Category FLIGHT and M ОТІOIM

n everyday speech, the words weight and mass are used to mean the same thing. To scientists and engineers, however, they have different meanings. Mass is the amount of matter an object is made of. Its weight is the downward force acting on it due to gravity. Mass is a number, but weight is a force. Mass is constant, but weight depends on the strength of gravity acting on the mass.

The weight and mass of an object are linked in the following equation: weight = mass x g

in which g is the acceleration the object would experience if it fell. At Earth’s
surface, this is 32 feet per second per sec­ond (9.75 meters per second per second).

For most of human history, an object’s weight meant its weight on Earth’s surface. As gravity is more or less the same everywhere on Earth, it did not matter if weight and mass were mixed up. It matters today, because spacecraft can be sent into Earth orbit and away to the Moon and planets, where the force of gravity acting on them may be quite different from the strength of Earth’s gravity.

Mass

If an object is given a push, it acceler­ates. The size of acceleration depends on the mass of the object. If a small mass and a large mass are pushed by the same force, the small mass accelerates faster. This is because the large mass offers more resistance to the force than the small mass.

The tendency of a mass to resist a change in its motion is called inertia. The bigger the mass, the greater the inertia. This is impor­tant when calculating how aircraft and spacecraft will accelerate when their engines produce a cer­tain amount of thrust.

The Eindecker monoplane (1915) devel­oped by Dutch engineer Anthony Fokker (1890-1939) was the first modern fight­er plane. It was agile, could fly at 83 miles per hour (134 kilometers per hour), and could dive steeply without its wings breaking off-an accident all too com­mon in less robust aircraft.

Most airplanes of this period were still made of wood and fabric. The first all-metal plane was the J-1, designed in 1915 by German engineer Hugo Junkers (1859-1935). After this model, progress

INTERRUPTER GEAR

In 1915, French pilot Roland Garros shot down a German Albatross air­plane. Garros surprised his opponent by firing bullets through the pro­peller. The French Morane plane had deflectors fitted to the propeller blades, which prevented bullet dam­age. The French plane was later captured by the Germans and stud­ied by Anthony Fokker, who invented an improved system called an inter­rupter gear. This device basically made the propeller fire the gun, so the spitting bullets synchronized with the spinning propeller.

was rapid. By 1917, the Sopwith Camel, a twin-gun biplane built in Great Britain, could reach speeds of 124 miles per hour (200 kilometers per hour) and climb to a height of around 22,000 feet (6,700 meters).

Fighter pilots, known as aces, devel­oped air combat tactics and maneuvers, such as the Immelman Turn, named after German flier Max Immelman, who was killed in 1916. The most famous fighter ace was the German Manfred von Richthofen, known as the “Red Baron.” Another German pilot, Oswald Boelcke, took the lead in drawing up group tactics and organizing pilots into squadrons. Starting in 1915, pilots flew in formation instead of wandering alone in the skies over battlefields.

The Wrights were now ready to try pow­ered flight. Charles Taylor, a mechanic in their bicycle shop, built a four-cylinder engine. The Wrights spent three months designing a propeller, a difficult problem that produced several heated arguments.

The brothers went back to North Carolina with their powered plane, the Flyer, in late September 1903. However, bad weather and mechanical problems plagued them through the entire fall. At last, on December 14, Wilbur attempted the first flight. The plane rose briefly into the air, but the engine stalled and the plane crashed into the sand. Fortunately, the repairs were easy.

On December 17, 1903, the weather allowed another attempt. At 10:35 a. m., Orville took off. In 12 seconds of flying, he cruised 120 feet (37 meters). The brothers had three more successful flights that day, two by Wilbur and one more by Orville. The last, piloted by

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Wilbur, was the longest (59 seconds) and went the farthest: 852 feet (260 meters). That evening, a powerful wind picked the Flyer up and smashed it into the ground. The brothers packed up the pieces of the aircraft and went home.

Most probes are launched by a multi­stage rocket from the ground. There are three ways to send spacecraft into space using a rocket: sounding trajectory, Earth orbit, and Earth escape.

