Category FLIGHT and M ОТІOIM

In World War I, the main role of the warplane was tactical. Generals want­ed airplanes to shoot at troops on the ground, help artillery guns locate tar­gets, and provide intelligence about enemy movements. Strategists soon realized, however, that airplanes could have a greater effect on a battle, by attacking enemy transportation routes, for example. Aircraft also could dis­rupt industry by bombing factories and cities far behind the frontline.

Most warplanes in World War I were land planes, flying from grass airfields. There also were seaplanes able to land on water, and naval avia­tion progressed rapidly. Ships were hurriedly adapted to carry seaplanes, and spotter planes were used in naval battles. Deck landing trials in 1917 led to the first aircraft carriers.

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SEE ALSO:

• Aircraft, Military • Aircraft

Carrier • Airship • Bomber

• Fighter Plane • Mitchell, Billy

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WORLD WAR I AVIATION ADVANCES

The war taught aviators many lessons: about fighter tactics, bombing, and the different roles that aircraft could per­form in war. World War I saw rapid progress in airplane design. When the war began in 1914, planes were made of wood and fabric, carried no guns or bombs, and flew slowly, at low heights.

By the end of the war in 1918, most planes were still biplanes, but they were much improved. By 1918, planes could fly as high as 24,000 feet (7,300 meters). Pilots carried radios to talk to the ground. Trainee pilots learned to fly on dual-control trainers. Interrupter gear had transformed the experience of air-to-air combat. Fast, single-seat fighters flew at about 120 miles per hour (190 kilometers per hour) and fired two machine guns.

Fighter planes were the aircraft with the most impact in World War I. There also were specialized airplanes, including ground-attack strike planes and naval airplanes able to land on ships or water. The development of large, multi-engine bombers was a significant step toward future warfare.

By the war’s end, all the warring nations had air forces of some form. These air forces that evolved during World War I would play a much larger role in the future.

Venus was the first planet to be reached by a space probe. In 1962, the U. S. probe Mariner 2 flew within 22,000 miles (35,400 kilometers) of Venus, but the Russians made the first remote-con­trolled landing in 1970, with their probe Venera 7. The U. S. Magellan spacecraft arrived at Venus in 1990. During a four-year stay, it sent back radar images of almost the entire planet surface.

In the 1960s and 1970s, U. S. Mariner probes investigated

О The identical space probes Voyager 1 and 2 continue to travel decades after they were launched. It is hoped that they will continue their journey farther into space. Voyager 1 has already reached the outer edge of the solar system.

the planet Mars as well as Venus and Mercury. Mariner 9, launched in 1971, went into orbit around Mars and sent back the first close-up pictures of the planet. In 1976, the United States land­ed Viking 1 and 2 on Mars.

In November 1996, Mars Global Surveyor became the sixteenth space probe to fly by, orbit, or land on Mars. The following year, 1997, the Pathfinder lander made a touchdown on Mars and released a robot rover named Sojourner. The little solar-powered rover had a spec­trometer to analyze the chemical compo­sition of the Martian soil and a camera to send back pictures of the surface.

An aircraft’s speed is measured in a vari­ety of different ways. Its speed across the ground is called its groundspeed. An airplane’s speed compared to the air through which it moves is called its true airspeed. If the air is moving (if there is a wind blowing), the groundspeed and true air­speed are not the same. The speed that appears on the airspeed instrument in an aircraft’s cockpit is the indicated airspeed. Pilots used to do complicated cal­culations to convert the indicated airspeed to the groundspeed, which is the speed the pilot needs to know for accu­rate navigation. Today, flight computers and electronic navigation systems take care of this task.

upersonic flight is flight faster than the speed of sound. The speed of sound in air is about 761 miles per hour (1,225 kilometers per hour) at sea level. Most planes fly more slowly than this, but the fastest can fly at two or three times the speed of sound. Many missiles fly at two to five times the speed of sound.

