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Jumbo Gel The introduction of jumbo jets in the early 1 970s helped relieve air traffic congestion at airports. 30 People react much the same when a jumbo jet lumbers up to the boarding gate: eyes widen, jaws slowly drop, and speech fails until the looming giant rolls to a stop just outside the window. To most passengers, a jumbo jet is huge outside, spacious inside, and a lot more comfortable to fly than lesser planes. To the airline industry, the jumbos, with their huge capacities of 300 to 450 passengers or more, are economy of scale. It is more economical, for example, for an airline to fly 400 passen- gers in a single jumbo jet than 100 each in four smaller planes. Jumbos also carry passengers much farther without costly refueling stops. Earlier jets with a range of about 4,500 miles, for example, had to refuel in Hawaii during a flight to Tokyo from San Francisco. Some of today's jumbos, flying 6,500 miles or more nonstop, make the same trip uninterrupted from San Francisco, Chicago, or even New York. Jumbo jets also help relieve air traffic congestion at airports, which can handle only a limited number of takeoffs and landings per hour whether the plane holds 4 or 400 passengers. In the mid-1960s, when the first jumbo jets were designed, air travel was growing at a rate that would double the number of passengers every five years. But the introduction of jumbos in the early 1970s reduced the immediate need to expand many airports or build new ones. In fact, no major U.S. airport has been built since 1974. The 747, DC-10, and L-1011 jumbo jets joined the commercial air fleet in 1970,1971, and 1972, respectively, forming a necessary link in a chain of events that introduced long- distance air travel to the masses. Since then the combination of rising fuel prices, lower air fares, and economical long-range jumbos has helped triple the annual revenue passen ger miles (one RPM equals one paying I passenger carried one mile) logged by the U.S. airline industry. In 1988,420 billion RPMs were logged, and the figure is project ed to surpass 760 billion by the year 2000. I The origin of the jumbo jet lies in the competition for the giant Air Force C-5A cargo plane, which began operational flights in 1969. The challenge of building the jumbos was to fit those design advances plus other state-of-the-art technologies into a huge ~ machine that was not only safer than earlier ~' airplanes but less expensive to operate. The 747, for instance, reduced the per-mile cost of carrying a passenger 20 to 30 percent. The jumbos also introduced an era of safer aircraft designed with a strong empha- sis on several redundant, or backup, systems. The failure of a single system would not cripple the airplane. For instance, designers of the 747 put four main landing-gear legs on the plane instead of the usual two and added a middle spar to the wings. If the front spar were damaged in a collision, the middle and rear ones could hold the wing together for E N G I N E E R I N G A N D T H E A D VA N C E M E N T O F H U M Q N W E ~ FA R E
l . landing. Designers of the L-1011 built in four separate hydraulic systems to operate the critical pitch control system elevators and stabilizers which moves the nose up or down. Three are powered by the plane's three engines and the fourth by an auxiliary power unit. The DC-10 was designed with five generators each of which can keep the plane flying-plus an emergency backup battery system. Three of the generators are powered by the aircraft's three engines, one by an auxiliary power unit, and the other by an extendable windmill. The jumbo jets owe much of their success to the high-bypass engine, which was introduced to military aviation on the C- 5A and to the commercial sector on the 747. They help the big planes fly farther, consume less fuel per passenger mile, and climb into the sky with a murmur compared with the scream of earlier jet aircraft. Advanced metal alloys, new cooling systems, and the high- bypass design help the engines deliver almost twice the thrust per pound of engine weight, while using 20 to 25 percent less fuel per pound of thrust than conventional turbojet engines. The core of a high-bypass engine operates like a pure turbojet, in which whirling compressor blades pull air into the engine's combustion chamber, where the air and fuel are burned. This creates a hot, rapidly expanding gas that thrusts the engine ~ U M B O ~ E T The sine of jumbo jets sets them apart from earlier airliners, and so does their increased emphasis on safety. The four main landing-gear legs on the 747 instead of the usual two are an example of the multiple backup systems that add a wide margin of safety to jumbo jets. 31
l ~ ~ - The high-bypass engine gets its name from the huge fan that thrusts air past the combustion chamber, the section where the technician is working. In the conical back end of the engine, expanding gas from the combustion chamber spins turbines on a shaft that turns the fan. Right: Nickel-alloy turbine blades cast as single crystals have greater strength and durability and higher temperature capability than blades made of earlier materials. The wavelike patterns, enhanced for this photo by chemical processing, are caused by minute chemical differences between parts of the crystal that solidify earlier in the casting process and those that solidify later. Large fans on the high-bypass engines of these DC-1 Os push air out the back of the silver engine cowlings while combustion chambers shoot expanding gas through the gray cones. 