Category FLIGHT and M ОТІOIM

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.

The calculations involved in space navi­gation are complex. The launch base (Earth), the space probe, and the probe’s target (a moon of one of the giant plan­ets, perhaps) are all moving through space. Ground controllers must calculate

launch speed and course precisely. If necessary, they make midcourse correc­tions by using computers to fire small rocket motors onboard the spacecraft. In this way, scientists can send a probe on voyages that will last for years.

When deciding on a launch date for a planetary probe, scientists choose a favorable “window,” usually when the target planet is at its closest. Moving between planets in space seldom involves traveling in a straight line. Keeping a space probe on the right course requires smart computing and

accurate gyroscopes on the spacecraft. The gyroscopes are used for inertial guidance to keep the space probe on course without reference to the Sun or stars. Instruments measure the slightest change in the spacecraft’s acceleration so that computers can calculate any adjust­ment to the course. When planning a multiplanet mission, scientists may be able to send the probe on a “slingshot” trajectory. This takes the probe around one planet and then uses the planet’s gravitational pull to accelerate it off to the next target. Pioneer 11 did this in 1975, swinging around Jupiter onto a path that took it to Saturn.

The Space Shuttle is about the same size and weight as a medium-size airliner, but crew conditions inside are cramped. The cargo bay takes up the middle of the spacecraft. The main engines and orbital maneuvering systems are in the tail sec­tion. The flight deck and living quarters, are confined to the forward fuselage. On the flight deck, two pilot seats are fitted with manual controls in addition to the automatic systems. The pilots have more than 2,000 separate displays and controls on the flight deck to monitor. The mid-deck area con­tains four crew sleep stations, the waste management system, a personal hygiene station, and a table used for work and meals.

The mid-deck also contains an area for scientific experiments.

О A large truss (support) for the International Space Station arrives at the Kennedy Space Center in 1999. The Space Shuttle carried the truss in its cargo bay to the space station, where it was installed by astronauts.

To carry out extravehicular activity (EVA), or spacewalks, the crew exits the Space Shuttle through an airlock, wear­ing spacesuits. The airlock has room for two crew members to change spacesuits. An EVA that involves a complex task in the payload bay may last up to 6 hours. One of the tools that Space Shuttle astronauts may use is the Remote Manipulator System (RMS), a 50-foot (15-meter) articulating arm, remotely controlled from the flight deck. The arm has a video camera near its tip, so the operator can see clearly when doing delicate assembly or repair work.

In 1977, Lockheed began the top-secret Have Blue project for the Defense Advanced Research Projects Agency (DARPA), the central research and devel­opment organization of the U. S. Depart­ment of Defense. Skunk Works came up with a design that looked like a pyramid with wings and two tails. It was almost invisible to radar when mounted on a pole on the ground, but would it fly? The plane’s unusual shape made it unstable, and it could not have flown successfully without the aid of comput­er technology. Computerized fly-by-wire systems were already in use on planes such as the F-16, constantly adjusting the flight controls to prevent the plane from losing stability and crashing. The secret stealth plane, designated the F-117, was equipped with such a system.

The F-117 was test flown in the Nevada desert in 1981. It was like no other plane. Anything that gave off a radar trace was eliminated from the air­craft, so antennae and sensors were designed to retract into the fuselage. The F-117 had no radar system of its

TECH ‘kTALK

THE F-117

The F-117 Nighthawk is a single-seat airplane powered by two General Electric turbofan engines. It flies at just below the speed of sound (Mach 1). Its principal weapons are two 2,000-pound (907-kilogram) laser-guided bombs, or air-to-surface missiles. The wings and fuselage are aerodynamically blended. They are made of a conventional material, aluminum, but are coated with spe­cial radar-absorbent materials. The Nighthawk weighs about 52,000 pounds (about 23,000 kilograms) when fully loaded.

own to betray its position. The cockpit was coated with a reflective material that radar beams bounced off in all directions. The engine intakes were screened, and exhaust gases were cooled by heat absorbers so that little trace showed on heat sensors.