Designing Supersonic Aircraft

As an aircraft approaches the speed of sound, the drag it experiences increases sharply. Aircraft designers deal with

О An F/A-18F Super Hornet breaks the sound barrier during a 2006 U. S. Air Force demonstration.

this problem by making supersonic aircraft slender, with swept-back wings and very powerful engines to overcome the extra drag.

The most powerful engines are noisi­er, however, and they burn fuel faster. These factors make it difficult to design a supersonic airliner, also called a super­sonic transport (SST). A slender aircraft cannot hold many passengers, and noisy engines are unpopular with people who live near airports. In addition, faster burning of fuel means that an airplane cannot fly as far. For these reasons, all the supersonic aircraft flying today are military aircraft. There have been only two supersonic airliners in the past: the British/French Concorde and the Soviet Tupolev Tu-144.

The Altitude Factor

One way to reduce the drag that a supersonic plane experiences is to fly much higher than other aircraft. The thinner air at higher altitudes causes less drag. Sub­sonic airliners fly at altitudes of around 30,000 to 40,000 feet (9,100 to 12,1200 meters), while the supersonic passenger airplane Concorde flew at altitudes of

50,0 to 60,000 feet (15,250 to 18,300 meters).

The atmosphere protects us from harmful radiation from space. The higher an aircraft flies, the more of this atmospheric pro­tection it loses. Concorde received double the radiation dose of sub-

sonic airliners, but it also flew more than twice as fast, so the exposure to radiation was about the same. However, storms on the Sun (known as solar flares) can produce a sudden increase in radi­ation in the upper atmos­phere. One instrument on Concorde was a radiation meter. If the radiation levels were too high, an alarm sounded, and the airplane was required to descend to below 47,000 feet (14,330 meters), where it had more pro­tection from the atmos­phere above it.

Launching a large rocket or Space Shuttle is a complicated process. It may take several weeks or months to prepare the vehicle for liftoff. The final countdown begins one to two days before the actual launch. Then, many events have to take place at the right time and in the correct order. Holds, or pauses, in the schedule are included in the countdown so that minor problems can be handled with­out delaying the launch itself. The last few minutes of a count­down usually are controlled by computer. Finally, the engines fire and produce powerful jets of gas that thrust the vehicle upward into the air.

Some rockets are programmed to make splash landings in the ocean so that they can be ret­rieved. Others are designed to burn up in the atmosphere after they have com­pleted their task of launching a space­craft of some kind.

All of the space­craft that carry astro­nauts and some unmanned space vehicles are carefully designed to make safe, controlled land­ings. The Space Shuttle is the only manned spacecraft that lands on a run­way. Other spacecraft have to slow their descent by other methods.

If a spacecraft descends through an atmosphere, whether on Mars or Earth, it can use the air resistance in the atmos­phere to reduce its speed. As the space­craft descends, it experiences drag, which slows it down. Then it can use parachutes to slow down even more.

When landing on solid ground, a spacecraft may fire rockets downward as a final brake just before touchdown. The manned Russian Soyuz capsules and Chinese Shenzhou capsules land on Earth in this way. In the 1960s and 1970s, the Mercury, Gemini, and Apollo missions also used space capsules to bring their astronauts back to Earth. The capsules had only parachutes to slow them down, and they landed in water.

Where there is no atmosphere, on the Moon for example, rockets are used to control the craft’s speed throughout the entire descent. The Apollo mission’s lunar landers used this method to land on the Moon. Another landing method was devised for the Mars mission. Just before the Spirit and Opportunity rovers landed on Mars, airbags inflated around them. They hit the surface, bounced and when they came to a halt, the bags deflated, the landers opened, and the rovers drove away.

In just one day of experiments, the wind tunnel showed the Wright brothers why their gliders were not performing well. The data they had been using to design their wings was not correct. When the Wrights corrected the errors they had discovered, the performance of their new wings and gliders improved.

