genx development emphasizes composites, combustor technology

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By Michael Mecham

COMPOSITE POWER

After waiting three decades for enabling technology to catch up with its design ideas, GE-Aviation is

deeply embedding itself in composite materials as a next-generation answer to making engines

quieter and cheaper to own and operate.

The payoff is coming in the GEnx engine, which made its first run on Mar. 19 when Serial No. 001

was started on test stand 6A at the company's Test Operation Center in Peebles, Ohio. By the time the trials concluded two

days later, the team had pushed the engine to 80,500 lb. of sea-level, standard-day takeoff thrust. Its top power requirements

thus far are for 75,000 lb.

The first fruition of GE's composite technology appeared in fan blades for the GE90 series for Boeing's 777 family, an engine

development investment that brought years of market struggle for GE but is now paying off. It is in the GEnx (pronounced

"gee-ee-en-ex"), a next-generation derivative of the GE90, that General Electric is pushing composite technology the most.

Where it can, GE is displacing aluminum alloys with composites for the same reason Boeing has used them in the 787's wings

and fuselage. Composites are lighter, so they cut fuel burn; they are noncorrosive, so they reduce maintenance expenses.

Besides in its 111-in. fan blades and the engine's forward fan case, GE employs composites for the engine's variable-bleed

valve ducts at the exit of the booster stage.

GEnx's second major design advance addresses the airline industry's drive to reduce oxides of nitrogen (NOx), the principal

constituent of smog. The engine introduces Twin-Annular Premixing-Swirler (TAPS) technology into production so

combustor temperatures can be lowered to reduce NOx emissions.

Of the 1,600 instrumentation lines on SN001, only 300 were needed for last month's initial run; the others will be put into

play over the next year. One interesting note from the initial tests is the first use by GE of imagers inside the combustor to

evaluate the performance of TAPS. Customers will see the initial results in a symposium here Apr. 26-27.

GEnx began life as a competitor to the Rolls-Royce Trent 1000 on the 787. The Trent began its own test cycle in February.

(Aviation Week & Space Technology will report on current Rolls-Royce technology programs in the next few weeks.) Under

competitive pressure, Airbus standardized on the 787 engines for their efficiency gains; it has been revising its airframe for

the A350 as a competitor to the 787 ever since.

Subsequently, in a sole-source contract, Boeing has tapped GEnx to power the 450-seat 747-8 challenger to the A380.

GE expects to certify the 787-8's GEnx-1B64 in the third quarter 2007 for a mid-2008 service entry. The GEnx-2B67 for the

747-8 will enter service a year later and the GEnx-1A72 for the A350, in mid-2010.

GEnx is the successor to the CF6 series. The heart of General Electric's large-engine lineup for 30 years, the CF6's thrust

ratings rose to 72,000 lb. from 41,000 lb. as it fulfilled assignments that included the Airbus A300/A310 and A330 series, the

McDonnell Douglas DC-10 and MD-11, and Boeing's 767 and 747. The CF6 also has seen widespread military usage with

the Boeing KC-10 tanker and Lockheed C-5 transport.

For their A350 and 787 programs, Airbus and Boeing want engine makers to drive 20% out of the cost of ownership for

airplane customers. Using the CF6 as a baseline, that translates into a 15% savings in specific fuel consumption (SFC). The

remainder comes from longevity performance, says GEnx General Manager Thomas A. Brisken.

In fact, GEnx will achieve a 15.4% cut in SFC over the CF6-80C2B6F, the powerplant of the 767-300ER, Brisken says.

GEnx also should have a 6.9% SFC advantage over the GE90-94B, which powers the 777-200ER. That comparison is

interesting because Boeing says a planned second stretch of the 787--to be called the 787-10 and likely launched this

year--will have a seating overlap with the 777-200ER. In most observers' view, that makes the first-generation 777 models

obsolete to the newer technology 787s.

Boeing accepted powerplants from all three major manufacturers--General Electric, Pratt & Whitney and Rolls-

Royce--when it launched the 777 family in 1990. But GE's application was the only entirely new engine--a move it made to

meet expectations of record-setting power demands to satisfy payload/range requirements in future models. GE's analysis

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was correct: at 115,000 lb. thrust, the GE90-115B is the world's most powerful aircraft engine (see p. 53). Boeing has made it

the sole-source engine for the top end of the 777 family--the high-capacity -300ER, ultra-long-range -200LR and -200

Freighter--that now generate the 777's best sales. GE reports capturing 153 of the 156 engine orders taken last year for the

777.