32 . ~ forward. The gas also spins a turbine, whose shaft runs back through the hollow engine core to turn the compressor blades. The high-bypass engine, however, takes advantage of the fact that for a given power level, a large volume of slowly moving air will generate more thrust than a small volume of fast moving air. In the high-bypass engine, the turbine also turns a wide fan at the front, which pushes a large mass of air past the combustion chamber. The ratio of air bypassing the combustion chamber to air flowing through it was 5:1 on the first commercial high-bypass engines and 8:1 on military versions. The fat cowling covering the large fan clearly distinguishes the high- bypass engine from the slender, less efficient turbojet. The new engines are much quieter, too. All the thrust of a turbojet comes from its exhaust, which shoots out at twice the speed of sound with an ear-splitting roar. But a high-bypass engine mixes quieter low- velocity air with the high-velocity exhaust to slow its speed and reduce its noise. The noise level of the first DC-lO's, for example, was about half that of earlier four-engine airliners. High-bypass engines are also more efficient than earlier turbojets in part because they operate at higher combustion tempera- lures. Turbines in the new engines tolerate burner-exit gas temperatures up to 2,800 degrees F. about 500 degrees greater than the earlier engines. Much of this higher tolerance results from new air cooling systems that allow turbine blades to operate in gas streams at temperatures higher than their material melting point. The blades are cooled by compressed air that is channeled around the combustion chamber and directed onto the whirling disk holding the blades. A pressure differential causes the air to flow into the base of each hollow blade, where a network of passages carries the air through- out the blade to cool it from inside. As air escapes through strategically spaced holes, it flows over the external surface in a protective film that insulates the blade from hot gas. Gas temperatures are 200 or more degrees hotter than the melting point of the blades. Some of the increased engine perfor- mance also came from an advanced process, called directional solidification, for casting the nickel-alloy turbine blades. Whirling blades under high centrifugal force tend to creep, or elongate, at turbine operating E N G ~ N E E R ~ N G Q N D T H ~ b D va N C E M E temperatures. Creep causes cracking along the boundaries between alloy crystals, which form when the blade is cast. In earlier blades, the crystals solidified in random alignments. Directional solidification, however, forces the alloy to solidify in long crystals that grow from one end of the blade to the other. This eliminates crystal boundaries across the blade and so reduces creep and cracking. Directional solidification begins by pouring molten alloy into a mold that sits inside a hot furnace. At the bottom of the mold is a water-cooled chill plate, where the alloy starts cooling and crystals begin forming. The mold is slowly lowered out of the furnace, causing the first crystals to continue growing up in parallel columns toward the top of the mold. Turbine blades made this way could operate at temperatures up to 100 degrees F hotter than conventional blades. The heat tolerance of turbine blades has since been raised another 100 degrees by making each blade out of just one crystal. With a single crystal and no boundaries, engineers could remove from the alloy several boundary-strengthening ingredients that, essentially, had prevented the blades from tolerating higher heat. The trick was to select a single crystal that would grow both vertically and horizontally to fill the entire mold. The problem was solved by putting a corkscrew bottleneck between the chill plate and the mold. As with directional solidifica- tion, several crystals start to grow from the chill plate. But the bottleneck is small enough that only the crystal with the best horizontal _ N ~ O F H U M A N W E ~ FA R E
l - - ~ 'a - - - - - ~ 3~$~ _~ ~:_ ~a I'm_ ~ i\ ~ .~\~ _..." and vertical growth properties can worst, through the corkscrew into the mold. The first single-crystal turbine blades went into engines for the midsize 767 jetliner in 1982 and subsequently into new jumbojet engines. Many of the engineering advances developed for jumbo jets have been incorpo- rated in smaller aircraft. High-bypass engines are now found on nearly all new commercial airliners of every size. The jumbos, in turn, are benefiting from new technologies developed for commercial aircraft in general: composite structural materials, automated landing and flight systems, and "all glass" cockpits in which instrument readings are displayed on color television monitors. Over the next 10 to 15 years, the current family of jumbos will continue to expand in size, range, and capability. Future jumbos may weigh 1 million pounds, compared with 870,000 pounds today. They will carry over 600 passengers more than 8,000 miles nonstop. Currently, only short-range jumbos carry that many passengers and long-range jumbos carry many fewer. In addition, the three jumbo families will be joined by a fourth, the A330/340, in the early 1990s. Supersonic transports, such as the Concorde, generate an unacceptable sonic boom that limits their use over populated areas. The next really advanced commercial aircraft therefore may be one that travels at hypersonic speeds at and above Mach 6 (six times the speed of sound). A proposed hypersonic plane, the National Aero-Space Plane, is expected to travel at speeds and altitudes where its sonic boom would create less of a problem. It will probably burn liquid hydrogen in a propulsion system that uses jet engines while in the atmosphere and rockets while in space. It remains to be seen, though, whether hypersonic planes will become economical for long-haul commercial transport. In the meantime, the jumbos and their descendants will undoubtedly remain the workhorses of the airlines into the twenty-first century. 1 r JUMBO JET L' 1 01 1 s, here under construction, have four separate hydraulic systems for added safety. They operate the elevators and stabilizers on the wings, which control the pitch of the jumbo's nose. 33