Sounding rockets were often fired into space during the 1940s and 1950s and are still used today. A sounding rocket can be fired to an altitude of about 100 miles (160 kilometers), at a

HIGH-SPEED LAUNCH

The fastest space launches have all involved space probes. In 1972 NASA’s Pioneer 10 was launched toward Jupiter at 32,400 miles per hour (52,130 kilo­meters per hour). In 1990, the probe Ulysses, on a mission to study the Sun, reached 34,450 miles per hour (55,430 kilometers per hour) during launch. New Horizons, launched in 2006 toward Pluto, was boosted to 35,800 miles per hour (57,600 kilometers per hour), as it left Earth’s orbit for deep space.

О A NASA sounding rocket is fired in 1988. Sounding rockets only reach the fringes of space, but they offer an inexpensive way of gathering data.

maximum speed of about 5,000 miles per hour (8,050 kilometers per hour). After its engine burns out, the rocket begins its descent back to Earth. Scientific instruments in the nose of the sounding rocket send information to the ground by telemetry (radio) or may be retrieved by parachute.

To enter Earth orbit, a rocket trajec­tory must be at an angle so that it flies parallel to Earth’s surface. When its booster motors cut out, the top­most stage of the rocket must be going fast enough to enter orbit and not fall back to the ground under the pull of Earth’s gravity.

To escape completely from Earth’s gravity and become a planetary probe, a spacecraft must reach a velocity of around 25,000 miles per hour (40,200 kilometers per hour). It will then fly away from Earth, gradually slowing down. It may go into orbit around the Sun, or it may be attracted by the gravitational pull of a planet, such as Mars or Jupiter.

At the end of its orbital mission, the Space Shuttle comes back to land on Earth. During reentry, the craft adopts a nose-up angle, and heat-absorbent tiles on the spacecraft’s underside shield the crew from the intense heat. As it descends, the Space Shuttle switches to horizontal flight, dropping fast through the air toward the landing strip. With no engine power during landing, pilots have described the Space Shuttle during air flight as a “flying brick.” A Shuttle

SHUTTLE FACTS

• Most Space Shuttle missions last between five and sixteen days.

• The longest orbital mission to date was STS-80, which lasted 17.5 days in November 1996.

• Early missions carried two-person crews, but the usual crew is seven. The Space Shuttle has room for ten.

• More than 800 people have flown into space on Space Shuttles.

• Each Space Shuttle mission is given a number preceded by the letters STS, which stands for Space Transportation System.

• The energy released by the Space Shuttle’s three main engines is equivalent to the output of thirteen Hoover Dams.

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pilot has only one opportunity to land the spacecraft-because it has no engine power for landing, the craft cannot fly around for a second try. Space Shuttle landings are usually made at the Kennedy Space Center in Florida or at Edwards Air Force Base in California. At Kennedy Space Center, the Space Shuttle lands on a 2.8-mile (4.5-kilometer) run­way, one of the longest in the world.

tealth is the ability to move in secret. A stealth airplane is designed to fly unseen, evading detection by enemy radar. Stealth tech­nology uses a combination of design factors, including materials, engines, and shape. A stealth strike aircraft can attack its target without warning. It also can fly reconnaissance missions without being detected.

Early Research

The research for stealth airplanes began in the 1950s. Air defense radar was developed during World War II (1939-1945). By the 1950s, air defenses had progressed so rapidly that almost any airplane flying over hostile territory was likely to be detected by radar. It then could be tracked and shot down with missiles. This was the era of the Cold War, when the United States and the Soviet Union were engaged in an arms race, during which both countries built up weapons supplies and advanced their military technology. Both sides used espionage, including spyplanes, to discover the other’s secrets.

At first, American strategists hoped that a high-flying airplane such as the U-2, developed by the Lockheed Corp­oration’s Skunk Works design team, could evade detection. This theory was disproved in 1960 when a U-2 spy plane was shot down over the Soviet Union. Later, expensive projects-such as the XB-70 Valkyrie bomber-were canceled

when it was realized that such airplanes left a large “blip” on radar screens, mak­ing them easy targets for missiles. By the 1970s, surface-to-air (SAM) missiles had

become so effective that few airplanes could escape being targeted once spotted on enemy radar.

Scientists did not give up, however. All airplanes, especially metal planes with heat-emitting jet engines, leave a track, called a signature, on a radar screen. The answer to escaping detection appeared to lie in finding a way to “cloak” the plane, thereby making it invisible to radar. Engineers looked for ways of reducing an aircraft’s radar sig­nature so that it would leave a smaller blip or not show up at all.

