Integrated propulsion systems: the engine connection

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Engine manufacturers worldwide are gearing up and teaming to prepare portfolios of optimised integrated propulsion systems to meet airframers' needs for re-engine or clean-sheet projects scheduled for mid-decade and beyond.

Gone are the days when mottos like "thrust you can trust", the 1980s tagline for Pratt & Whitney, summed up the desired features from engine suppliers. Instead, airframers have evolved to the point where they are defining top-level interfaces and performance requirements that, by default, demand the efficiencies best provided by an integrated propulsion system that manages the aircraft's energy demands while minimising the weight and drag of engine-related components.

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Chief among the demands are fuel burn reductions of 15-17% for mid-decade projects and up to 25% for the 2025 timeframe. Although engines may account for a large percentage of the gain, an optimal integrated propulsion system can add percentage points.

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 © Pratt & Whitney

Pratt & Whitney's PW1000G geared turbofan is on the leading edge of integrated propulsion systems set to debut on the Comac C919

"GE Aircraft Engines spends billions of dollars to get a point of efficiency [from an engine]," says Steve Walters, president of Nexcelle, a new integrated propulsion system joint venture between GE and Safran. "For pretty nominal non-recurring costs for the development of an IPS, you can get multiple points of fuel efficiency."

DESIGN ACTIVITIES

Major engine manufacturers and their joint ventures are immersed in integrated propulsion system design activities to compete in the most effective way for a handful of new or updated ultra-fuel-efficient aircraft models being pondered or pursued by Airbus, Boeing and others.

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 © Rex Features and Tim Bicheno-Brown /Flightglobal
Airbus concept shows possible configuration using rear-mounted open rotors

Among them, Rolls-Royce is studying integrated propulsion as part of its Option 15-50 project for the next-generation single-aisle aircraft, while P&W is investigating options through its parent company United Technologies' propulsion systems integration centre, a laboratory created five years ago to model the systems integration aspects of UTC's contributions to the Boeing 787.

However, GE and Safran appear to be placing the largest bet by launching the Nexcelle partnership in June. It is modelled on CFM International, the Franco-US 50/50 partnership between GE and Snecma that has produced the most prolific engine in commercial aviation history, the 18,500-34,000lb-thrust (82-150kN) CFM56 family.

Nexcelle combines the experience of GE's Middle River Aircraft Systems (MRAS) group, provider of aerostructures and nacelles for several GE engine models - including nacelles for the CF34-10A engine for China's ARJ21 regional jet and thrust reversers for the GEnx-2B engine for the Boeing 747-8 - with Snecma's Aircelle subsidiary, which builds the nacelles for most of the Airbus family aircraft and several business jets.

Many core competencies of the two companies overlap, but each has specialities - MRAS with electric nacelle components and Aircelle with an electrical thrust reverser for the A380 - that will bring a holism to the team. The companies can also reach back into the core competencies of their parents, both of which will have access to the intellectual property developed in the partnership, some of which may also be used for business jet applications.

Nacelles typically include the engine inlet, fan cowl, thrust reverser and associated equipment, plumbing, wiring and electronics.

Bidding wars are likely to begin fairly soon, fuelled by the demanding performance requirements of an atypical entrant - the Commercial Aircraft Company of China (Comac) with its 130- to 200-passenger single-aisle Comac 919 twinjet.

The 919 is set for a first flight in 2014 and service entry in 2016. MRAS officials say the Comac integrated propulsion system will "push the state of the art", including electric anti-ice protection for engine inlets rather than the traditional hot bleed air, and an electrical thrust reverser actuation system (ETRAS), an advanced technology that debuted on the A380, as well as other new features.

In a related development, China's AVIC and Nexcelle agreed on 23 September to explore a new joint venture to "consider a broad range of nacelle and components manufacturing and design opportunities" for current production aircraft and new designs ranging from "business jets to large airliners".

Integrated propulsion system requests coming on the heels of the 919 will include Embraer's KC-390 tanker transport, to be powered by two 27,000lb-thrust turbofan engines. First prototypes are due to fly in 2015. Integrated propulsion systems will also be included in some form if there is to be a re-engining of the Airbus and Boeing single-aisle fleets around mid-decade because of the further slipping of next-generation single-aisles to post-2020.

Nexcelle's technology roadmap gives a glimpse of how much there remains to optimise on today's commercial airliners. The effort is designed to mature all required technologies to be able to build a prototype nacelle and pylon to be mated with a "Leap X" CFM56 turbofan engine for a full-scale test by the end of 2012, says Eric Masse, vice-president of engineering for Aircelle.

"We have started to work all these demonstrations to make sure we can meet a TRL 6 [technology readiness level of a prototype] to make sure we can meet the requirements of OEMs," he says, cautioning that it is not clear when the OEMs will "push the button" for a new product.

Leap X is CFM's advanced technology programme to build a next-generation turbofan with 16% less fuel burn for the engine alone, compared with today's CFM56s. The company is promising additional savings if the engine is matched to the airframe and systems using its integrated propulsion system concepts.

Nexcelle is investigating more than a dozen new technologies to decrease fuel burn, noise and maintenance, while increasing the reliability of the integrated propulsion system. As well as increasing the use of electric actuation rather than hydraulic or pneumatic, engineers are moving toward weight-saving and maintenance-friendly monolithic structures, improved acoustics and a variety of design tools to better harmonise the wing, pylon, nacelle and engine combination.

Key projects for the joint venture include developing a one-piece "O" duct for the thrust reverser, which eliminates weight and complexity compared with today's two-piece ducts; designing an electric engine inlet heater that is embedded in the composite material used to form the inlet; and reducing generator requirements, hence fuel burn, by optimally managing power needs for the fan nozzle control, engine inlet anti-ice, ETRAS and other systems according to the phase of flight.

