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Ever-widening range of forces driving direction of engine tech

 - November 8, 2007, 2:26 AM

The debate about the design of future commercial aircraft engines broadened this year as concerns mounted over the effect aircraft engines may be having on global warming, while the cost of aviation fuel rocketed and noise became ever more of an issue.

The biggest surprise of the year so far has come from Rolls-Royce (Stand W206) and CFM International (Stand W424), who revealed they are looking at contra-rotating open-rotor engine designs, which are reminiscent of General Electric’s unducted fan research in the late 1980s. While both still base their main future engine research on conventionally configured turbofans, the efficiency advantages offered by open rotors cannot be ignored. Even the European Commission has thrown its hat into the ring with two major new programs (see below).

Pratt & Whitney (Stand E518), however, remains doggedly committed to its geared turbofan (GTF) and is closing in on tests of the full-scale prototype engine. In October, the company celebrated the decision by Japan’s Mitsubishi Heavy Industries to use the GTF on its new regional jet, a landmark decision that will give a huge boost to P&W’s confidence.

The stakes are enormous, since the winning concept will establish the direction of aircraft propulsion for decades. While P&W has what it calls an “advanced turbofan” concept ready if the GTF fails, it has put a huge amount of time and money, not to mention credibility, into the geared fan solution. In so doing, it is gambling its future in the most lucrative sector of commercial aircraft propulsion–replacement engines for the Airbus A320 and Boeing 737 families.

Adding fuel to the story is the increasing pressure on reducing engine emissions resulting from proposals such as those from the European Commission to impose on aviation a carbon-emissions trading scheme that would set limits on the amount of carbon dioxide (CO2) that could be emitted.

Engine-emission reduction has drawn a predictably staunch response from the air transport community. Association of European Airlines (AEA) general secretary Ulrich Schulte-Strathaus commented earlier this year that while the AEA agreed with the general principle of reducing emissions, any scheme should be “workable and affordable” and “should not distort competition.”

Rolls-Royce pointed out that it spends around $2 million a day on “research aimed at improving environmental performance.” It might have added that one of the most important of these is improved fuel burn, and with the proportion of airline direct operating costs accounted for by kerosene rising from 13 percent in 2001 to 27 percent today, there is clearly a strong incentive to do so.

At CFM International, the emphasis is on its LEAP56 technology program, which centers on engines in the 18,000- to 33,000-pound thrust class. The work centers on high-performance compressors and turbines and low-emissions combustors, which will bring a 10- to 15-percent cut in fuel burn, 15 percent lower maintenance costs and 15dB lower noise.

The main thrust of the effort is directed at producing a much higher bypass ratio engine of 9:1 with a new woven-composite fan driven by an advanced core based around an ultra-high-pressure core and a single-stage high-pressure turbine. Other potential advances include a composite fan case, the latest three-dimensional design compressor and turbine blades, ceramic matrix composite turbine nozzles and an advanced low-pressure turbine with titanium aluminide blades. The combustor would be the latest development of the TAPS design, which has contributed to progressive reductions in emissions in the CFM56 series.

These advances, while specific to CFM, are not dissimilar to those being pursued by the other engine manufacturers as they aim for reduced fuel burn, noise and emissions. The general idea is the same: to increase the work done by each component within the engine. The technologies which are deployed to do so are essentially similar. All of the manufacturers use three-dimensional flow analysis to better understand the aerodynamics of the engine, and all have access to the latest materials, either to withstand higher temperatures or provide lighter, stronger components.

The last major leap forward in the configuration of commercial aircraft engines came with the introduction of high-bypass ratio turbofans in the late 1960s, first by Pratt & Whitney, then Rolls-Royce and GE. All knew it was the way to go, but all suffered huge technical challenges getting their versions into service. Today, however, all three, along with their specialized offshoots–CFM International and International Aircraft Engines–provide the industry with extremely reliable, high-performance turbofan engines.

The question facing them now is: do we remain with this tried and tested format, or do something that will provide a quantum leap in propulsive efficiency like the big fan did with low-bypass-ratio turbines?

Pratt & Whitney, with its GTF, has clearly decided on the latter path. However, its main competitors, having essentially ruled out the geared solution as being too heavy and potentially unreliable, have in recent months become more interested in the open-rotor concept.

The advantages of contra-rotating open rotors are well known: an ultra-high bypass ratio of up to 35:1 could provide a huge boost in propulsive efficiency and a consequent 10- to 15-percent reduction in fuel burn. Such were the perceived advantages that during the late 1980s and early 1990s, following major fuel price increases, several manufacturers–including General Electric, Allison and Russia’s Progress–tested contra-rotating designs.

Rolls-Royce, lacking support from major state-owned research organizations such as NASA, was absent from the party–but had built up considerable contra-rotating propeller experience with the venerable Avro Shackleton, which is now providing useful data for its current research.

The contra-rotating engines that emerged, while proving to have superior propulsive efficiency, failed on one major point–they were far too noisy. Passenger acceptance of a return to “propeller-powered” aircraft was also considered unlikely.

Since then, however, huge advances have been made in the understanding of aerodynamics, which could bring open rotors back to the table. Rolls-Royce, for one, “has recognized that the open-rotor concept presents a significant opportunity on smaller aircraft,” said Nick Peacock, the UK group’s manager responsible for technology acquisition for civil aerospace.

“In terms of designs, there are an awful lot of options on the table,” he added. “It is a very dynamic situation. Our priority is that, given the external pressures from aircraft manufacturers, airlines and political bodies, we must respond. The message is that we need to accelerate technology acquisition.”

Whether the engine industry is ready with an open-rotor solution when the market demands a new generation of narrowbodied aircraft depends crucially on the results of research taking place over the next few years. “Timing is the $64,000 question,” said Peacock. 

Europe Backs Open-Rotor Research

Two major European Commission research programs sponsored under its Seventh Framework Program will make major contributions to open rotor know-how in Europe and will lead to decisions later in the decade as to whether there is a future for such engines.

The first, led by Rolls-Royce, is the three-year, $50 million Validation of Radical Engine Architecture Systems program (also known as Dream), which brings together 47 partners from 13 countries. It aims to “develop new engine concepts based on open-rotor contra-rotating rotors” and has a target of a 10- to 15-percent cut in fuel burn over the best engines of today and a 3 dB noise reduction.

The go-ahead for Dream came in October and the program is due to begin in January. The project is split into five areas: whole engine architecture (Snecma); geared open rotor (Rolls-Royce); direct drive open rotor (Snecma); innovative systems (MTU) and alternative fuels demo (Turbomeca).

With a wider scope, the $2 billion Joint Technology Initiative is due for launch at the end of this year or early next year, and under the Clean Sky component, Rolls-Royce and Snecma will be joint leaders of a seven-year effort during which four or five full engine demonstrators will be built to demonstrate a number of possible, and still undecided, configurations. “It’s an opportunity to learn and to engage with our European partners,” said Nick Peacock, Rolls-Royce manager for technology acquisition for civil aerospace. “We’re still flexible on the configuration.”

The work follows the highly successful ANTLE demonstrator program funded through the European Commission’s $130 million Efficient and Environmentally Friendly Engine program, launched in 2000.