Gearing Up for the GTF...geared turbofan (GTF) concept is already highly proven in the civil...

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There’s little question that Pratt &Whitney’s new PurePower PW1000G fam-ily of geared turbofan engines represents

a remarkable technological achievement andmany pundits are touting its potential as amajor contender in tomorrow’s narrowbody-propulsion market.

The PW1000G has already been chosen topower the Bombardier CSeries, Mitsubishi MRJand Irkut MC-21 narrowbodies, and — asrumours proliferate that Bombardier intends tostretch the CSeries into a 150-seat design —

analysts see the PW1000G as a strong candi-date to power any re-engined A320 or 737 thatAirbus or Boeing might respectively offer.

However, few observers have noted that thegeared turbofan (GTF) concept is already highlyproven in the civil aviation market — albeit inthe form of engines rather less powerful thanPratt & Whitney’s new offering. As do all GTFs,these engines employ planetary gearboxes —driven by the low-pressure spool — to de-cou-ple the low-pressure turbine (and low-pressurecompressor) from the fan. This allows both the

� Aircraft Technology - Issue 105 �

REGIONAL PERSPECTIVE

Pratt & Whitney’s new PurePower PW1000G geared turbofan has already been chosen for threenew narrowbody programmes. Speculation is growing that the GTF could feature on a re-enginedA320 or 737, and even on a future widebody. Chris Kjelgaard looks at its prospects.

Gearing Up for the GTF

AT105_RegPersp:AT105_Regional 8/4/10 17:39 Page 86

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fan and the low-pressure spool to revolve attheir optimal rates — fast for the low-pressurespool, quite slowly for the fan.

Geared turbofans already inservice

Both the LF502/LF507 and the TFE731engine families (now made by Honeywell, butoriginally developed by Avco Lycoming andGarrett respectively) are geared turbofans thathave clocked up many millions of hours of flighttime — and new versions of the TFE731 in par-ticular continue to sell well, according to BillStorey, founder and president of aerospaceresearch firm Teal Group. While the LF502 suf-fered serious reliability problems — which ledto the development of the LF507 — theseweren’t to do with its geared fan, says Storey,and the GTF concept has proved reliable inservice.

So why all the fuss about the PW1000G?The answer is that it’s a much bigger, morepowerful GTF than any previously designed.Storey says that where Pratt & Whitney (P&W)has made a breakthrough in bringing to marketa new GTF family is in managing “to get the

weight of the gearbox down to a scale where itmakes the concept viable” for large-aircraftapplications. P&W duly patented this new gear-box technology, “so competitors can’t jump onit,” he notes.

The PW1000G’s fan and coreIn pursuing the narrowbody propulsion mar-

ket, P&W chose to develop a GTF rather than amore conventional turbofan because it believedits GTF design could offer the best “valueproposition” in two ways, says Paul Adams,P&W’s senior vice president of engineering.

First, because the PW1000G’s fan rotates30 per cent more slowly than that of a conven-tional turbofan, P&W could make the engine’s18-blade fan diameter very large without run-ning into the blade-tip shockwave problemsthat destroy a conventional engine’s efficiencyabove about 80 per cent of maximum thrust.The large fan diameter allows a much higherbypass ratio than would be possible with a con-ventional turbofan, increasing the PW1000G’spropulsive efficiency. The slower-turning fanalso minimises the chances of blade damagefrom bird ingestion, since most ingestion dam-



We think we have the mostdurable blades, in combinationwith [operating] temperaturesmore conservative than thecompetition. The PW1000G willrun at lower temperatures thanwe think the competitors aregoing to run at.—Paul Adams, senior vicepresident of engineering, Pratt& Whitney

Final assembly of a PW1000G demonstrator engine.


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age is caused by the speed at which the fan isturning, not the speed at which the bird is fly-ing.

In developing the PW1000G, P&W alsodesigned in a new, very durable core — basedon a core it developed with MTU, one of P&W’spartners in A320 supplier International AeroEngines — optimised for the high-cycle narrow-body operating environment. Because the low-pressure spool in the GTF can run at its optimal

speed without effecting the propulsive effi-ciency of the fan, P&W was able to remove sev-eral low-pressure turbine (LPT) stages: ThePW1000G has three, rather than the six orseven normally needed. Similarly, P&W coulddo away with two low-pressure compressor(LPC) stages, using three in the PW1000Grather than the conventional five.

Dropping five or six LPC/LPT stages pro-duced an engine with less than 50 per cent ofthe low-pressure stages in a conventional tur-bofan, and 1,500 fewer blades, says Adams. Atthe same time, P&W introduced into thePW1000G a new eight-stage high pressurecompressor, each stage of which is a single-piece, integrally bladed rotor (or “blisk”).

The PW1000G’s core also has anadvanced combustor based on P&W’s low-emissions TALON-X design. This features a“floatwall” of inner-lining panels — which canexpand and contract independently, reducinglining wear — as well as a rich-quench-leancombustion cycle to prevent nitrogen oxidesforming. Behind the combustor, P&W hasemployed new turbine cooling technologies,including advanced thermal-barrier coatings,powder-metal blade alloys, new cooling-air-path geometries within blades, and turbine-casing active clearance control.



