high before. As they passed

29,0 feet (8,850 meters), Glaisher became paralyzed. Then he lost con­sciousness. Coxwell lost the use of his arms and had to use his teeth to pull the rope that released hydrogen from the balloon and let them descend. They both survived.

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Using Pressure

Aircraft and spacecraft can use different kinds of pressure in their operations. When a large liquid-fuel rocket is fired, pumps feed fuel to the engine. Pumps are too big and heavy for small rockets and spacecraft, so they use pressure instead. A high-pressure gas, such as helium, pushes the fuel from its storage tank to the engine.

Aircraft use high-pressure oil in their hydraulic systems. Hydraulic systems are those operated by liquid. Liquids cannot be squashed as much as gases because their molecules are already close together. This causes liquids under pressure to transmit force from one place to another. A digging machine

works in this way. Oil pumped through flexible hoses operates mechanical pushers called rams, which move the digger’s arm. Modern automobile brakes also work by hydraulic power. An air­craft’s hydraulic system uses oil pressure to move its control surfaces and raise its landing gear. Cargo planes use hydraulic pressure to operate their cargo doors and loading ramps. Rockets use hydraulic power, too, when they swivel their engine nozzles for steering.

SEE ALSO:

• Air and Atmosphere • Altitude

• Astronaut

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There are several ways of steering a rocket or rocket-powered spacecraft. One way is to use swiveling fins, like an airplane’s control surfaces, in the atmos­phere. A rocket must be traveling fast before its fins begin to work, because they only work when air is flowing over them very quickly. Other rockets have swiveling vanes in the rocket exhaust. When the vanes swivel, they deflect some of the engine’s exhaust jet. The entire jet can be deflected by swiveling the engine itself or just the nozzle. Most modern rockets have swiveling engines, also called gimbaled engines. The Space Shuttle’s main engines are gimbaled.

A rocket or spacecraft also can be turned or steered by means of thrusters.

When the Space Shuttle’s solid rocket boosters fall away, thrusters push them away from the spacecraft. The Space Shuttle uses forty-four thrusters in its nose and tail for attitude control when flying in space.

Rockets are used for braking as well as steering. Braking rockets also are called retro-rockets. When an orbiting spacecraft is ready to land, it fires off rockets in the direction in which it is traveling. The thrust slows the space­craft, and gravity begins to pull it down.

The Soyuz spacecraft uses retro-rock­ets for landing. It fires retro-rockets just before it touches down on the ground to cushion its landing.

United Aircraft discontinued the Clippers, but the company funded Sikorsky’s effort to achieve his long-

"I have never been in the air in a machine that was as pleasant to fly as the helicopter. It is a dream to feel the machine lift you gently up in the air, float smoothly over one spot for indefinite periods, move up or down under good control, as well as move not only forward or backward but in any direction."

Igor Sikorsky

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held dream of building a helicopter. This time, that effort succeeded, with the help of new, lightweight materials and a staff of engineers.

On September 14, 1939, Sikorsky climbed into the first helicopter model, the VS-300. He always insisted on tak­ing the first flight of any completely new design. The helicopter worked— it rose vertically, hovered, and returned to land. The flying machine had a single rotor with three blades driven by a 75-horsepower engine. Sikorsky’s heli­copter was not the first to reach the air, but it was the first successful flight of a helicopter with a single rotor. Because most helicopters follow that design, Sikorsky is considered the leading pioneer of the helicopter industry.

The design needed improvement, however. Sikorsky tried another version with two small rotor blades in the rear. On May 13, 1940, that machine rose into the air, but was difficult to move for-

ward. The next model had just one smaller rotor blade in the rear. This ver­sion flew smoothly and on May 6, 1941, it set a record by staying aloft for more than an hour.

Some shapes move through air more easily than others. Angular, boxy shapes catch more air. They also break up the smooth flow of the air, making it turbu­lent and chaotic. Slender, gently curving shapes create less drag than angular shapes, because air can flow around them more smoothly. Objects that air flows around smoothly are described as streamlined.

Airplanes are streamlined. Anything on their surface that might stick out into the air and cause unnecessary drag is smoothed out wherever possible to reduce drag. A plane’s metal skin is held

in place by fastenings called rivets. Airplanes used to be held together by rivets with round heads. The round heads stuck out and caused some drag. Today, the most streamlined aircraft are held together by rivets with flat heads that do not stick out. A plane’s metal skin is also polished or painted to give it a smooth surface that air can flow over easily.

All but the smallest and slowest planes have wheels that fold up inside them after takeoff. Doors close over the wheels to give the plane’s body a smooth, streamlined shape. If the wheels stayed down, they would spoil the plane’s streamlined shape and create a lot of drag. The doors and windows are also designed to be level with the plane’s skin.

