New engines promise leap in economy


The commercial aviation sector will see a clutch of new aircraft designs entering service over the next decade or so, including the Boeing 787, Airbus A350XWB, Bombardier CSeries and Comac C919.

All of these programmes aim to tap the latest engine technology to offer significant leaps in performance, fuel-efficiency and reductions in noise and emissions. Furthermore, Boeing and Airbus are both considering the future of their single-aisle offerings, pondering successor programmes to the highly successful Boeing 737 and Airbus A320 families while considering offering the existing aircraft with a new engine as a more efficient interim solution for carriers that do not want to wait for an all-new design.

As reported elsewhere in this issue (see feature, airbus unviels single aisle strategy), Airbus may make a decision within months to proceed with the so-called A320 New Engine Option (NEO), which it hopes will be sufficiently attractive to undermine the business case for the all-new Bombardier CSeries. The Canadian aircraft will be designed to seat 110 to 149 passengers, competing directly with smaller variants of the A320 family.

Airbus is looking to evolutionary engine solutions such as CFM International’s Leap-X and Pratt & Whitney’s Geared Turbofan (GTF) PurePower P1000G as potential powerplants for the A320 NEO.

The manufacturer sees a market window of opportunity for such a re-engined aircraft in about 2015. Then, by the end of the next decade, the manufacturer will be in a position to proceed with designs emerging from the current A30X study into an all-new, future single-aisle model that could even be powered by open-rotor engines, now in the early stages of development.

CFM solution

CFM International, the joint venture of General Electric and Snecma best known for its CFM56 turbofan family, last year finalised the architecture of its Leap-X turbofan. Since then, the manufacturer has been carrying out testing in the second phase of demonstrations on the first engine core, known as the eCore.
CFM plans to certificate the engine in 2014, prior to entry into service in 2016 on the Chinese-developed Comac C919 jetliner. With Airbus and Boeing looking at new single-aisle designs closer to 2020, the manufacturer has said this would give it more time to refine the engine’s technology and add more efficient components to make it a credible candidate to power the next generation of narrowbodies.

The LEAP-X is designed to burn 16 percent less fuel than the CFM56, with up to 60 percent lower nitrous oxide (NOx) emissions and noise levels at 15dB below the current standard.

The new turbofan will have a bypass ratio of about 10, compared with its predecessor’s figure of 5 to 6. It will also have a core pressure ratio of 22 – double that of the current engine – with a two-stage high-pressure (HP) turbine driving a 10-stage HP compressor. The low-pressure (LP) turbine blades will be made of ceramic matrix composite (CMC) materials, saving about 150kg (330lb) of weight, the manufacturer says.
The first eCore had completed more than 30 hours of testing as of the end of April, and was scheduled to be finished by mid-May. It is the first of three test units being built, with the second expected to begin testing by mid-2011.

CFM plans to have a full engine demonstrator running in 2012, according to company executives.
A crucial element of the new engine is its 18-blade, 1.8m diameter composite fan and casing. The fan is produced using three-dimensional woven resin transfer moulding, which allows the blades to be carefully shaped for optimum aerodynamic efficiency. The new material also allows a weight saving of about 450kg per aircraft, as well as bolstering durability and fatigue resistance, the manufacturer says.

Fan testing

In the first quarter of 2009, the company began ground-testing a woven RTM fan on a CFM56-5C engine core at its Villaroche facility in France, with acoustic and crosswind trials following in the second quarter at GE’s Peebles site.

The first results from those tests were “in line with expectations” CFM said late last year, adding that the results would be used to develop the second generation of fan designs. Endurance trials were then carried out in Villaroche in the second half of last year.

The company is working to optimise blade aerodynamics in the LP turbine by going through multiple stages of computational fluid dynamic design and analysis. The turbine with its CMC blades, is to be demonstrated on a CFM56-7B engine core this year.

The eCore design builds on all the experience CFM has from previous HP cores – from the GE90, through the GEnx, to CFM56 technology enhancements developed under the TECH56 programme – to yield a high level of compressor efficiency and stall-free operation “with a high level of probability”, the company says.

