Guy Norris/LOS ANGELES
While the International Civil Aviation Organisation (ICAO) moves relentlessly towards an undefined Stage IV noise limit for airliner operations in the 21st century, the industry is hard at work to find ways to meet it. Many suspect the new requirements will call for an average rise of 2-3 decibel (dB) per certification point, or an accumulated 8-10dB increase overall.
Getting there by the expected 2002-3 timetable is possible, say most engine makers, airframers and research agencies like NASA. Most of the unknowns concern the exact timing and maturing of technologies to reach, and possibly exceed, the targets, and the economic justifications for doing so. Further off, as in any battle of diminishing returns, more fundamental questions are being asked about how much further noise reduction can reasonably go.
Leading the race for answers in the USA is the noise reduction element of NASA's Advanced Subsonic Technology (AST) programme - a five-year-old effort to reduce noise by 10dB relative to 1992 production technology, by next year. The AST attacks the five main elements of aircraft noise: engine acoustics, nacelle air acoustics, airframe-generated noise, interior noise and community noise impact. Engine sound was to be reduced by 6dB, acoustic liner performance in engine nacelles was to improve 50% and airframe noise was set a goal of 4dB. Interior levels were expected to be reduced by 6dB. Community noise impact, through modelling and reduction by means of advanced aircraft operations, was expected to generate the equivalent of a 10dB reduction - or roughly half the noise experienced by the average airport neighbourhood in 1997.
"Current status is 4dB for engines," says William Willshire, NASA's AST noise reduction programme manager. "In liners we have got 35% of the 50%. In airframes there are three main components: flaps, slats and gear. We've met the 4dB goal for flaps and work is under way on slats and gear, but things look promising. For the interior we have reduced tones by 6dB and for broadband we are at 4dB, but all at the laboratory scale," adds Willshire.
An estimated community noise reduction level of around 5dB has been achieved so far by combining US Federal Aviation Administration integrated noise models with "some newly identified low noise operations", he adds. The agency appears confident that sufficient progress has been made towards helping meet the next wave of ICAO limits.
Validation comes next. A full-scale static test is being planned to evaluate the engine and nacelle noise reduction technologies, while a 26% model of a Boeing 777 will be tested in the 12 x 24m (40 x 80ft) windtunnel at NASA Ames in California. Interior noise technology validation will be more easily achieved using laboratory and aircraft cabin tests, while there are no plans to validate the community noise reduction in the current programme.
Quiet power
"One of the home runs we've had with engines has been sweep and lean of stators," says Willshire. Originally conceived by General Electric, the idea of swept and leaned outlet guide vanes is yet to be tested on a full-scale engine, but is expected to result in big noise reductions. "It might be a little premature, but in the windtunnel it dramatically reduced fan tones," he adds. A critical advance which has helped bring ideas about topics like swept/lean stators to the fore is the availability of new computational fluid dynamic (CFD) modelling techniques. The three-dimensional modelling ability of these systems gives researchers the first true understandings of the mechanics of noise generation and how to tackle it.
"We think the fan wake impinging on the stator is what causes the noise. This comes off the fan in a three-dimensional, whirling spiral pattern, but hits the traditional two-dimensional stator," says Willshire. "This is being led by the predicative code model. So we are learning to minimise noise through design." Phil Gliebe, GE's principal engineer in acoustics technology, adds: "The problem was that we didn't understand the physics. Now we have better analytical tools." As far as the advanced stator design is concerned, some applications could already be on the horizon, including possible retrofit of CFM56 engines. The integral fan frame of larger engines, such as the GE90, and its large spacing between axial stages and stators, mean that applications in this thrust class are less likely.
Computer modelling has also made a dramatic impact on studies of quieter nacelle linings. "We have improved the process in every link of the chain, so we are able to mathematically come up with a better acoustic impedence of the liner. Computer modelling has been the key because it helps all the way through to how you manufacture it," says Willshire. One example, although not quite as attractive as swept/leaned vanes, is reducing the number of liner splices inside the inlet duct through improved design. "The more splices, the more noise," he says. "I think we'll meet the 50% metric." Alignment of linings is also changing, again helping reduce noise. "With improved modelling we have the ability to align the linings with the high speed grazing [air] flow and produce better optimal impedence."
