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OCR for page 30
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
OCR for page 31
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
OCR for page 32
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
OCR for page 33
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- -
- -
~ 3~$~ _~
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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
Representative terms from entire chapter:
combustion chamber
