Andrew Doyle/LONDON
THE DRIVE TO establish latest-generation cabin noise suppression technology on turboprop-powered regional aircraft is likely to spark a fierce battle between manufacturers clamouring to offer airlines new levels of cabin quietness. While most airframe manufacturers agree that they must offer the technology, they have taken widely differing approaches to its implementation.
Cabin-noise levels are becoming an increasingly important aspect of turboprop-aircraft manufacturers' marketing campaigns, with their regional-airline customers chasing high-yield business passengers who, in the past, may have found travelling on turboprop aircraft a less-than-attractive proposition.
These manufacturers have long sought ways to eliminate the cabin noise and vibration caused by the fluctuations in air pressure generated by spinning propellers. Over the years, the development of digital avionics has resulted in significant progress towards this goal, improving comfort levels through auto-stabilisation and more efficient flight management. Quieter engines have also been developed, along with sophisticated propeller synchro-phasing techniques.
NEW SYSTEMS UNDER DEVELOPMENT
These improvements alone, manufacturers believe, have not satisfied the expectations of modern airline passengers who demand the same comfort levels in small turboprop aircraft as those provided by wide body jet airliners. Noise suppression technology may now be mature enough to close the gap.
As a result, active noise control (ANC) systems are under development for the Aerospatiale/Alenia ATR-42-500, de Havilland Dash-8Q and Jetstream 41. The Saab 2000 and Saab 340BPlus, meanwhile, lead the field, having entered service last year with ANC fitted as standard, while the Dornier 328, which features a sophisticated passive system, entered service in 1993.
Ultra Electronics' Noise and Vibration Systems division, which is behind the noise suppression technology on four of these types, developed the active system for the Saab 2000. Saab decided to replace the tuned vibration dampers included in the original design with an ANC system, following extensive in-flight cabin noise measurements. The Swedish manufacturer says the switch was made "...due to weight considerations".
By the time the 2000 entered service, however, Saab had failed to achieve the cabin noise level of 76dB(A) that had been promised to launch customer Crossair and the manufacturer has since been working to achieve the lower level.
"We had a few teething problems," admits Dr Andrew Langley, Ultra's technical director, who believes that one of the principal problems encountered on the Saab 2000 was installing the system and eliminating elusive "rattles and buzzes".
"It requires a high-quality installation to work properly," he says.
The division's managing director, Dr Colin Ross, believes that Saab's decision to set a target of 76dB(A) for the 2000 spurred competitors into action. "As a consequence, the other manufacturers have followed suit," he says. "The market now expects low noise."
Ross concedes that quieting the Saab 2000's cabin was "...a difficult task, probably more difficult than Saab had envisaged," but he adds that installation problems have now been overcome. The Swedish manufacturer got the level down to 78dB(A), but found that certain higher-frequency sources such as aerodynamic noise and the air-conditioning system, which could not be controlled by the active system, needed to be tackled using more conventional sound insulation.
Ross contends that a purely passive system, as used on the Dornier 328, although cheaper and more reliable, would not have been adequate for the Saab 2000, mainly because it is faster and has larger, more powerful propellers.
"Passive [alone] was not able to do the job," Ross says. "To achieve those figures we had to use ANC. There were some passive measures designed from the start, but, with passive, you are able to get some noise reduction, but not a great deal unless you add a lot of weight."
It will be "difficult", adds Ross, for makers of turboprop aircraft of a similar speed of that of the 2000 to achieve cabin-noise levels approaching 76dB(A). An almost identical system to that of the 2000 is now fitted as standard on the Saab 340BPlus, which was one of the first aircraft to enter service with an active system.
Ultra is also developing an active system for Bombardier's de Havillands Dash 8Q, although this system differs in that no loudspeakers are required. So-called active tuned vibration absorbers (ATVAs) are used to initiate out-of-phase counter-vibrations, resulting, claims Bombardier, in sharply reduced cabin vibration and noise levels. Bombardier and Ultra have been involved in the development of ATVAs for the past two years.
"Loudspeakers can only address noise, whereas ATVAs can address noise and vibration," says Langley, although there is "very little difference" between the two systems except that they use a "different type of actuator". The possibility of using tuned dampers in a less complex, purely passive, system for the Dash 8Q was rejected, he adds. "Active technology offers such a big improvement over what you can achieve passively... there's a real case for it," Langley concludes.
ADDRESSING VIBRATION AT SOURCE
Ross claims that an active system such as the noise- and vibration-suppression (NVS) system selected for the Dash 8Q can offer noise reduction of up to 14dB(A), whereas a purely passive system may achieve a reduction of 2-3dB(A). Another advantage of using ATVAs is that as they address vibration at source, the acoustic characteristics of the cabin trim are less of a factor.
Hunting Aviation, which recently won a contract to supply complete cabin interiors for the whole Dash 8 family, is providing a "turnkey" package for the Dash 8Q, incorporating Ultra's NVS system and its own passive noise-suppression technology. Propeller synchro-phasing is also being introduced on the type for the first time, and Hunting claims that cabin noise will be reduced by around 12dB(A) compared with previous Dash 8 interiors.
Ultra is also developing an active system for the Aerospatiale/Alenia ATR 42-500, similar to that of the Saab 2000, and is studying other possible twin-turboprop aircraft applications.
Langley also confirms that the company has identified jet airliners with rear fuselage-mounted engines as potential candidates for active cabin-noise reduction, with Ultra having carried out trials using a McDonnell Douglas MD-80 some four years ago. "It's a very busy field at the moment," he says.