The first wind tunnel was such a suc­cess that the Wrights built a bigger one, 6 feet (about 2 meters) long. A fan blew air through it at about 30 miles per hour (48 kilometers per hour). One problem they had was ensuring that the air
flowed smoothly through the tunnel. Wilbur said, “Our greatest trouble was obtaining a perfectly straight current of air.” It took nearly a month to solve this problem. They did it by blowing air into the tunnel through a honeycomb. Modern wind tunnels have a similar part, called the settling chamber.

Seeing the Air

The supports that hold a model in today’s wind tunnel are fitted with instruments that measure the forces experienced by the model. Researchers who use wind tunnels have to find addi­tional ways to measure the airflow so that they can see how it is behaving. One of the oldest methods is to stick short tufts of wool-like material all over

the test object. The way the tufts lay flat, or stick up, or flutter shows the airflow. Another testing method is to release fine streams of smoke into a wind tunnel. The smoke follows the airflow and shows whether it is smooth or turbulent. In a third method, the model being stud­ied in the wind tunnel is painted with a pressure-sensitive liquid that changes when air blows against it. Another type of tunnel has small holes, called pressure taps, drilled at important points. The air pressure in the holes is measured.

Most of these methods for studying airflow disturb or change it in some way. Lasers are now used to study airflow in wind tunnels without disturbing it. A laser produces an intense beam of light of only one wavelength. When light

О The world’s largest wind tunnel at NASA’s Ames Research Center was used to test a parafoil designed to deliver a new type of manned spacecraft back to Earth.

bounces off something, its wavelength changes if the object is moving. When a laser beam is fired into a wind tun­nel, it is reflected by specks of dust in the air and is changed by their motion. A fine mist of oil or air is sometimes added to the air to make its move­ments easier to detect.

World War II (1939-1945) was the first major war in which aircraft played a decisive part. Hardly any of the battles, on land or sea, were fought without airplanes. During the war, the speed and offensive power of warplanes increased dramatically.

Warplanes of World War II included fighters, fighter-bombers, bombers, trainers, transports, seaplanes, and a variety of specialized and general air­craft, including the first helicopters. Aircraft supported armies on the ground, dropped bombs on cities, and patrolled the oceans to attack ships and sub­marines. Planes dropped paratroopers, carried soldiers and supplies into combat zones, evacuated the wounded, and made reconnaissance flights.

Air Forces

When war began in Europe in September 1939, the strongest air forces in Europe were Germany’s Luftwaffe and Britain’s Royal Air Force (RAF). The United States as yet had no separate air force-its

warplanes were flown by the U. S. Army, U. S. Navy, and U. S. Marine Corps.

New ideas about the use of air power had been put forward in the 1920s by Italy’s Giulio Douhet (1869-1930), who predicted that the bomber would be the key weapon in a future war. Similar ideas were advanced by Billy Mitchell (1879-1936), an advocate of an inde­pendent air force for the United States. British air force commanders, such as Hugh Trenchard (1873-1956), supported these ideas.

The Germans tested some new ideas by sending pilots and planes to fight in the Spanish Civil War (1936-1939). The German Luftwaffe was equipped with around 5,000 planes, mostly medium bombers, dive bombers, fighters, and troop transports. The Luftwaffe was intended to support fast-moving armor and infantry on the ground. This blitzkrieg (lightning war) strategy worked well during the German invasions of Poland (1939) and of Belgium, Holland, and France (1940).

The Germans enjoyed air superiority during these invasions. Poland’s air force was out-of-date, and there were only a few modern French fighters, such as the Dewoitine D.520, to match those of the Germans. Britain sent planes to aid France but found that its airplanes
(such as the Battle light bomber), were ineffective and were soon shot down. With the fall of France to the Germans, British fighter commander Hugh Dowding gathered his squadrons for the next battle.

