Climate change is becoming a driving force in the long-term planning of the next generation of commercial airliners. The issue of what should be done about greenhouse gas emissions created by airliners has split the aviation industry into several camps.
However, manufacturers no longer question the relevance of climate change for long-range technology planning.
Paul Adams, vice-president of engineering for engine manufacturer Pratt & Whitney, notes how swiftly the issue has ascended in priority for aviation technologists. Even five years ago, with relatively little regulatory attention paid to global warming, the industry sought technology solutions that "optimised on operating-cost criteria and that drove a balance on maintenance cost and fuel burn", he says.
With the spike in fuel prices accompanied by a windfall of political support for finding solutions - or culprits - to the problem of global warming, the leading aviation technology houses are fully aware of the issue.
"We definitely recognise that there has been a world-view change as it relates to the environment and emissions, and that's happened in the last five years," Adams says.
"We do think this is a permanent change on emissions and the question is, how severe is it going to be?"
The shift in attitude has come with the general acceptance that global warming is caused by the amount of carbon emitted into the atmosphere, of which the aviation industry contributes about 2-3%.
"Five years ago there was a lot of debate about carbon," says Chet Fuller, general manager for marketing at General Electric Aviation. "There's not a lot of argument about whether carbon contributes to climate change today."
The aviation industry's relatively recent acceptance of the issue will have a far-reaching impact on aircraft technology, and most especially for aircraft engines.
But the most fundamental change is a potential break from the nearly four-decade-old trend of introducing technology improvements, including those for fuel efficiency, in small increments.
The urgency and demands of the climate change issue may force the industry to respond with a leap in technology, similar to the move from turbojet to turbofan engines in the late 1960s.
For the first time, industry officials say, a new class of aircraft and engine technologies may be developed, with environmental performance a primary factor governing the design process.
ll other major traditional design criteria - speed, cost, maintainability, safety and so on - could be subordinated to near-secondary status to achieve environmental goals.
"Beyond 2015 we think there are technologies that can yield a 25% fuel burn reduction," says Fuller.
In the aircraft business, seemingly slight efficiency improvements are hailed as breakthroughs. For example, International Aero Engines, the partnership between Rolls-Royce and P&W to produce the V2500, is investing hundreds of millions of dollars to develop and certify the SelectOne technological insertion package, mostly with the aim of boosting fuel efficiency by a single percentage point over the standard V2500-A5.
th fuel costs averaging a quarter of operating expenses across the industry, that improvement is worth the investment for many carriers.
Trade-off
But for each marginal increase in fuel efficiency, cruise speed or maintenance cost, there is often a trade-off that must be made in some other area of either aerodynamic or operational performance. Sometimes, the trade-off can be accepted, while other times it must be resolved with technological or operational improvements.
Take, for example, fuel efficiency: engine designers know the simplest way to reduce fuel burn is to raise the air pressure as it flows through the engine fan and enters the combustor. The higher the pressure of air in the combustor, the lower the amount of fuel needed to yield the desired thrust.
Applying this concept to the climate change issue is clear enough. Carbon dioxide emissions are a function of fuel burn. In fact, 0.45kg (1lb) of jet fuel is responsible for producing about 1.43kg of CO2. The process of combustion fuses two slightly heavier oxygen atoms from the air flowing through the fan on to a single carbon atom contributed by the jet fuel, hence the non-linear increase in total weight.
Logically, it should follow that raising the air pressure inside the combustor would be the easiest and fastest way to reduce CO2, and therefore curb a widely accepted gaseous contributor to global warming.
Balancing act
But aircraft engines produce several different types of potentially climate-altering emissions. The problem is, reducing one type of greenhouse gas in the engine exhaust means increasing another. Such is the case with increasing air pressure as part of jet engine combustion. Increasing the air pressure also raises the level of heat inside the engine core.
More heat means that more air exits the combustor as nitrogen oxides. It is not well understood how NOx directly contributes to global warming, but it is generally accepted that it creates more ozone, which serves to trap the planet's warmth within the atmosphere.
