Paul Seidenman/SAN FRANCISCO
Airport noise regulations worldwide are becoming ever stricter, putting airframe and engine manufacturers under increasing pressure to deliver quieter aircraft. In the USA, NASA's Advanced Subsonic Technology (AST) programme is a joint government-industry research effort which names aircraft noise reduction as a primary objective. Running since 1992, the programme is funded through to September 2001.
"Under AST, our goal is to reduce community noise impact by 10dB, using aircraft produced in 1992 as a baseline standard," says Bill Willshire, noise reduction manager for the AST programme at NASA's Langley Research Center in Hampton, Virginia. "We're looking at the engine, nacelle aero-acoustics, acoustic/aerodynamic integration and system prediction, interior noise and community noise impact," he says.
Willshire says that among the predetermined objectives is at least a 6dB effective perceived noise level reduction for engines, and a 50% improvement in noise reduction efficiency for engine nacelle liners. For community noise impact, NASA is seeking a 2dB reduction by using advanced aircraft operational procedures, including high-lift take-offs, curved approaches and more precise flight trajectories.
REDUCING SOURCE NOISE
According to Peter Batterton, chief of the Subsonic Systems Office at the NASA Lewis Research Center in Cleveland, Ohio, research has been directed towards addressing what he terms "source noise reduction", mainly with respect to fan and jet noise. "We are examining aerodynamic design technologies for nacelles that will enhance fan noise absorption. This could include using lighter sound absorbers and making improvements to the inner cowling surface," he says. Batterton adds that research emphasis is on modern, high bypass-ratio turbofans. "We are looking at today's engines and asking ourselves: 'Where do we go from here?' We are not trying to fix old problems," he says.
To address jet noise - the roaring sound caused when high velocity exhaust gases meet stationary ambient air - Batterton says that NASA is exploring new technology for tailpipe liners and for devices to mix the engine core exhaust with slower-moving fan airflow.
"Although promising, mixers do present some performance trade-off and, for this reason, enhanced mixing capability with fewer performance penalties, such as thrust loss, is a primary goal," he notes.
Batterton adds that emission reduction is also part of the engine programme. "Our focus is on nitrogen oxides [NOx] which are produced when air is exposed to the high temperatures within the engine and which contribute to local smog problems," he says. "Reducing NOx means burning off more oxygen, so there is less of it to produce NOx. We are looking at new metal alloys capable of withstanding higher temperatures to do this," he says
Philip Gliebe, a principal engineer in acoustic technology for GE Aircraft Engines, and a member of the noise reduction working group and industry steering committee of AST, reports that GE is exploring engine noise suppression concepts, including the use of active "anti-noise" techniques. "We are actively researching active noise control, although there are still technical hurdles to clear to make it feasible. For passive noise control, we are concentrating on new fan blade and fan outlet guide-vane designs," he says.
Gliebe adds that GE considers the development of nacelle inlet and exhaust liners a high priority. "We are making a significant effort with our suppliers to develop liners that will give us the same, or greater, noise suppression, but at reduced manufacturing costs," he says.
Larry Craig, Boeing Commercial Airplane Group's chief engineer for noise engineering, who also chairs the industry steering committee for the AST noise reduction programme, says that Boeing is now pursuing windtunnel experiments involving the landing gear, slats and flaps. "Our experiments have shown that flap edges are a primary source of airframe noise, so we are focusing on flap edge vorticex strength reduction," he says. Other tests to identify landing gear noise sources show that most of the noise was coming from smaller components, including brackets and tubes. "We're looking at design changes to relocate the components out of the direct airstream," he says.
While the emphasis of research into noise reduction is looking towards the future, older-technology aircraft are not being neglected. Along this line, two companies have found what they see as viable alternatives to expensive engine hushkits.
Bellevue, Washington-based DuganAir Technologies has developed its Quiet Wing System. This uses increased performance improvement and drag reduction - along with some engine modifications - to make existing aircraft Stage III-compliant. According to president Robert Olson, the firm has received US and Canadian supplemental type certificates (STCs) applicable to all Boeing 727 models, weights and engine variants. Olson explains that the Quiet Wing concept includes the installation of 1.2m-tall winglets in combination with a flap droop system - or the flap droop without the winglets. Engine modifications include installing a Pratt & Whitney exhaust mixer and an enhanced, acoustically treated tailpipe.
QUIET CONSEQUENCES
"The Quiet Wing provides a 30% rise in climb rate, decreases stall speed and increases cruise speed from Mach 0.8 to M0.84. Because the modifications to the wings and flaps reduce drag and increase lift, less power is needed to climb, and less power means less noise," Olson says.
Since achieving US certification last April, 45 kits have been sold, including seven to launch customer Kelowna Flightcraft of Kelowna, British Columbia. More recently, Amerijet, a Fort Lauderdale, Florida-based cargo operator, has ordered 14 shipsets. At $1.1-$2.2 million each, Olson says that the modification is about half the price of hushkits.
In another development, Seattle-based Raisbeck Commercial Air Group has sold the increased gross weight version of its Boeing 727 noise abatement system to American Airlines, which will apply it to 52 of its 78 727-200s. Raisbeck certificated the system in August. Under the $58 million American contract, first deliveries are scheduled for September.
The Raisbeck kit brings the aircraft up to Stage III noise standards, without any changes to the airframe, or to take-off and landing weights. Noise reduction is brought about by reducing drag through changes to flap and slat settings. The programme, developed under an STC, also includes flight manual changes.
REDUCED DRAG
James Raisbeck, chairman and chief executive, says that three other 727 kits are also available. They include a standard gross weight -200 kit, certificated in December 1996, and a standard gross weight version for the 727-100, certificated in June 1997.
"By April 1999, we expect to certify a heavy gross weight -200 kit for 727-200s with take-off weights of between 184,500lb [83,800kg] and 209,500lb," he says. "Although this will mandate the addition of an acoustical tailpipe, it will be mounted in the area of the thrust reversers, and not touch the engine," he adds. Raisbeck is quoting between $695,000 (standard -100 kit) and $1,795,000 (heavy -200 kit) for the modification.
Noise reduction programmes, with emission control efforts, are going to be a continuing process as the industry recognises that today's Stage III standards will not set the final limits. As Willshire says: "We have to look at technologies that address near-term requirements and provide solutions for the next 20 years."
Source: Flight International
