18
GA
/ Vol. 5 / No. 2 / FEBRUARY 2013
by Cesaré de Villiers
Business news
General
Electric GE90
world's most
powerful turbofan
The General Electric GE90-115B en-
gine is an advanced ultra high bypass
turbofan engine that was developed
especially for the twin engine Boeing
777 airliner and is used exclusively in
the 777-300ER, 200LR and 200F.
It is regarded as the ultimate
turbofan engine and holds the
Guiness World Record for the most
powerful engine in aviation for thrust
production a massive 115,000 lbs.
of thrust, though on the test stand it
has been run up to 127,900 lbs. of
thrust to demonstrate the incredible
performance/reliability margin and the
potential for future growth. The GE90
also holds the record for the highest
pressure ratio of any aviation engine
in the world at 45:1. That means that
the engine's compressors compress
atmospheric air to approximately 45
bar, or to 661 psi before feeding that
high pressure air into the burner.
Efficient and reliable
Along with tremendous power, the
GE90 is the most efficient engine in
the sky, with specific fuel consump-
tion rivaling that of a typical large
Diesel engine. High efficiency occurs
due to the high compression ratio,
high turbine inlet temperature, and
high bypass ratio, which converts
most of the high energy combustion
gas into shaft horsepower to drive the
massive ducted fan at low rpm to pro-
vide the major portion of the engine's
total thrust.
Air entering the engine nacelle
is drawn up by the gigantic, 128 inch
diameter all composite fan. Of interest
is the fact that the diameter of the
fan and nacelle are essentially the
same diameter as the whole fuselage
on a Boeing 737. The single stage
fan features all carbon composite
construction with specially contoured
blades for high efficiency. The fan
turns at a maximum speed of only
2,550 rpm, but flows enough air in
doing so to make tremendous thrust.
It is an advanced, larger diameter
unit made from composite materials
and is the first production engine to
feature swept rotor blades. Air from
the fan can follow one of two paths.
A vast majority of it enters the bypass
duct, and travels around the core of
the engine where it is exhausted out of
the rear to provide thrust. The engine
features a 9:1 bypass ratio, so 9 times
more air travels through the bypass
duct than actually enters the core of the
engine.
The air that does enter the core
is compressed further by the low
pressure compressor or booster, a
four stage axial compressor made of
titanium blisks, and which turns on
the same shaft as the fan. After the
air is compressed by the low pressure
compressor, it is then fed to the 9
stage axial high pressure compressor,
which turns at a much faster 10,850
rpm and brings the airflow up to the
design pressure ratio of approximately
42:1 and higher. The high pressure
compressor is a 9 stage axial design
with a high pressure rise per stage
concept. The blisks are made of
titanium and nickel based alloyed
stainless steels in the latter stages
due to the ultra-high temperatures
of compression. Variable geometry
compressor stators as well as
compressor air bleed at various stages
ensure surge-free operation throughout
the flight envelope.
Compressed air is fed to the
through flow double annular burner,
with duplex fuel nozzles and high
temperature exotic alloy construction.
The double annular combustor (DAC)
is a new concept developed by GE with
the intention of improving fuel burn
and reducing harmful emissions. The
double annular combustor features an
additional inner combustion ring, into
which the fuel nozzles extend. At lower
power settings, the outer ring is being
fed fuel only, but as power increases,
the inner ring is introduced and the
fuel is burned in stages for more
complete combustion. Hot combustion
gases are cooled to a temperature
upwards of 2,700 degrees Fahrenheit
by dilution air in the combustor, before
being fed through the high pressure
nozzle to the cooled, two stage high
pressure axial turbine.
High tech axial turbine
The axial turbine is made through pro-
prietary construction techniques, but
it can be safely assumed that General
Electric has utilized all of the latest in
high tech, high temperature construc-
tion, with transpiration cooled blades
made of single crystal formed high
temperature surface treated alloys.
Unfortunately, there is little infor-
mation available on the composition of
the hot section components, due to the
ultra-competitive nature of the busi-
ness. The high pressure turbine drives
the high pressure compressor and the
accessory gearbox, which drives all of
the ancillary systems that keep the en-
gines running and the aircraft in the air.
Accessories include low and high pres-
sure fuel pumps, multiple oil pressure
and scavenge pumps, an AC generator
for electricity, hydraulic pumps to
actuate the aircraft's control surfaces,
and a pneumatic turbine starter. After
the two stage high pressure turbine, the
combustion gas is expanded through
the low pressure turbine, which is a six
stage axial design, featuring special
high temperature titanium alloys and
transpiration cooling.
Both the high and low pressure tur-
bines feature Active Clearance Control,
an electronically controlled system
which uses compressor bleed air to
heat and cool the turbine blade roots,
allowing them to expand and contract
slightly to control turbine blade tip
clearance over the whole power range,
allowing for better sealing at high and
low power settings.
After the combustion gases leave
the low pressure turbine, there is
still ample energy left to provide a
very healthy kick of thrust out of the
core exhaust tailpipe, although a vast
majority of engine thrust is developed
by the huge, slow moving fan at the
front of the engine. The fan air that
enters the bypass duct around the
core engine is exhausted through the
bypass duct nozzle, concentric with
the core exhaust. The bypass exhaust
wraps around the core exhaust and
actually helps to quiet the exhaust roar
significantly. In a turbofan engine such
as this, most of the engine noise is
actually produced by the fan, and the
contoured, swept back fan blades even
make the fan less noisy.
Largest engine in
aviation history
The GE90 series are physically the
largest engines in aviation history
with the largest variant, the GE90-
115B, having a fan diameter of 312cm.
As a result, GE90 engines can only
be airfreighted in assembled form
by outsize cargo aircraft such as the
Antonov An-124 Condor, presenting
unique problems if, due to emergency
diversions, a 777 were stranded in a
place without the proper spare parts.
If the fan is removed from the core,
then the engines may be shipped on
a 747 Freighter. Apart from its size,
the GE90-115B is powerful enough to
fully operate GE's Boeing 747 testbed
on its own power during a maximum
thrust low speed flight testing on
board the aircraft
Records
According to the Guinness Book of
Records, at 127,900 lbf it holds the
record for the highest thrust (though
it is rated at 115,300 lbf). This thrust
record was accomplished inadvert-
ently as part of a one-hour, triple-
red-line engine stress test. In order to
accommodate the increase in torsional
stresses an entirely new steel alloy
(GE1014) had to be created and then
machined to extreme tolerances.
The new record was set during
testing of a GE90-115B development
engine at GE Aviations' Peebles
Test Operation, which is an outdoor
test complex outside Peebles, Ohio.
It eclipsed the engine's previous
Guinness world record of 122,965
pounds of thrust. In October 2003, a
Boeing 777-300ER broke the ETOPS
record by being able to fly five and a
half hours with one engine shut down.
The aircraft, with GE90-115B engines,
flew from Seattle to Taiwan as part of
the ETOPS certification programme.
On November 10, 2005, the GE90
entered the Guinness World Records
for a second time. The GE90-110B1
powered a 777-200LR during the
world's longest flight by a commercial
airliner, though there were no fare-
paying passengers on the flight, only
journalists and invited guests.
The 777-200LR flew 21,601km
in 22 hours, 42 minutes, flying
from Hong Kong to London "the
long way": over the Pacific, over
the continental U.S., then over the
Atlantic to London.
The longest flight by a commercial
airliner with passengers is 18.5
hours, flown by an Airbus A340-500
aircraft on a daily non-stop flight
from Newark to Singapore on
Singapore Airlines. ·