Category FLIGHT and M ОТІOIM

Stealth airplanes rely almost entirely on being nearly invisible to radar. This fact brings with it certain limitations that can put stealth aircraft at a disadvan­tage. First, neither the F-117 nor the B-2 carries defensive armament, so they cannot defend themselves. Second, most high-performance jet airplanes rely on engine afterburners to boost their speed in combat. Afterburning produces high­ly visible emissions of exhaust gases, however, which make the airplane more detectable. Stealth aircraft, therefore, lack afterburners and so do not have supersonic performance. This makes them vulnerable to faster fighters once they are detected. Third, the design of stealth aircraft may protect them from enemy missiles that use radar, but it offers no protection from other weapons. Fourth, the unorthodox shape and slow speed of stealth planes make them inferior to conventional fighters when engaged in aerial dogfights.

The stealth aircraft’s sophistication also can be a disadvantage. The elec­tronic fly-by-wire system required to keep a stealth plane like the F-117 flying adds to both the cost and weight of the aircraft. Advanced computers on the airplane are also a potential risk since electromagnetic equipment gives off radiation that can be detected by sensors on the ground, revealing the presence of the plane. All stealth airplanes need meticulous maintenance, since the air­craft skin must be kept flawless to pre­serve its radar anonymity. Even a scratch from a pebble thrown up during a landing, or weather damage to the paint, may increase the radar signature.

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SEE ALSO:

• Aerodynamics • Aircraft, Experimental • Aircraft Design

• Control System • Radar

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A pilot lands an airplane by first reduc­ing power so that the plane begins to slow down and lose altitude. Closer to the runway, the engines are throttled back to reduce the airspeed even more. To make up for the loss of lift, the plane’s nose is raised to increase the angle of attack of the wings. The flaps and slats used for takeoff are deployed again, and the landing gear is lowered.

Lights and radio beacons on the ground show the way to the runway. The plane also receives two radio signals from the runway: one is the localizer and the other is the glide slope. The localizer is transmitted along the run­way. It tells the crew whether the plane is flying to the left or right of the run­way or is flying down the centerline. The glide slope shows whether the plane is descending at the correct rate. Just after touchdown, spoilers spring up from the wings to reduce lift and keep the plane on the ground. Jet planes often use reverse thrust to help them slow down.

Helicopters

Helicopters do not need runways to land and take off. Like rockets and airships, they are capable of vertical takeoff and landing. This gives helicopters the abili­ty to land in and take off from small spaces or remote locations where there are no airports.

Helicopters and VTOL aircraft (verti­cal takeoff and landing aircraft other than helicopters) take off vertically by directing their engine power downward to thrust them upward. The helicopter pilot uses the collective pitch lever and the engine throttle to get the aircraft off the ground. The collective pitch control tilts the rotor blades, increasing their angle of attack and producing more lift. The throttle increases the engine speed, which makes the rotor spin faster. To land, these controls are used to reduce lift.

Airships

An airship is balanced so that it is neu­trally buoyant on the ground. Its weight is balanced by the lift produced by the lighter-than-air gas inside it. When it is ready for takeoff, an airship’s propellers and the elevators in its tail swivel up to drive the airship upward. Modern air­ships are filled with helium, and they

also have air-filled bags called ballonets inside them. As an airship climbs, the outside air pressure falls, and the gas inside the airship expands. Valves pop open and let air escape from the ballonets so the helium can expand safely.

To land, the propellers and elevators tilt down to drive the airship down. The outside air pressure rises and compresses the gas inside the airship. The airship’s engines force extra air into the ballonets to keep the airship fully inflated as it descends.

The first wind tunnel was designed by the British engineer Francis Herbert Wenham and it was built by John Browning at Greenwich, England, in 1871. The wind tunnel was 12 feet (about 4 meters) long and 18 inches (45 centimeters) high and wide. Air could be blown through it at up to 40 miles per hour (64 kilometers per hour).

The swirling, unsteady airflow inside the first wind tunnels made it very diffi­cult to obtain reliable measurements. The first wind tunnel to provide useful aerodynamic data for designing an actu­al aircraft was built by Orville and Wilbur Wright in 1901.

The brothers were disappointed with the performance of the early gliders they had built. They needed a way of testing different shapes and angles of wings instead of relying on data provided by other people. The Wrights tried testing small model wings fixed to a bicycle, but they really wanted to test each wing in a controlled environment with exactly the same airflow. So Orville Wright built a

О While many wind tunnels use models to test new designs, this wind tunnel in the 1950s could hold full-scale aircraft.

TECH^TALK

PARTS OF A WIND TUNNEL

A wind tunnel has five main parts:

• The drive section pushes air through the tunnel.

• The settling chamber straightens the airflow.

• The contraction zone speeds up the air.

• The test section contains the object to be studied.

• The diffuser slows the air down.

V.

wind tunnel from an old box. It was only 18 inches (46 centimeters) long. A fan driven by their workshop engine blew air through the box, and a glass window on top showed what was hap­pening inside.

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.