Growing public objections to aircraft noise have fuelled major initiatives by manufacturers to find ways of reducing the problem. We look at developments

Anger at airport noise is near boiling point. In September, Frankfurt Main Airport received 56,330 noise complaints from local residents - more than one every minute of every day and a 30% increase on the same period in 2001. Residents near London's "rural" Stansted Airport are submitting 100 objections a day to tentative proposals for three new runways at the site. Brussels Airport is proposing to spend €152 million ($151 million) on appeasing disgruntled neighbours unable to sleep because of night-time operations. And in France, Patrice Paris‚, head of civil aviation programmes at civil aviation authority DGAC, says: "The promises agreed over the last 20 years with populations near airports have been considered ineffectual."

Against this rising tide of public frustration, airports are imposing stricter noise limits. Even as aircraft and engine manufacturers claim their products are quieter than ever, all have embarked on programmes to achieve step-change reductions in aircraft noise over the next decade, and some radical thinkers are looking well beyond 2020.

Most manufacturers point to a 20dB noise reduction per passenger from the turbofan aircraft of the 1960s to today's Airbus A340-500/600 and Boeing 777. Most agree the driving force behind this improvement has been, and will continue to be, turbofan technology.

They also agree it is in the propulsion arena that most progress will be made over the next 20 years. With fierce competition to bring technologies to the marketplace, manufacturers are juggling with a bewildering array of features such as two- and three-degree-of-freedom acoustic linings, "scarfed" inlets and "fluidic" chevroned nozzles, all of which are being improved rapidly. Further into the future, more obscure concepts arise, such as inflatable linings, "weeping" inlet lips and even loudspeaker-generated anti-noise.

For future airframes, manufacturers propose faired landing gear and smooth wing trailing-edges, and are extolling the virtues of a "quiet" blended wing body (BWB).

Quiet engines

The highest-profile recent engine noise-reduction programme was the Boeing/ Rolls-Royce Quiet Technology Demonstrator (QTD). This 777-200ER testbed with R-R Trent 800s has an area-maximised (AMAX) fan inlet acoustic lining, which increases lining area by 30%, and chevroned nozzles (serrated to aid flow mixing).

Fan noise reductions of up to 13dB and jet noise reductions of 4dB have been claimed (Flight International, 20-26 November 2001). Boeing and R-R also observed a significant reduction in the "buzz-saw" fan noise using the AMAX lining. Buzz-saw noise is caused by fan blade shocks propagating upstream and collapsing, releasing acoustic energy. The acoustic effect of inlet lining is extremely sensitive to breaks in coverage, and Airbus's head of acoustics and environment, Michel Pacull, says: "A one-piece lining with just one splice could yield a further 6dB fan-noise reduction, while a zero-splice lining could yield a 30dB reduction."

A zero-splice lining would require substantial development of installation and repair techniques, but R-R is already taking advantage of its experience with new linings. A version of the new inlet lining is already being used on R-R Trent 500s fitted to the new Airbus A340-600, and Boeing hopes to have AMAX-lined Trents on its 777 by next year. The lining on the A340 is a two-piece construction, with two splices splitting the circumferential nacelle lining.

Further forward, at the inlet lip, a combination of scarfing and lip-lining is being studied to help reduce fan-noise forward propagation. The scarf is a relatively simple extension of the inlet lower lip, reducing aircraft-overhead noise by shielding the ground from the fan. Boeing hopes to demonstrate a 10dB fan-noise reduction next year using a scarfed Pratt & Whitney PW4098 on a 777 testbed.

Belur Shivashankara, Boeing technical fellow, environmental performance strategy, says: "Windtunnel trials have shown no performance losses at high angles of attack and low speeds, but crosswind performance still has to be evaluated."

Ric Parker, director of research and technology at R-R, more cautiously predicts a 2-4dB fan-noise reduction from scarfing. He also highlights the need for additional forward-fuselage acoustic insulation to counter the now upward-directed noise.

"There are potential aerodynamic and weight penalties, and the scarf would be non-retrofitable," he says. R-R is pursuing its own scarfed inlet studies under the EU's Silencer programme.

Another "simple" geometric solution is used on the fan-exhaust ducting, where optimising the curvature of the fan-duct walls can minimise noise propagation and attenuation. Boeing has developed its Cduct software for designing duct lining in this way. At the inlet lip, Boeing is studying a surface treatment involving thousands of tiny "weeping" holes, through which it plans to pump hot bleed air. This disrupts the reflection of noise from the lip area and provides an energised boundary layer to help the inlet linings.