Above: Pratt & Whitney is hoping that shouldeither a re-engined A320 or 737 appear, thePW100G would be selected to power it.


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“We do think we have the most advancedcombination [of cooling technologies],” saysAdams. “And we think we have the mostdurable blades, in combination with [operating]temperatures more conservative than the com-petition. The PW1000G will run at lower tem-peratures than we think the competitors aregoing to run at.”

Other key technologiesTwo other technologies are important in the

PW1000G, says Adams. One is the composi-tion of the engine’s fan blades, still largelysecret. Adams says the PW1000G has “ahybrid metallic fan blade” that P&W has devel-oped over the past two years. “It is actuallylighter and higher-efficiency than a composite

fan blade” the same size would be, he claims.“We think we have got a concept that meets allstructural criteria and is significantly betterthan a composite fan blade. We have done full-scale, full-speed birdshots and we’re inextremely good shape. We don’t see any signif-icant risks with the fan blade.”

Another key technology is the PW1000G’sgearbox. P&W expects the gearbox, which ismade of high-strength gear steels, to be “verylow-maintenance”, says Adams. “We’re expect-ing the gear will be less than two per cent of themaintenance cost of the product. It’s designedto be full-life without any additional mainte-nance outside normal maintenance periods.”

Combining “a very efficient core designedspecifically for the high-cyclic narrowbody mar-ket” with the propulsive efficiency created bythe PW1000G’s large fan and its high-strengthgearbox will create “a step change” which willprovide “the best value proposition” for themarket, he says.

That market includes aircraft “up to theA321 or the large-737 class”. P&W has alreadyrun demonstrator GTF engines at 30,000lbf, isdeveloping a 30,000lbf PW1000G for the IrkutMC-21 — with a 12:1 bypass ratio, like theCSeries powerplant — and Adams says P&Wsees the PW1000G’s competitors as beingengines offering “more than 30,000lb”.

The company has been running a fullPW1000G core since late 2009, and reportedly



Above: The full PW1000G core is tested.

source: Pratt & Whitney


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has scheduled the first CSeries engine to runin August and the first MRJ engine in October.With the CSeries expected to enter service in2013, P&W will bring the PW1000G to marketsignificantly before 2016, when CFM has saidits LEAP-X will be ready for its first application,China’s COMAC C919 mainline narrowbody.

The PW1000G’s market and itscompetition

This could help P&W in winning a positionon a re-engined 737 — and a re-engined A320.“Whether or not it proves to be a competitiveadvantage, it gives [P&W] good reason to thinkthey have a head start on the single-aisle war,”says Storey. “They may even stumble in by theback door if Bombardier builds a 150-seater.”Bombardier might well do so, he thinks: “Therehas always been a gulf from the regional jetsinto the single-aisle widebodies [such as theA320 and 737]. The CSeries might be a span-ner between the two, if they were to stretch it.”

CFM International’s new LEAP-X engine isP&W’s most obvious medium-term competition.“LEAP-X has been by far the most vocal inthere,” says Adams. But he believes CFM istrading away some of the vaunted maintainabil-

ity of its existing CFM56 engine family to meetcustomers’ fuel-efficiency demands for a re-engined A320 or 737.

LEAP-X is “more like a widebody engine”,claims Adams. “The interesting thing here is, inorder to compete with the GTF product, theLEAP architecture has had to change signifi-cantly from the CFM56 architecture to a wide-body architecture,” optimised for fuel-efficiencyand low-cycle operation rather than high cyclesand durability. “We think the competition hashad to compromise maintenance cost,” hesays.

Meanwhile, Adams thinks that the openrotor’s noise, installation and blade-contain-ment challenges, along with the need to pro-vide a variable pitch mechanism for each of itstwo contra-rotating blade rows, will rule out anopen-rotor design in the medium term.“Variable pitch is 10 times less reliable than agearbox,” he says. “And square that” for tworows of blades.

Phased improvementsSome have pointed to the apparent gap

between the 12 per cent fuel-efficiencyimprovement P&W is promising from 2013 with



In 10 years the gearedturbofan can be 7.5-10 percent better than it is now. Wethink that for at least the next15 years we can continue torun that out at the historictrend or better.—Paul Adams, senior vicepresident of engineering, Pratt& Whitney

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the PW1000G and the 15-16 per cent benefitCFM says it can offer from 2016 — and the 30per cent improvement Rolls-Royce is claimingfrom 2018-2020 with an open rotor. However,Adams says that, “apples to apples,” all threeengine manufacturers are essentially talkingabout the same levels of benefit being avail-able at the same dates.

P&W’s initial 12 per cent benefit with thePW1000G is only a starting point, he explains.In developing the new engine, P&W hasadopted a programme to introduce “technolog-ical injection” packages at phased intervals tokeep the programme “fresh”, both for new-buildengines and upgrades to in-servicePW1000Gs: “We think we get to the samenumber at the same point in time” as the othermanufacturers.