EXPLOITING THE CORPORATE MARKET
Ultra sees the corporate and general aviation market as a major opportunity, and, earlier this year, announced that Elliot Aviation of Illinois would become the exclusive distributor of its UltraQuiet installations in North America. Initial development efforts focused on the Beechcraft King Air series of turboprop aircraft, and US Federal Aviation Administration certification for the King Air C90 and 200 models was gained in August. Installations have now begun, costing $25,000 for the C90 and $30,000 for the 200.
One aircraft, which Ultra has not managed to get on board, however, is the Jetstream 41. Jetstream Aircraft carried out head-to-head flight trials between an active system from Ultra and one supplied by GEC-Marconi Avionics. The latter won the contract. Jetstream is committed to purchasing ten sets of the ANC system from GEC-Marconi, although it will be offered to airlines only as an option.
The company's Jetstream 31 chief designer, Tony Furner, who is also responsible for special projects such as active noise control, says that the technology is "ready" despite several earlier false starts. Jetstream first flew active- noise technology on a British Aerospace 748, followed later by trials on a Jetstream 31.
"Everything was too heavy," says Furner. "Keeping the weight down is a major thing." The weight of such systems has been brought down to an acceptable level, however, (due mainly to advances in the processing power of microchips) and the next big challenge is "...getting space to fit the equipment, "Furner says. The speakers must be made as small as possible, since there is a relatively small space available between the aircraft's cabin-trim panels and the outer skin.
VERY COMPETITIVE WITH JETS
Nick Godwin, Jetstream's executive vice-president and director of marketing, plays down the importance of noise control relative to the complete aircraft package, however. "Noise doesn't feature highly in the overall perspective," he says. "I think turboprops are getting to the point where they are very competitive [with jets]. The ANC system was introduced to take it to another level." Jetstream anticipates particular interest in ANC from corporate operators of the Jetstream 41, however.
The company has also introduced passive vibration dampers on the Jetstream 41, which are attached to the five fuselage frames most affected by prop-wash. These are also available for retrofit to earlier aircraft.
GEC-Marconi says that it recognised a need for an ANC system, which could control "...a large number of unrelated tones". Passive techniques, it adds, are "...effective at higher audio frequencies of 500Hz and above", while passive techniques are "...less effective at the lower frequencies generated by turbine engines".
The company claims that its ANC system is unique in that it uses a high-fidelity template of the cabin noise to generate the canceling signal rather than the less-sophisticated sine- wave approximation of other systems.
It carried out flight trials of its ANC system on a Beech King Air 200 in 1993, followed by further testing on a Cessna Caravan. In 1994, trials with Jetstream and Indonesia's IPTN resulted in the system being selected for the Jetstream 41 and being evaluated on the latter's CN-235 regional turboprop.
According to John Aplin, marketing manager of GEC-Marconi's Flight Controls Group, the company's research laboratories are now working on more advanced "broadband vibration control systems", using an extension of existing control algorithms.
In the USA, meanwhile, Lord began working on ANC systems in the early 1980s, but was forced to freeze their development because some key components rendered them impractical. "The enabling technology just wasn't there," says Doug Hodgson, manager of Electro-Mechanical Systems at Lord, citing the lack of digital signal-processors as one example.
The technology is now "coming to maturity", he says, with the equipment being smaller and lighter, coupled with advances in speaker and actuator technology and control algorithms.
Lord's NVX systems, consists of three main technologies: ANC, active structural control, and active isolation control. The systems differ in the types of actuators and sensors used, as well as in their placement in relation to noise and vibration sources.
Lord says that NVX systems use an accelerometer for error sensing, as opposed to the synchronisation signal used on other systems. This patented control method, the company claims, allows the system to "...control both narrow-band and broadband [background] disturbances".
According to Hodgson, ANC is most suitable for "...an application that is dominated by low-frequency noise in a situation where vibration is not deemed to be objectionable". As ANC does not tackle vibration at source, however, it is "the least preferable from a technical point of view", he says, and is likely to be used only where "...it might be difficult to apply either of the other technologies".
Active structural-control systems are most effective for regional-turboprop applications, says Hodgson, and Lord began what are thought to be the first flight tests of such a system earlier this year, on an Embraer EMB-120.
Active isolation-control, meanwhile, is being pitched at rear-fuselage-mounted jet-engine-powered aircraft. This technology could be applied to turboprop aircraft, but "...the noise in turboprop aircraft is not so much engine vibration as prop-wash," says Hodgson.
A NEW APPROACH
Dornier took a different approach to noise suppression when designing its 328 regional turboprop and, theoretically, may have come up with the best solution, at least from an operational point of view. A range of measures was designed into the aircraft from the outset, including vibration absorbers, advanced insulation materials and tuned trim panels. Vibration-isolation devices are attached to the engines, and solid-state propeller synchrophasing is also employed.
The aim, says Dornier, was to achieve a purely passive "fit-and-forget" system requiring zero maintenance, and the result was noise levels below 78dB(A) in 75% of the cabin "...without [the use of] complex electronic devices".
Reinhold Birrenbach, vice-president for the Dornier 328 programme, says that the system was chosen because it was more cost-effective and reliable than an active system, but he admits that there is a weight penalty. "It may be easier to go for an active system," he says, "but you have to trade off cost, reliability and weight against how much noise reduction you may need."
Modern noise-suppression systems are undoubtedly performing as their designers intended, but are airlines willing to pay the premium for quieter aircraft cabins? "The market is answering that question," says Ultra's Langley. "It is a very complicated equation, but operators want quiet aircraft and they are prepared to pay a premium for that. The airframer also has an aircraft that is more competitive."
Source: Flight International