Initially, the F-117 was not an easy aircraft to fly. Two early prototypes crashed, in 1978 and 1980, but the pro­gram continued with two more test air­craft. The first F-117A was handed over to the U. S. Air Force in 1982 and went operational the following year. It was still top secret, flying only at night. The public became aware of the mystery plane, named the Nighthawk, in 1989, when it took part in operations against Panama. F-117s also flew missions against Iraq during the Gulf War in 1991.

The Space Shuttle’s tail is a complicated structure that contains a total of thirty – three rocket engines and thrusters. There are three main engines that are used only during launch. Two smaller Orbital Maneuvering System (OMS) engines boost the spacecraft into orbit, change its orbit, and slow the spacecraft down to begin its return to Earth. In addition, there are twenty-four pri­mary thrusters and four smaller thrusters, called vernier thrusters, for small changes in speed and attitude control.

The Space Shuttle’s tail fin, or vertical stabi­lizer, stands more than 26 feet (8 meters) high. Its rudder is unique.

О NOTAR helicopters, with no tail rotor, offer reduced noise and vibration. They are also safer to fly in tight situations where a tail rotor might meet an obstruction.

The first privately operated space plane, SpaceShipOne, has an unusual tail. When the space plane reaches the highest point in its flight-about 70 miles (110 kilometers)— its twin tail booms swivel up so that they stick straight up from the small wings. This jackknifed position enables it to reenter the atmosphere safely. The reentry is ballis­tic, which means SpaceShipOne falls back into the atmosphere under the action of gravity alone, like a rock falling to Earth. No engine power is used. The jackknifed shape causes a lot of drag, which slows the craft down. At an altitude of 50,000-60,000 feet (15,250-18,300 meters), the pilot swivels the tail booms down again, and the craft glides down to land on a runway.

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It works not only as a rudder but also as a speed brake. When it swivels to the left or right, it works as a rudder. To work as a speed brake, it splits in two and opens like a book. Half of it swivels to the left, and the other half swivels to the right.

SEE ALSO:

• Aileron and Rudder • Control System • Pitch, Roll, and Yaw

• Stability and Control

Date of birth: June 1, 1907.

Place of birth: Coventry, England.

Died: August 9, 1996.

Major contributions: Invented the jet engine; built a jet engine used in an air­plane that set speed and altitude records. Awards: Knight of the Order of the British Empire; Albert Medal; Order of Merit; Charles Stark Draper Prize; SAE Aerospace Engineering Award.

he son of a mechanic, Frank Whittle joined the British Royal Air Force (RAF) at the age of six­teen. His work with model airplanes caught the eye of an officer who recom­mended him for officer training. During training, Whittle wrote a detailed essay about the possibility of developing a new kind of aircraft engine that would not turn a propeller. He wrote that planes could reach higher altitudes and faster speeds if exhaust from the engine provided the thrust.

Higher-ranking offi­cers dismissed Whittle’s ideas, but he continued to pursue them. His ini­tial plan used a piston to compress air, but he con­cluded that such an engine would weigh too much. Whittle developed a new approach using a turning turbine. Once again, however, his supe­riors rejected the idea. In 1930, Whittle patented the idea himself.

Little happened with Whittle’s idea until he was approached six years later by Rolf Dudley- Williams and James Tinlin, about the possi­bility of developing his engine. The three formed a company-Power Jets Limited-in 1936 and began work on building a model of Whittle’s
engine plans. On April 12, 1937, they tested the engine, which was mounted on a stand on the ground. It worked per­fectly. A scientist who learned of the success convinced the RAF to provide funding to develop an airplane that used the new engine.

That work took place slowly. A suc­cessful test of a newer version of the engine in 1939 spurred quicker work. The prototype plane, built by Gloster, arrived late in 1940, and Whittle built the engine at Power Jets facilities. The engine was tested successfully late in 1940. On May 15, 1941, the Gloster plane, with Whittle’s engine inside, flew for 17 minutes. It reached a top speed of 340 miles per hour (545 kilometers per
hour). The success convinced RAF offi­cials to move ahead with the aircraft.

In 1944, the British government took control of Whittle’s company. In 1948, Whittle resigned from the company and from the RAF due to ill health. For the next twenty years or so, he worked as a consultant for various companies. Some of his work focused on jet engines. Whittle also designed a new kind of drilling head for oil drills. In 1976, he moved to the United States, where he taught at the U. S. Naval Academy.