"The hot air provides anti-icing protection and has the added bonus of preventing the build-up of bugs," says Shivashankara. Fabrication of test sections began in 2001, and Boeing plans full-scale tests of its treated lip on a 777 next year.

Liners are also employed further back on the engine, in the exhaust ducts of the fan and the core. Engineers are trying to improve the range of noise frequencies the liners can suppress, to aid broadband noise reduction and to suppress tones at a range of power settings and flow velocities. General Electric is studying variable-depth liners and active liners.

"We can vary the liner depth depending on the frequencies we want to absorb by inflating it with air from the fan," says Mike Benzakein, general manager of advanced engineering at GE. Traditional lining material is built from one or more layers (degrees of freedom) of honeycomb structure, tailored to absorb certain frequencies, but useless against others. The system is at the laboratory stage and Benzakein hopes to run ground tests next year, "although the appropriate engine type is yet to be determined". Snecma's director of R&D Christian Mari outlines a similar vision for CFM International's CFM56. "The Mechatronics active liner will have adaptive impedance as a function of engine power settings," he says.

At the business end of the engine, chevrons can serve to improve the mix of fan and jet exhaust gases. They diffuse the boundaries between flow types by causing the exhaust to "spill" over the chevrons' edges. Chevrons are emerging from the testbeds of several manufacturers and look set to become common in the near future.

GE has been testing them on its CF34 powering the Bombardier CRJ700. "They will be available on the basic-configuration Embraer 170," says Benzakein. Next year Airbus is to flight test an A321 fitted with a chevron-equipped CFM56, and Honeywell is proposing them for its TFE731 turbofans on the Dassault Falcon 900EX.

As a result of successful flights of its QTD, Boeing is now trying to optimise the chevron design, for possible introduction on the 747-400XQLR or other future types. Boeing wants to take advantage of the measured noise reductions, but is working on minimising thrust loss in the cruise phase. "We're looking at shape-memory Nitonol [bimetallic nickel plus titanium] that will bend into the flow at high-thrust conditions, but straighten out in cruise," says Shivashankara. He estimates the QTD's fixed chevrons, which protrude about 10mm (0.4in) into the fan exhaust, "cause up to 1% thrust loss".

GE is also looking at what it terms "fluidic" chevrons, but is investigating composite materials instead of metals. "The structure cannot be life-limited to less than overhaul," says Benzakein, who sees the exhaust environment as particularly harsh for composites.

Some technologies developed primarily for overall engine efficiency also have a knock-on beneficial acoustic effect. Three-dimensional swept fan blades allow smoother airflow and minimise shock upstream of the blades, but also lead to less noise being generated by disturbances in the flow. R-R's Parker says that, by further refining the design of the blades, "fan tone intake noise could be reduced by 4dB at take-off". But he cautions that the benefits must be realised without compromising blade stability or stall margins. R-R has studied combinations of fan blade numbers, tip speeds and even forward-swept fan-blades, as its acoustic modelling capabilities improve.

Similar gains could be realised by further optimising the position and sweep of the outlet guide vanes at the fan exhaust, which Parker predicts "could yield reductions of up to 5dB in fan tone noise and 3dB in fan broadband noise". Snecma is studying active stator blades that would match flow conditions as part of its noise demonstrator programme, and Mari says it has "already tested small-scale versions".

Meanwhile, GE is studying blade wake and vortex reduction. The company is conducting a joint project with NASA to analyse the generation of blade wakes and blade tip vortices. "In principle, the solutions should work at any speed," says Benzakein, "but they'll have to be tailored to one condition. In a year we'll know more about what's possible."

Loud airframes

Engines have historically been the noisiest part of an aircraft, but Airbus's Pacull says that "for the A340-500/600 on approach, the airframe contributes 2dB more than the engines". He adds: "Of that, the landing gear, slats and flaps are the biggest contributors to noise."

Recent tests with phased-array ground-based microphones have enabled engineers to determine with great accuracy the individual sources of an aircraft's noise signature. Boeing cites the example of eliminating the 777 wing leading-edge anti-ice hole tones as a result of its Montana QTD tests (Flight International, 22-28 October).

In September, Boeing conducted an extensive phased-array measurement of nearly 500 flights at Amsterdam's Schiphol Airport, using 323 microphones (Flight International, 30 July-5 August). Preliminary results suggest that on the 777, for example, major noise sources are the outboard flap edges and nose landing gear (NLG).

"Extending the landing gear adds 15dB to a generic airframe's noise signature, excluding engines," says Ulf Michel, senior scientist at the DLR's Institute of Propulsion Technology in Germany.