When an object tries to move through air, the air pushes back. This resistance to motion is called drag. All aircraft experience drag as they move through air. When an airplane turns, the rising wing experiences more drag than the falling wing. The extra drag is caused by

SPECIAL AILERONS

Light aircraft and planes with long wings, such as gliders, suffer from the worst adverse yaw. Designers can make adverse yaw less of a problem by using special ailerons.

One type, called a Frise aileron, cre­ates more drag when it tilts up than when it tilts down. When a plane with Frise ailerons turns, both wings create extra drag, and so there is little or no adverse yaw.

Another way to deal with the problem is to make the aileron in one wing tilt down just a little, while the aileron in the other wing tilts up a lot. These ailerons are known as differential ailerons. The rising wing creates less drag because the aileron is not tilted downward as much. As a result, the yaw problem is reduced.

The Tiger Moth biplane had differen­tial ailerons. More modern light air­craft, such as the Cessna 152, also use differential ailerons.

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the downward-tilted aileron. This force, called aileron drag, acts like a brake, slowing down one side of the plane. It turns the plane’s nose in the wrong direction-the opposite direction to the turn. This effect is called adverse yaw. Yaw means turning to the left or right.

An airplane’s rudder is used to con­trol yaw. The rudder swivels to the left or right. A pilot corrects adverse yaw by turning the rudder to point the plane’s nose in the correct direction. If a plane banks to the right in order to turn right, for example, its nose yaws to the left. Adding some right rudder corrects this.

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SEE ALSO:

• Aerodynamics • Biplane • Lift and Drag • Pitch, Roll, and Yaw • Tail

• Wing

Airport controllers work from a control tower, a tall building with a good view of the run­ways. They use their eyes, as well as radar, to scan airspace. Incoming airplanes waiting to

О In the United States, military airplanes are controlled by military air traffic centers based on land and on ships at sea. These U. S. Navy controllers are monitoring incoming aircraft aboard the aircraft carrier USS Abraham Lincoln.

land circle in a vertical “holding stack” above the airfield. When a controller directs the lowest plane in the stack to start its landing approach, the next plane descends to take its place.

Once an airplane has touched down, the controller directs it to an exit taxi­way, clearing the runway for the next landing. At large, busy airports, for extra safety, radar also tracks planes on the ground. With an increasing number of flights every year-and tightened security checks at airports-air traffic controllers have a heavy workload. Delays at airports may happen when there are simply too many aircraft for the controllers to handle smoothly.

Modern navigational aids have increased air safety. The first naviga­tional aids for pilots were illuminated beacons on the ground. Next came radio
stations transmitting signals, which a pilot could use to fix a course. Modern airplanes have onboard radar and iner­tial guidance navigation systems, with computers that can fly the plane and plot a course automatically, using data from the satellites of the Global Positioning System (GPS) and other navigation systems. Most modern airliners can, if necessary, land automat­ically without any help from the pilot.

SEE ALSO:

• Airport • Altitude • Communication

• Navigation • Pilot • Radar

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One of the most useful military aircraft is the helicopter. Developed toward the end of World War II, helicopters were used in the Korean War (1950-1953) and the Vietnam War (1954-1975). They have been used in all conflicts since.

Helicopters can land combat troops, carry weapons and supplies, evacuate wounded, and fly around a battlefield to support ground troops and destroy tanks. Helicopters rescue air force pilots shot down over enemy territory as well as civilians in trouble on land or off­shore. Naval helicopters may take off from the decks of naval ships to carry out reconnaissance and anti-submarine patrols and to attack enemy ships.

Transportation

The aircraft used by the military to carry troops and equipment are known as transports. Some transports are huge. The U. S. Air Force’s biggest transport is the C-5 Galaxy; only slightly smaller is the C-141 Starlifter. During the Gulf War (1990-1991), the U. S. Air Force air­lifted more than 577,000 tons (523,340 metric tons) of supplies and nearly 500,000 personnel over distances of up to 7,000 miles (11,260 kilometers) to the Middle East combat zone.

The C-130 Hercules is used on shorter – range missions. This sturdy four-engine turboprop transport has been around since 1954. One of its jobs is to drop paratroops, but it also flies as a heavily armed “gunship.” To extend their range, many military airplanes can be refueled in the air by flying tankers, such as the U. S. Air Force’s KC-10.

Military airplanes provide trans­portation wherever people are in danger or in trouble. They fly emergency aid to the victims of hurricanes, earthquakes, and other natural disasters. They evacu­ate civilians from war zones. They bring food, medicines, tents, and other sup­plies wherever there are floods, famines, or fighting.