Since the 787-10 will blur the line between the top end of the 787's seating chart and the lower-end of the 777's capacity, GE

is in an enviable position. With 777-200 and -200ER sales fading, the Pratt & Whitney PW4084/4090 and Rolls-Royce Trent

800 that power them are finding it increasingly difficult to find new customers.

Although it faces competition from the Trent, the GEnx has had a brisk startup, securing 578 orders from 17 customers,

making it "the fastest selling wide-body engine, ever," Brisken asserts. This is quite a comeback from the late 1980s, when

then-GE CEO Jack Welch was considering abandoning the GE90 because of its billion-dollar development budget. Then-GE

Aircraft Engines President/CEO James McNerney, Jr., who now holds that same post at Boeing, persuaded Welch that the

effort would pay off. The company's persistence fulfilled Welch's maxim that GE's products should rank first or second in

their markets.

"They should look pretty good," says Cowen & Co. aerospace analyst Kai von Rumohr. "On the '87 and the A350--Boeing

versus Airbus--GE looks pretty good any way you want to go."

Including its partnership with Snecma in CFM International--which holds sole powerplant rights on Boeing's 737 Next

Generation series and competes on Airbus's A320 family--General Electric's engine strategy leads the industry.

The heritage of composite fan blades dates to 1971 when GE built the TF39 for the U.S. Air Force's C-5 fleet. While the idea

of lightweight composite blades was appealing, manufacturing techniques weren't mature enough then, nor was there

sufficient computing power to design them, recalls GE composites expert David W. Call. By 1985, as improved epoxy resins

became commercially available, GE had a set of "OK" blades for its GE36 Unducted Fan project. But the GE36 died for lack

of airline interest.

Advanced military structures fueled GE's research advances, and by the 1990s its composite blade technology was ready for

commercial production with the launch of the GE90-76B on the 777. By then, 3D aerodynamic modeling was allowing the

company to make wide-chord fan blades with an airfoil geometry for a high-flow, swept configuration. Their inherent low

density make solid composite blades 10% lighter than a hollow titanium blade, says Call, who manages the GEnx's

cold-section integration: "Overall, composites are 66% lighter than titanium . . . and 100% stronger."

The GEnx blades follow the GE90-115B design methodology. They are made with 400 plies of prepregged tape with the plies

thinning out from the base to the tip. Since sharp-edged composite materials tend to fray, the blades use replaceable titanium

cladding for the leading, tip and trailing edges. This edging also spreads the energy of foreign object damage (FOD) into the

fan's composite material. After a decade of performance and 6.5 million engine flight hours, the GE90 has experienced only

three composite blades removed from service due to birds or FOD.

Most FOD is centrifuged into the fan duct by the fan blades. GE also has installed "variable-bleed valve doors" at the booster

to suck FOD out to the fan duct and prevent it from entering the low-pressure compressor.

The fan blades--made by CFAN, a GE/Snecma joint venture in San Marco, Tex.--were designed for long-term strength. "I

don't think we will ever, ever fail a blade because the composite fan blades are so reliable," says Brisken.

GEnx uses Teflon wear pads for the blade's dovetail plate. Teflon doesn't fret like metal, and the pads don't have to be relined

like metal ones, says Call.

When the GE90 was in testing, the FAA turned to its experience with helicopter rotor blades to set a certification standard,

Brisken recalls. The blades were given a 30,000-cycle life limit, which translates to about 30 years for typical 777 usage.

GE's goal is to achieve the unlimited cycle certification standard that titanium blades hold.

As fan sizes grew bigger to improve engine bypass ratios for quieter operation, so has the weight of engines. Ironically,

advances in cooling technology, improved hot-section materials and better aerodynamic loading are reducing engine core

sizes. The result is a growing shift between the weight of the engine core and the fan and its casing. The fan for a CF6-80C2-

series for A330s, 767s and 747s produces a 5.3 bypass ratio and accounts for 21% of the engine's total weight. The GEnx-1B

for the 787 has a 9.5 bypass ratio, and the fan accounts for 33% of the engine's weight.

Since it had already reduced weight with composite fan blades, GE wanted the fan casing to go on a composite diet. But its

early attempts to build a single-wind composite case weren't strong enough. It uses composite fiber braid in a herringbone

pattern from A&P Technology of Cincinnati that resembles shoelaces. The strength issue was solved by laying a third braid

in the center of the overlapping laces.