The company began initial eCore testing in mid-2009 at GE’s altitude test facility. The manufacturer sais at the time that those tests would analyse aerodynamics, performance, aeromechanical response, operability and the performance of the engine’s new Twin-Annular Premixing Swirler II (TAPS II) combustor, which improves the efficiency of fuel burn and reduces harmful emissions.

TAPS II is a more advanced version of the TAPS combustor found in GE’s GEnx engine, which is to power the Boeing 787 and 747-8. The new combustor will be key to achieving the engine’s target 50-60 percent reduction in NOx emissions.

CFM says it is also looking beyond the LEAP-X, at open rotor engine architecture.

“With an open rotor you can add 10 percent more fuel-burn benefits, so getting to a 26 percent reduction,” says Fabienne Lacorre, the manufacturer’s general manager for product strategy and market. Some LEAP-X technology can be adapted to the open rotor design, but there will still need to be dedicated technology acquisition to develop such an engine, Lacorre adds. Challenges to be overcome include installation and integration with the airframe and fan acoustics issues.
[Subhead:] PurePower programme

Rival engine manufacturer Pratt & Whitney (P&W) has taken a different approach to next-generation engines, with its Geared Turbofan (GTF) technology already having undergone flight testing on Airbus A340 and Boeing 747SP testbeds.

The GTF engine, now known as the PurePower PW1000G, offers a 15 percent reduction in fuel burn compared with today’s equivalent powerplants. It has also been selected as the exclusive powerplant for the Mitsubishi Aircraft MRJ regional jet and Bombardier’s CSeries jetliner, scheduled to enter service in 2014 and 2013, respectively.

“The only challenge that we see for the conventional turbofan is that the fan is actually connected to the LP turbine section, and that creates some sort of compromise in performance,” Shigeaki Nakano, P&W’s vice-president of sales for Greater China, said in a presentation in 2009. “To resolve that, we have decoupled the fan and the LP turbine by putting a gear inbetween.”

The Fan Drive Gear System (FDGS) comprises a central sun gear connected to the LP turbine and compressor shaft, surrounded by five star gears, which then drive an outer ring gear, which drives the fan. This allows the fan to spin two-thirds more slowly than the engine’s low-pressure compressor and turbine.
That permits an increase in fan diameter, which means an increase in bypass ratio, boosting fuel-efficiency and reducing noise. Meanwhile, the LP turbine and compressor are free to spin faster, so they need fewer stages and fewer blades to do the same amount of work – this simplifies manufacture of the engine, making it cheaper to build and maintain.

Today, the General Electric GE90 155B has the highest bypass ratio of an in-service engine, of between 10:1 and 11:1. GTF technology allows P&W to take even smaller engines to a bypass ratio of at least 12:1.

Simulations performed at various airports suggest that the GTF engine may yield a 73-77 percent reduction in noise footprint. The manufacturer says that while a Boeing 737NG or Airbus A320 aircraft fall about 5dB below the International Civil Aviation Organisation (ICAO) Stage 4 noise standard. By comparison, Nakano says the MRJ will be 15dB below, while the CSeries is expected to be 20dB below the limit.

In 2009, Pratt & Whitney officials confirmed that they had found no thrust limitations to the GTF technology, which means it could be offered as a solution for larger aircraft as well, including any potential successor to Boeing’s 777.

In April, the manufacturer formally inked a deal with Russia’s Irkut to begin preliminary design activities on the PW1000G as the powerplant for the manufacturer’s planned MS-21 narrowbody jetliner. The aircraft is being jointly developed by Irkut, Yakovlev and Tupolev as a replacement for Tupolev Tu-154 and Tu-204/214 aircraft.

Pratt & Whitney will power the MS-21 with a higher-thrust variant of its engine, designated PW1400G, rated at 25,000-32,000lb thrust.
Full engine tests on the PW1000G for the CSeries and MRJ aircraft are scheduled to begin this year. The first flight of the MS-21 is expected to take place in 2014, with entry-into-service in 2016.


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