NOx is also an air pollutant, and hot temperatures inside the engine while airliners taxi are known to affect local air quality adversely.
Another trade-off created by increasing the air pressure is a maintenance penalty. The same hot spots inside the combustor that produce spikes in NOx levels also create more wear and tear on the turbine blades located behind the combustor, so airlines have to inspect and replace the blades more frequently.
Aerospace engineers have dealt with each of these trade-offs with the introduction of each new generation of high-bypass ratio turbofans.
In the mid-1990s, for example, engine companies developed the "rich burn, quick quench" process that allows combustion to occur at higher pressures, but without the hot spots that produce NOx and maintenance penalties.
The science of materials used for turbine blades also has progressed, yielding longer service lives in higher temperatures.
For the past four decades, aviation technology has improved steadily in the form of slight increments, with each improvement balanced by the need for improvements and offsets in other areas.
This holistic approach to aerospace design is now the principle challenge aerospace engineers face to meet the rising demands of environmentalists, policy makers and an increasingly conscious travelling public.
An end to step change advances?
The advent of turbojet-powered airliners that so revolutionised the civil aviation market in the 1950s also created an environmental nightmare.
Airline passengers had never before travelled in such speed or comfort, but this progress came initially at the expense of fuel efficiency by any reasonable measure.
The breakthrough technology arrived in the late 1960s to early 1970s with the high-bypass ratio turbofan, such as the GE-built TF39 for the Lockheed C-5A Galaxy and the derived CF6 for civil airliners.
The turbofan allowed more air to bypass the engine core than travel through it, greatly boosting fuel efficiency while dramatically reducing noise.
If the first-generation engines produced bypass ratios of 5:1, such as the early CFM International CFM56 models, today's engines are typically double that amount and should increase further to 15:1 by the middle of the next decade.
Ever since, the aviation industry has improved by small increments instead of great leaps. Engine technology has been the greatest focus in terms of fuel efficiency, but improvements in aerodynamic shaping, the movement towards greater use of lighter composite structures and recent advances in airspace management have continued gradually but consistently since the early 1980s.
Bill Glover, Boeing's managing director for environmental strategy, says that each new generation of airliners produces a 15% overall improvement in fuel efficiency over the preceding generation.
However, with some exceptions, the components of each new generation of improvements are often the sum of hundreds of small innovations developed piecemeal for existing aircraft fleets.
Boeing says that its new 787 now achieves the same level of fuel efficiency as a family car on a cross-country trip.
The Intergovernmental Panel on Climate Change, created by the United Nations and World Meteorological Organisation, estimated in a 1999 report that the aviation industry has shown a 1-2% improvement in fuel efficiency annually, and that is expected to remain constant into the future, even as air traffic volumes rise at forecasted rates of 3-5% annually.
The track of incremental progress extends beyond fuel efficiency and environmental impact. Regulators expect each new generation of aircraft to come with improved safety features. Operators demand that parts are sturdier and need less maintenance.
Passengers want to see a healthier cabin environment, with purer air quality, lower noise levels and better entertainment. Aerospace engineers design each new generation of aircraft with this holistic approach in mind.
The question is, how long this trend can last? The two largest airline manufacturers have widely different answers.
In Boeing's 20-year outlook period, the company foresees no need to break from the path of incremental improvements. Nor does it foresee any technological barriers to new efficiencies that can be gained from maximising future aircraft sharing the same basic configuration as the existing fleet.
Lightening loads
Glover points to the recent adoption of composite materials for major structural components of the 787, to include the fuselage barrel. The entry into service of the 787 will provide a new source of data on opportunities to improve and refine existing composites, as well as reveal additional structural areas that can be lightened by converting from metals to composites.
Composites can improve fuel efficiency in two ways. First, the lighter material reduces overall weight. Second, it creates stronger structures that can be designed in new ways to maximise aerodynamic performance.
"I think we have a lot more opportunities in structural efficiency," Glover says. "We're at the beginning of an improvement cycle."