Mathematicians came up with a com­puter program, called Echo, that was able to predict the radar signature left by different airplane shapes. Studies showed that a body shape made of flat panels, or facets, could take almost all the radar energy that was hitting it and radiate that energy away from the ground, making the airplane virtually invisible to defense radars. The trick was to design an airplane of this shape, somewhat like a flying diamond, that could fly fast enough and high enough to be effective.

A helicopter’s tail has no rudder or elevators. Its main rotor controls alti­tude, so elevators are not needed. Its tail rotor controls yaw, so a rudder is not needed either. The sideways thrust of the tail rotor stops the helicopter from spin­ning in the opposite direction to the main rotor. Increasing or decreasing the tail rotor thrust makes the helicopter turn, or yaw.

Not all helicopters have a tail rotor. A NOTAR (short for NO TAil Rotor) heli­copter has a jet thruster at the end of its
tail boom. It blows air out of a slot in the side of the boom. Some helicopters do not have a tail at all. These helicopters have two main rotors instead of one, and the two rotors spin in opposite directions. The turning forces they apply to the helicopter are equal and opposite. No overall turning force, or torque, is applied to the helicopter, and so no tail rotor or thruster is needed.

If an object with a mass of 132 pounds (60 kilograms) is weighed, the scales show a weight of 132 pounds (60 kilograms). If the same object is taken to the Moon and weighed there, it weighs only

О Gravity is weaker on the Moon than on Earth because the Moon is smaller and has less mass. Astronauts on Apollo missions set up scientific experiments on the Moon to find out about its force of gravity and other aspects of its environment.

22 pounds (10 kilograms), because the Moon’s gravity is only one-sixth the strength of Earth’s gravity. However, the object’s mass has not changed. It is still 132 pounds (60 kilograms).

The pound mass and the pound of weight, or pound-force, are therefore different. The pound mass never changes, but the weight of the pound mass depends on the strength of gravity acting upon it. In the metric system, the unit of mass is the kilogram, and the unit of force is the newton. Gravity acts on a mass of 1 kilogram with a force of 9.8 newtons. So, a 1-kilogram mass actually weighs 9.8 newtons. It is impor­tant to know the weight of an aircraft or rocket because it shows how much lift it must generate to take off.

WEIGHTLESSNESS

The strength of Earth’s gravity weak­ens with distance. The farther that something is from the center of Earth, the weaker the force of gravi­ty it experiences, and so it weighs less. This means that airline passen­gers and astronauts in space weigh less the higher they go.

This does not explain why astro­nauts are able to float about in space. Astronauts are weightless not because the force of gravity has fall­en to zero where they are. In fact, the force of gravity acting on astronauts in Earth orbit is just a fraction less than the force of gravity at Earth’s surface. Orbiting astronauts float about because they are in a state of free fall, like skydivers.

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Pilots and generals soon realized that planes could do more than just fly over the battlefield firing at one another. They also could drop bombs. The first air bombs were little bigger than grenades and were dropped by hand. Bombs, with fins to stabilize them as they fell, grew steadily bigger: from 10 pounds (4.5 kilograms) in 1914 to 1,600 pounds (726 kilograms) in 1918. Bombs were carried in racks and were dropped using bomb – sights that took into account the plane’s height and speed.

At first, bombs were dropped on military tar­gets only, but in 1915 German Zeppelin airships began bombing London and other British cities.

О Handley Page two-engine bombers were used to fly long distances and attack German ground targets and warships.

These bombings were the first air raids on civilians. The Zeppelins scared many people, but the giant aircraft were vulnerable to fighter planes once they were intercepted. Their size made them an easy target for machine guns.

Some news reports about the flight appeared, but the details were often wrong. The Wrights issued a statement but gave few details, hoping to protect their work. They already were planning a better machine.

In the spring of 1904, the Wrights launched a model named the Flyer II near Dayton. They made many flights that summer, becoming more skilled at piloting. On September 20, 1904, Wilbur had a spectacular flight. He traveled more than 3 miles (4.8 kilometers), made circles in the air, and stayed aloft for more than 1/2 minutes.

The following year, the brothers flew another new model. On October 5, 1905, the Flyer III covered more than 24 miles (39 kilometers) and was airborne for nearly 40 minutes. It would be a further three years before a European matched these achievements.

О This is the telegram that arrived in the Wright brothers’ family home in Dayton, Ohio, announcing the first successful powered flight on December 17, 1903.