Generally, “turbomachinery [efficiency]improves at 0.75-1 per cent a year” throughmanufacturers’ continuing research, saysAdams. “The reason we stepped to the gearedturbofan was that we saw we were actually get-ting nearer to the limit ... with traditional con-cepts. We had to work harder and harder to getthe improvements. The GTF-style architectureallows us to continue that rate” more easily.

“In 10 years the geared turbofan can be 7.5-10 per cent better than it is now. We think thatfor at least the next 15 years we can continueto run that out at the historic trend or better.”

Will the 737 or the A320 use thePW1000G?

If a re-engined A320 or 737 appears, it willbe important to P&W to be on it, Storeybelieves. P&W was the sole provider for thefirst generation of 737s with the JT8D, but sub-sequently “Pratt & Whitney played themselvesout the game and they’re keen to get back in,in a big way, and to be at least one of theengines on the Airbus or Boeing plane. It wouldmean a lot for [P&W’s] bottom line.”

The PW1000G’s large fan size - the diame-ter of the CSeries fan is 73in, compared withthe CFM56-7B’s 61in, the IAE V.2500A5’s63.5in and the CFM56-5B’s 68.3in — will cre-ate challenges if it is chosen to re-engine the737. “It certainly fits pretty easily with theA320, even now,” says Storey. “At the time theA320 was designed [Airbus] probably antici-pated larger and larger engines” being used.But the same isn’t true for the 737, whosedesign started life some 20 years earlier.



In developing the new engine, Pratt & Whitney has adopted a programme to introduce "technological injection" packages at phased intervals to keepthe programme "fresh", both for new-build engines and upgrades to in-service PW1000Gs.


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However, Storey says senior P&W execu-tives have assured him the PW1000G could fitunder a 737 wing. “I think the possible road tothat is a somewhat lengthened nose gear,”says Storey. “It’s probably not feasible tolengthen the main gear on the 737 — youmight as well redesign the plane.” He declinescomment on how much longer the 737’s nosegear would need to be. But reports suggestthat, assuming the nacelle design was flat-tened like that of the CFM56-7B, a four-inchnose gear extension would produce about atwo-inch clearance for a PW1000G.

IAE and the PW1000GA320 re-engining presents a different prob-

lem: P&W is, along with Rolls-Royce, one of thetwo largest partners in IAE, which has wonmore than a 50 per cent market share on theA320 family with the V2500. Airbus hasstressed it wants any P&W path to market on are-engined A320 to be through IAE.

Asked for comment, IAE responds: “Weintend to offer an engine solution that meetsour customer’s needs and we’re in ongoing dis-cussions with Airbus and our shareholders todetermine the right approach to meet thoseneeds. We are considering all available tech-nology options and will offer the best solutiononce Airbus has clearly defined the aircraft’srequirements.”

Presenting P&W’s view, Adams says: “Weboth clearly state we like IAE and it’s a goodchannel to market, and we continue to workwith Rolls-Royce to see if there’s a resolution tothe problem. There are some philosophical dif-ferences and we’re not sure where it will comeout.” He confirms “we have done studies with

both airframers” and notes “a 737 approachwouldn’t necessarily have to be through IAE”.

What this means, says Storey, is that “Rolls-Royce obviously hasn’t jumped on the band-wagon. It’s always been a triple-spoolproponent and possibly also of an open rotor. Ican’t see Rolls-Royce coming out [for anA320/737 re-engining] with its own 25,000-30,000lb engine ... so I would think [Rolls-Royce’s presence] would be either through IAEor nothing. That would lead me to believe Pratt& Whitney thinks it is bringing the most to thetable and would want a bigger share” of a newGTF-based IAE engine for the A320 than the32.5 per cent it now has on the V2500.

Ultimately, says Storey: “I think both Boeingand Airbus are going to prefer two engineoptions, and if they’re not both open rotors, Ithink they will be the LEAP-X and P&W. It seemsRolls-Royce is the one not in lockstep.” WhileBoeing might not be actively favouring twoengine options, “if the LEAP-X doesn’t deliveras advertised and Airbus has options with boththe LEAP-X and the geared turbofan, Boeingwould be at a disadvantage. My assumptionnow is that both manufacturers will offer twoengines, and they’ll be the same two engines”.

The GTF as a widebody engineAdams confirms Embraer is also consider-

ing the PW1000G in its “studies of what itsnext product would be like”. But even moreinteresting is Adams confirmation that “theoverall concept of the geared turbofan is scala-ble up to pretty much any size”, including apotential 100,000lb-thrust engine for a 777-300ER replacement — or perhaps for theA350XWB-1000.



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Pratt & Whitney playedthemselves out the game andthey're keen to get back in, ina big way, and to be at leastone of the engines on [are-engined] Airbus or Boeingplane. It would mean a lot for[P&W's] bottom line.—Bill Storey, founder andpresident, Teal Group

“We would architect a widebody product dif-ferently to the narrowbody product,” saysAdams. “For long missions, we would makesome design trades that advantaged fuel burnversus high-cycle maintainability, consistentwith low-cycle operations. The disk designwould trade weight for cycle life. We would runthe engine hotter, with higher operating ratios.The cooling technology would be different: thedesign margin in the turbine disks, how muchcooling air, what kind of cooling.” �


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