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SEE ALSO:

• AWACS • Bomber • Fighter Plane

• Helicopter • Missile • Radar

• Stealth

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n airship is a lighter-than-air craft that can be propelled, like a balloon with an engine. An air­ship also has a rudder and fins for steer­ing. Some airships have rigid sides, while others are soft until filled with gas, like a balloon. Airships were used in the first controlled, powered flights.

The First Airships

Before airships were invented, people had developed balloons for air travel. Balloons, however, are not steerable, and they drift with the wind. In the nine­teenth century, aviators tried to build balloons that could be controlled.

French inventor Henri Giffard (1825-1882) built the first airship in 1852. He constructed a cucumber­shaped balloon 144 feet (44 meters) long. The only engine available at the time was a steam engine. Suspended beneath the balloon was a platform on which Giffard fixed a small steam engine that he designed himself to make it as light as possible. The engine drove a propeller, which pushed the airship along at 5 miles per hour (8 kilometers per hour). In this airship, Giffard flew

for 17 miles (27 kilometers). His airship had no way of turning in flight, unfor­tunately, because it had no steering mechanism.

Charles Renard and Arthur Krebs addressed this drawback in 1884. Their airship, La France, had an electric motor plus a rudder and elevator for steering. The inventors proved their airship’s superiority by flying a circular course over Paris, which no balloon could do. This airship was known as a dirigible, from a French word meaning “steer­able.” The name dirigible came to be used for airships in general.

Other airships soon took to the skies. In 1888, Dr. Karl Wolfert of Germany tested the first airship powered by a gasoline engine-an engine already being tested in early automobiles.

The lunar module began its descent toward the target landing site on an area of the Moon called the Sea of Tranquility. By gazing out of the small window, Armstrong was able to choose a
flat landing area. Probes on the lunar module’s legs signaled when it was about 5 feet (1.5 meters) above the dusty surface. The engine cut out, and the Eagle landed on the Moon at 4:17 p. m. (Eastern Daylight Time) on July 20. After touchdown, Armstrong radioed to Mission Control in Houston, Texas: “Houston, Tranquility base here. The Eagle has landed.”

Armstrong and Aldrin wore space – suits to protect them from the Moon’s environment. There is no atmosphere on the Moon—to survive, the men needed the oxygen and steady temperature and air pressure provided by the suits.

Armstrong opened a hatch and climbed down a ladder onto the powdery surface, followed by Aldrin. The astro­nauts’ first steps on the Moon were recorded by a TV camera on the side of the lunar module. Armstrong said,

“That’s one small step for man, one giant leap for mankind.” The astronauts spent two hours on the Moon’s surface, collecting rock samples and setting up scientific equipment. Mission accom­plished, they reentered the Eagle.

The first manned Mercury spaceflight, launched by the United States in 1961, was ballistic. A rocket carried the space capsule into space. Forty-two seconds after liftoff, the rocket shut down, and the capsule separated from it. The cap­sule’s momentum carried it on upward. The capsule did not go into orbit around Earth. Gravity slowed its upward flight until, at a height of 118 miles (190 kilometers), it stopped climbing and began falling. Parachutes opened to slow the capsule’s fall before splashdown in the Atlantic Ocean.

THE BALLISTA

The word ballistic comes from a Greek word meaning "to throw." A weapon called the ballista was invented in ancient Greece in about 400 b. c.e. It was a throwing machine, like a huge crossbow. It hurled heavy stone balls or spears.

A spacecraft coming back to Earth can follow a ballistic trajectory, which simply means that it falls through the atmosphere. A ballistic reentry is uncomfortable for astronauts, however. The strong forces produced by drag slow the spacecraft down suddenly, like a car braking hard. Also, once the spacecraft begins its return to Earth on a ballistic reentry, it cannot be steered toward a particular landing area.

Another way to return to Earth is to use the spacecraft’s shape to create lift instead of letting it fall back to Earth. A spacecraft produces lift simply by tilt­ing, like a kite. As it plunges through the atmosphere, its angle to the oncoming air lifts it up.

A spacecraft returning from space in this way slows down more gently than in a ballistic reentry. By changing the amount and direction of its tilt, the
spacecraft can then be steered through the atmosphere toward a chosen landing site. The Space Shuttle comes back to Earth in this way.

SpaceShipOne, the first private spacecraft, uses a ballistic reentry. When its rocket engine shuts down after launch, at a height of 150,000 feet (45,720 meters), the spacecraft’s momen­tum carries it up another 150,000 feet (45,720 meters). From that point, SpaceShipOne falls back through the atmosphere. The pilot then retakes control and flies the spaceship like a glider to a landing on a runway.

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SEE ALSO:

• Gravity • Lift and Drag • Space­flight • Space Shuttle • Takeoff and Landing