Norton DePinho, marketing and strategy leader for GE Aviation Mechanical Systems, says the move to resistive heating elements woven into the composite engine inlet will eliminate the double-ducted tubing going forward from the engine to the inlet as well as the air duct system for anti-ice protection.

DePinho says the new system will also eliminate the burst-duct potential from high-pressure bleed air. MRAS has already successfully tested the technology, he adds.

Engineers are also developing acoustic treatments to reduce the integrated propulsion system noise to as much as 15dB below Stage 4 standards, says Aircelle's Masse. "The A380 is state of the art, but we are developing, in partnership with engine manufacturers, new technologies for even greater noise reductions."

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 ©GE MRAS
GE's Middle River Aircraft Systems will build up to eight ARJ21 nacelles a month

Masse says Nexcelle will perform outdoor ground testing of the new concepts with a CFM56 engine in 2010. Nexcelle is also working to increase the percentage of composites in the nacelle to as much as 70%, up from 60% in the current A380 - its most advanced nacelle system to date - to decrease weight and complexity.

Although an integrated propulsion system does involve components, GE's Walters says it should not be thought of as a product, more a process. "If you think about the traditional process, requirements flowed from the aircraft to the pylon, and the pylon in many cases was manufactured and developed by a separate entity," says Walters.

"Systems requirements were placed on the nacelle system and, in turn, on the engine. Each of those different organisations and teams were designing in a different environment to support requirements at each of those interfaces," he adds.

By contrast, Nexcelle will design and certificate an integrated propulsion system in a single environment with a specification negotiated by CFM with the airframer.

WEIGHT REDUCTIONS

"What we've seen in our development process is a great amount of aerodynamic improvement and drag reduction and significant weight reductions," says Walters. "In the traditional process, as requirements are flowed down, there is a build-up of redundancies. We cut that out.

"Fundamentally, we're taking it from a subsystem design approach to a system design approach. That occurs only when you have teams working under common practices."

How much can be gained? Walters says an integrated propulsion system can go "far beyond" even the state of the art in the A380 or Boeing 787. "We believe there is very significant fuel burn, significant maintenance and direct operating costs in the aftermarket with the integrated propulsion system," he says. "With engines like the Leap X, we can get well into double digits from the fuel burn standpoint and triple digit-type weight numbers."

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At P&W, a key research area being investigated with sister company Hamilton Sundstrand is the optimal management of power in the more-electric aircraft that will doubtless follow the 787. Hamilton Sundstrand, in its role of providing nine systems to the Dreamliner, including the electrical power generation and start system, has evolved from a component supplier to an aircraft systems designer and manufacturer, says Paul Adams, senior vice-president of engineering at P&W. "They benefited from their experience on the 787," he says.

As well as setting up a systems integration laboratory, the 787 work became the foundation for tools that will help P&W develop integrated propulsion system for future products. "We see this as being a continued long-term trend," says Adams. "For the next generation of aircraft, whether built by Bombardier, Embraer, Irkut or Comac, they will continue to look at broader levels of integration and we will see continued emphasis on propulsion providers, rather than engine providers."

Adams says electrical demands are likely to increase, calling for new ideas to balance engine performance, fuel efficiency and power needs. On the 787, the Hamilton Sundstrand power generation and start system generates 1.45MW of power, five times the capability of the Boeing 767, says the company.

The trend is driving the development of integrated engine and power controllers that can optimise the work split between the electricity generated by an engine's high-pressure turbine (HPT) spool and low-pressure turbine (LPT) spool generators.

Adams says that generators have historically taken power from the HPT spool only, but the 787's requirements necessitated taking power from the LPT too, a need that applied to both the GE and Rolls-Royce engine options for the aircraft. "As these power levels get larger and larger, the impact on high power extraction on engine performance gets more dramatic," he says.

At issue is a "speed mismatch" that can occur, says Adams. As power is extracted from the HPT, the engine control system compensates by increasing fuel flow to keep the speed constant. But when the LPT spool is tapped for electricity as well, a speed mismatch can result between the two shafts, leading to "stability and transient performance issues", he says.

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To control this phenomenon, Adams says designers are likely to have to implement a multi-variable control strategy that optimises the system response to multiple inputs rather than a single input, similar to the system used on the P&W F135 engine for the US military's Lockheed Martin F-35 Joint Strike Fighter. In the F135's case, the LPT is tapped to power the aircraft's lift fan is engaged for vertical or short take-off and landing operations. Adams says the F135 is the first production use of a multi-variable controller in an engine.

POWER EXTRACTION

Adams says P&W's plan to develop "a class of very-high and ultra-high bypass ratio engines" will put "larger demands" on the system for power extraction. The geared turbofan slows the speed of the fan by two-thirds with regard to the LPT, using a series of star and ring gears, creating a high bypass ratio as more air flows outside the engine core. This leads to better fuel efficiency and less noise, says P&W. The company's PurePower geared turbofan series will initially power the Bombardier CSeries and Mitsubishi MRJ regional jet.

Demands for more fuel efficiency for the next-generation single-aisle will result in a larger fan size that will become a design consideration in terms of interaction with the wing, noise management and flight profiles, says Adams.

GE and its partners are working on "multiple proposals" for a CFM56-based IPS for next-generation single-aisle aircraft, but not necessarily just the clean-sheet designs. "There are other programmes more near term, with Airbus, Boeing and other airframers," says Walters, referring in part to potential re-engining programmes.

"It doesn't have to be a clean-sheet design with regard to the aircraft. The perimeter and what is integrated changes a little bit, but clearly the value in a propulsion system with a new engine and a new integrated nacelle can be had," Walters adds.