But he adds that even with completely faired legs, only 3dB could be recovered from an NLG noise signature, and 2.3dB from the main landing gear's output. Airbus UK is designing landing gear fairings for testing on an A340-300 next year.

For the 747-400XQLR, Boeing proposes drooping the ailerons during flap deployment to create a more uniform trailing edge and minimise flap-edge noise.

Sensitive airports

Manufacturers are eager to exploit all other avenues of noise reduction. Through its Air Traffic Management group, Boeing is encouraging airports to adopt low-noise operational procedures. The company claims that by adopting a continuous descent approach (CDA) procedure, the area of the 65dBA ground footprint of a 747-400 at London Heathrow Airport could be reduced by 16.5%. Germany's DLR and Lufthansa have studied using 5¡ glide slopes instead of the current 3¡, and claim overall reductions of 3dB on the ground are achievable in the short term.

"Of 600 major airports surveyed around the world, nearly 400 now have noise-abatement procedures of some degree, and over 200 have curfews," says Boeing's Shivashankara. As an indication of increasing public noise perception, he adds: "Both numbers have doubled since 1990."

But BAA's Rick Norman says Heathrow does have CDA procedures in place. "At altitudes of less than 6,000ft [1,800m] over London, no more than one level segment is permitted during a CDA descent," says Norman, Gatwick flight evaluation manager for BAA. A level segment is defined as a 2nm (3.7km) path with less than 250ft altitude reduction. Although BAA has set a target that 90% of night-time operations should adhere to CDA procedures by 2005, Norman points out: "CDA is susceptible to high wind conditions." Airports are keen to do everything in their power to reduce noise impact, because the alternatives are expensive. World Health Organisation (WHO) guidelines issued in 2001 stipulate that maximum acceptable noise levels in a sleeping room should be 45dB no more than 10-15 times a night. Armed with these guidelines, residents are forcing airports to amend their operations or pay for acoustic insulation.

But any practical discussion of tolerable noise levels is often bogged down in detail. When considering the residential areas affected by noise levels, Brussels assumes a 25dB noise reduction across a household window. The Germans disagree, putting the figure at 15dB. Why the difference? The Germans assume the windows are open, Brussels considers them ajar.

The International Civil Aviation Organisation's Chapter 4 noise regulations for new aircraft come into force on 1 January 2006. They will require aircraft to be 10EPNdB (effective perceived noise decibels) quieter than today's Chapter 3 standards - an effective halving of noise energy. Yet local regulations at airports such as Heathrow, already considered the noise benchmark for heavy jets, are likely to be even stricter than these.

Airbus's A380 engines, the GE/P&W Engine Alliance GP7000 and the R-R Trent 900, had their fan diameters increased late in the design cycle in 2000, once it became apparent that the A380 would not meet Heathrow's strict QC2 requirements. Even now, the A380-800 freighter will suffer from maximum take-off weight restrictions at Heathrow when it enters service in 2008.

But manufacturers are optimistic and with the authorities have ambitious targets for their technological solutions. For example, R-R has instigated its Vision 10 programme, which aims to reduce engine noise 10dB by 2010.

R-R says such improvements would apply to any engine required for Boeing's proposed Sonic Cruiser or, for that matter, any new aircraft programme. Its even more ambitious Vision 20 goals are aligned to the wider European ACARE (Advisory Council for Aeronautics Research in Europe) initiative, and require a "halving of perceived noise levels by 2020".

Integration hopes

In its studies for Vision 20, R-R is looking at the effects of alternative airframe/engine integration concepts. Parker says the BWB concept being pursued by Boeing and Cranfield University "could act as giant inlet scarf effectively shielding the ground from inlet noise, and wing wake would shield rearward-radiated noise".

He adds: "There would be no ground clearance issues limiting engine bypass ratio." For the BWB, R-R is studying a contra-rotating aft fan engine, and the positioning of the aft fan would reduce core jet noise by as much as 5dB.

Another futuristic proposal is put forward by Ulf Michel at Germany's DLR, who proposes active noise control for engine rotor-stator interaction. "Loudspeakers would make cancelling anti-noise, with possible reductions of 24dB on a 340Hz base frequency," he says.

It is unclear how many of the technologies outlined here will ever see the light of day. What is clear is the driver - air traffic is growing at about 5% a year, and overall noise levels are supposed to remain constant, at worst. Technological advances will be found if the need exists.

Around €750 million of the EU's 6th Framework money is available for aeronautics research in Europe until 2006, and noise is one of the hot topics. With €110 million allocated to Silencer, the involvement of NASA in the USA and the competition between the major manufacturers, the problem will not go away quietly.

Source: Flight International