The case uses these 3/10-in.-thick triaxial braids in a ±60-deg. weave mixed with biaxial braids, which are easier to lay in

corners and bends. In an automated process, the braids are woven flat against a fan casing tool. A thickened middle is


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weaved in to provide a fan-blade containment layer, eliminating the need for the Kevlar blanket used on aluminum cases.

Resin is added to the outside of the case once the braids are laid.

THE COMPOSITE FAN case grew out of development work GE did on the Boeing Sonic Cruiser, a 787 predecessor. More

than 100 ballistic impact panel tests were conducted to validate the design. The tests demonstrated that a composite case was

more resistant to ballistic FOD damage than aluminum cases.

The GEnx's aft fan case, which shrouds the outlet guide vanes, remains an aluminum alloy because it isn't a major weight

item. But Call expects it to become composite eventually.

The forward fan case carries a one-piece interior acoustic liner from GKN Aerospace of Huntsville, Ala., that sandwiches

glass epoxy on a Nomex honeycomb core. It's a payoff of the Boeing/NASA/GE QTD2 noise abatement studies (AW&ST

Sept. 5, 2005, p. 44).

Long-term testing showed GE that composite fans run more efficiently--and are quieter--with fewer blades. Besides cutting

weight, lowering the blade count reduces "scrubbing"--the drag of air as it slides over a blade's surface, Brisken explains.

Experiments in reducing the number of blades brought the GEnx count down to 18 (compared with 22 on the GE90-115B).

GEnx is 30% quieter than a baseline CF6.

For the first time, GE has added a seventh stage to its low-pressure turbine, but it reduced the blade count of each stage. The

configuration gave a 1% boost in LPT efficiency that translates directly into a 1% improvement in fuel burn, Brisken says.

Lowering the blade count kept GEnx's weight constant.

The new engine uses a 10-stage high-pressure compressor (the GE90 has nine stages) that produces a 23:1 pressure ratio.

Testing showed that running the high-pressure turbine clockwise and the low-pressure turbine counterclockwise produces a

gentler transition in air coming off the HPT and improves efficiency. GE used the concept for its F120 engine for the 1990s

Joint Advanced Strike Technology effort. GEnx is the company's first commercial application.

Like the GE90, the GEnx's hot-core propulsion section can be removed while the fan module remains on the wing. "I tell

customers, just consider it part of your wing," Brisken says.

GENX'S OTHER big improvement over the GE90 is its TAPS combustor. While the industry has made substantial reductions

in carbon monoxide, unburned hydrocarbons and invisible smoke emissions, NOx has been a more difficult pollution target

because higher engine operating temperatures control other pollutants. NOx production increases exponentially as

temperature increases. The highest levels occur in a stoichiometric transition from fuel-rich to fuel-lean conditions in the

combustion chamber. The TAPS solution is to pre-mix air and fuel before combustion to assure even burning at lower

temperatures.

That solution grew out of work GE did with NASA in the mid-1990s on the Advanced Subsonic Transport program. The

initial approach was the Dual Annular Combustor (DAC) first applied on upgraded CFM56-5 engines to help A320s meet

European pollution standards. The GE90 employs DAC, too. Its radial combustor domes were successful in cutting NOx

levels, but the design has durability issues, and is expensive and heavy, says Rick Stickles, manager of GEnx's combustion

design.

The TAPS approach simplifying the fuel-air mixing process has demonstrated itself on a CFM56-7B through a 4,000-cycle

endurance test. Its relight capability was proved at pressures equivalent to 30,000 ft.

THE NAME "twin annular" refers to the fuel-air mixture, not the combustor. There's only one combustor, but it is fed by two

annular fuel/air swirlers made of cobalt castings by Parker Hannifin Gas Turbine Fuel System Div. in Mentor, Ohio. A central

swirler acts like a pilot light that is sufficient to power the engine for idle and taxi operations. An outside (main) swirler

encircles the first and kicks in with throttle advance for takeoff, climb and cruise. In descent, the main swirler is inactive.

TAPS brings to the GEnx a 30% improvement in NOx emissions over the baseline CF6. The lower combustion temperatures

also will improve the engine's maintenance demands.

GE-Aviation has used a composite front end and improved combustor technology to tackle the industry's drive for lower

engine operating costs, improved fuel burn and better emissions control.

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genx development emphasizes composites, combustor technology

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