Airbus, on the other hand, has a different mindset when it comes to the prospect of continuing incremental progress indefinitely. In the Airbus view, there is a point where the benefits of taking such small steps in the area of fuel efficiency vanish or incur unacceptable costs to make the initial improvement worthwhile.
"The holistic approach [to technology improvement] will be reaching the limit of what it can achieve," says Rainer von Wrede, Airbus's director of environmental affairs.
"We can get more fuel efficiency but then it will cost on the speed of the aircraft. We can make the aircraft lighter but then it will be - just, for instance - wider."
Von Wrede estimates that the aviation industry will reach that limit in "a decade or so". Ten years in the aviation industry represents about half the lifespan of a generation of aircraft and is within the current product development cycle for the manufacturers of engines, systems and structures.
By this reckoning, both manufacturers and regulators will be forced to make some difficult choices in the decade ahead, or face potential penalties for taking no action.
The Airbus forecast period also may roughly coincide with the projected schedule for replacing the 737 and A320 fleets with a single-aisle aircraft, creating a rich opportunity for a leap in efficiency of aircraft design and performance.
"Since the beginning of the jet age we had improvement in all aspects of aircraft design," Von Wrede says. "This is going to vanish in the years ahead. We have to make choices on what we improve and what we don't improve."
Leap-ahead technologies
Reducing CO2 emissions by 35% is achievable with the next generation of aircraft to enter service by the end of the next decade.
This great leap in fuel efficiency, however, would be possible only if airlines and regulators can accept penalties in maintenance cost and noise levels, as well as perhaps no further reduction in NOx emissions.
The penalties could be mitigated to some extent by advancing technology, but the changes required to enable that degree of CO2 reduction will require both operators and regulators to make concessions.
The two most significant technical changes for the step-change in CO2 emissions are a major tweak to conventional aircraft design and the introduction of a new kind of engine technology.
Airbus, Boeing and the engine manufacturers have been contemplating such a deviation from airliner design and technology for more than a decade, but until now have not seen clear signs that the market is willing to accept the associated penalties.
Engine for change
The key adjustment in airliner design involves the placement of the engines. The traditional method of hanging engines under the wing or on each side of the aft fuselage increases drag, and therefore lowers fuel efficiency by a marginal amount.
Some of that decrease can be mitigated by adopting nacelle shapes that optimise laminar airflow, but there remains a penalty. Airlines have been willing to accept this extra cost because that placement offers the most accessible place for inspecting, maintaining and replacing the engines. The industry's maintenance infrastructure is also designed with that approach in mind.
A more logical location to place engines in terms of fuel efficiency, however, is above the aft fuselage, where most aircraft today have the vertical stabiliser assembly.
Airbus has released a concept design of an environmentally friendly aircraft, showing two engines located between a split V-tail. This design greatly reduces the drag compared to hanging the engines under the wing.
Also, the wing can be fully optimised for lift, without even the structural supports necessary to carry the engine nacelle. But again, this approach carries a penalty in terms of maintenance cost for airlines, which would need to adopt new tools and methods to access the engines.
This ultra-efficient design goal would also require a leap to open-rotor technology for engines. In the late 1980s, GE tested the GE36 open-rotor concept, featuring two sets of unshrouded, counter-rotating blades at the front end of an engine nacelle.
Compared to a ducted turbofan design, this approach could dramatically improve fuel efficiency by enabling bypass ratios of 36:1. The GE36, however, failed to spark market interest after fuel prices dropped and regulators started pushing the aviation industry to reduce noise levels.
Silent revolution
With unshrouded blades, the open-rotor concept now pursued again by GE and R-R may require regulators to accept an increase in noise levels, with the trade-off being a huge benefit for fuel efficiency.
GE says the noise issue can be mitigated by installing devices on the aircraft to emit frequencies that are aimed at cancelling the open rotor's noise emissions. However, Airbus engineers remain dubious that the noise issue can be abated by new technology.
"These kind of engines on an aircraft would achieve the step change in fuel efficiency," says Von Wrede. "But in all circumstances [this would be] to the detriment of an improvement in noise levels."
Source: Flight International