Gliebe says modelling allows a more holistic approach to fan/nacelle noise treatment. "It has to be designed as a coupled system. Modelling gives you a better way of knowing how to relate fan noise to liner noise." Willshire takes it a step further, saying: "There is a paradigm shift going on today. Engines have been designed for the 'cut-off' condition of the engine, in which the blade path frequency of the fan does not pass out of the nacelle. If we are successful, they will no longer have to do that. Liners are designed to absorb discrete tones and, if we are able to eliminate that at source, the engine can be designed for cut-on, rather than cut-off."
Cut-off refers to the proper selection of numbers of blades and vanes to reduce noise emissions. By eliminating that restriction, engine designers will have more flexibility to improve compressor and turbine configurations. Discrete tones, or single-frequency noises produced by the movement of blades in the air, fan wake, and hot exhaust gases, are one of the two noise categories under attack. The others are broadband tones, which cover a wide range of frequencies produced by movement of air over blades and from combustion.
"Liners can have a new purpose. They no longer need to be optimised for one, two or maybe three fan tones. Now they need to be broadband devices. It represents a new era of engine design," says Willshire. The results may not produce vastly different looking engines, he adds, but linings designed to combat broadband frequencies could be thinner, and nacelles could be trimmed tocut weight and drag.
Despite the potential paradigm shift for the front arc of the engine noise arena, GE is exploring traditional approaches for further reductions in the aft arc. Future low-pressure turbine designs, for example, will "probably" be configured to ensure certain stages are "cut off" and that the blade passing frequency tone does not pass out of the exhaust. "We'd rather do it the smart way and cut off the tone, and when we do that we won't need exhaust liners which are heavy and expensive," says Gliebe.
Exhausts form an entire study area in their own right. GE is focused on a chevron nozzle design which has demonstrated 3.5dB jet noise reductions in lab tests to date. The design, which resembles the serrated edge nozzles of stealth aircraft engines, reduces acoustic energy by more than 50%, says the company. "We are looking at it for the CF34-8D [for the Fairchild 428JET]," says Gliebe. Other retrofit contenders are under study. Another area of focus related to core noise continues to be the low-emissions combustor design. As these are "generally running on the ragged edge of stability" as they try to control the fuel/air mixture, GE considers them a potential noise source. "It could produce an acoustic resonance or screech, and we can't let that happen because it would begin to break up," he adds.
As engines become relatively quieter, so the question of airframe noise grows - particularly for the larger widebody aircraft either in service or on the computer design screens. NASA's focus, and that of the leading US airframe maker, Boeing, is on three main areas of noise generation - flaps, slats and gear, and their combined effects on approach noise in particular. "We have a system where we can pinpoint noise sources on aircraft within a windtunnel setting and an anechoic chamber," says Larry Craig, Boeing noise and emissions chief engineer.
In terms of flap and leading-edge slat noise, Boeing is working on CFD analysis to "change designs" to alter noise characteristics. The plan is to attack "separation bubbles" that have been identified between the slat and the wing. Like the flaps, where noise has been identified as emanating from the inboard trailing edge, the main problem area is the trailing edge of the leading edge slat. Various revised configurations are being evaluated, including saw-tooth trailing edges, strategically placed serrations and vortex generators. Some primary research work has been undertaken by NASA Langley with the University of Southampton in the UK, and has pointed to significant noise reductions through steadying of flow.
Thickness alterations
Boeing, which is anxious to avoid the added manufacturing complications of unusual configurations, hopes that current tests may show that relatively innocuous alterations to thicknesses may suffice. It keeps an open mind, but Craig adds that altering thickness would be a "little more palatable from the manufacturing perspective". The noise produced by large landing gear trucks is also under study, with Boeing focusing on the low-frequency noises generated by air flow over hydraulic lines, wiring and detail around the tyres and axles.
Boeing believes its current work, like that of the NASA programme as a whole, is aimed at "more aggressive" targets than those being set by ICAO. The vast cost of some retrofit programmes for older equipment, however, continues to plague all concerned. As Pratt & Whitney sums it up: "The question is not really what is technically feasible. It should be what is environmentally needed, and what is economically practical? We should ask what can the industry afford to do, and then look at what is technically practicable."
Source: Flight International