Much of what scientists now know about the four “gas giant” planets-

Jupiter, Saturn, Uranus, and Neptune – came from NASA’s two Voyager probes, launched in 1977. Voyager 1 flew past Jupiter and Saturn before leaving the

DISTANT VOYAGERS

Voyager 1 and Voyager 2 each carry a gold disk showing the location of Earth within the Milky Way galaxy. The golden record also contain sounds and images chosen to portray the diversity of life on Earth. It is meant to communicate with any intelligent life-form that might col­lect one of the Voyagers. The Voyager spacecraft will take about 40,000 years to approach another star, however, and the probes are minute compared to the vastness of interstellar space. The chances of any alien life-form finding one of the probes is therefore remote.

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solar system. Voyager 2 journeyed on to visit Uranus in 1986 and Neptune in 1989. Jupiter has some of the wildest weather in the solar system, with winds up to 300 miles per hour (480 kilometers per hour). Jupiter also spins faster than any other planet. As a result, its day is less than 10 hours long.

In 1995 the Galileo probe orbited Jupiter and sent a small, cone-shaped lander plunging down into the atmos­phere through clouds of ammonia ice crystals. The probe survived for an hour, sampling the hostile atmosphere, until it was destroyed.

The Lockheed SR-71 Blackbird holds the official world airspeed record. On July 28, 1976, it flew 2,188 miles per hour (3,530 kilometers per hour) near Beale Air Force Base in California, with Eldon W. Joersz at the controls.

Other aircraft have gone faster than the Blackbird, but they do not qualify for the official world airspeed record because they cannot take off and land under their own power. In 1967, the X-15 rocket plane reached a top speed of 4,520 miles per hour (about 7,270 kilo­meters per hour), or Mach 6.7. It is the fastest manned aircraft that has ever flown, but it is launched in midair from beneath the wing of a B-52 bomber.

A spacecraft has to be boosted to a speed of about 17,500 miles per hour (28,000 kilometers per hour) to go into low Earth orbit. The highest speed ever attained by a manned space­craft is 24,791 miles per hour (39,900 kilometers per hour). The Apollo 10 spacecraft reached this speed during its return from the Moon in 1969. The fastest space probe, and also the fastest human – made object of any kind, was the Helios 2 solar space probe. It reached a speed of 157,000 miles per hour (252,700 kilometers per hour) in the 1970s.

SEE ALSO:

• Supersonic Flight • Velocity

The speed of sound in air depends on the temperature of the air. Sound travels faster through warm air and more slow­ly through cold air. The air high above the ground is very cold, so the speed of sound is lower there than at sea level.

As an aircraft climbs higher above the ground, the air gets colder. At a height of about 35,000 feet (about 10,700 meters), where airliners cruise, the air is as cold as -76°F (-60°C). At this temperature, sound travels through air at about 660 miles per hour (1,060 kilo­meters per hour). An airplane flying at, for example, 715 miles per hour (1,150 kilometers per hour) in warm air near

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OF SOUND IN AIR

Speed of Sound

660 mph (1,062 kph)

685 mph (1,102 kph) 714 mph (1,149 kph) 728 mph (1,171 kph) 741 mph (1,192 kph) 755 mph (1,215 kph) 762 mph (1,226 kph)

768 mph (1,236 kph) 775 mph (1,247 kph)

the ground is subsonic (below the speed of sound). The same plane flying at this speed at its cruising altitude would be supersonic. To avoid confusion, scien­tists invented Mach numbers.

An aircraft flying at the speed of sound flies at Mach 1, whatever its actu­al airspeed is. Mach 2 is twice the speed of sound, Mach 3 is three times the speed of sound, and so on. An aircraft’s Mach number is calculated by dividing its speed by the speed of sound in the air through which it is flying. An airliner such as the Boeing 777 flies at about Mach 0.84. Fighter planes such as the F-16 fly at up to Mach 2. A rare handful of manned self-launching aircraft, such as the Russian MiG-25R, can fly faster than Mach 3.

MACH 1 AT DIFFERENT ALTITUDES