Paris Air Show

Researchers in search of a truly silent aircraft

 - December 13, 2006, 7:07 AM

You couldn’t be in a better place than Le Bourget during airshow week to appreciate–if that’s the right word–aircraft noise. Yet a comparison between the takeoff rumble of the newest airliners and the thunderous departures of the latest military models amply demonstrates the progress in noise suppression made by the civil aircraft industry. And this progress continues, aimed at the eventual development of truly silent aircraft.

When introduced in the early 1960s, airline Boeing 707s, Douglas DC-8s and de Havilland Comets were extremely noisy, leading to intensive work to develop quieter, more efficient engines. In parallel, the International Civil Aviation Organization (ICAO) developed noise standards and introduced effective perceived noise decibel (EPNdB) measurements that provided a yardstick against which the noise levels of various aircraft could be measured. A decibel is a measure of noise intensity where an increase of 10dB sounds to the human ear like a doubling of the noise, and vice versa.

With this yardstick, ICAO established maximum permissible levels of aircraft takeoff noise and, over time, has progressively lowered the upper noise limits by 10dB steps as improving engine technology allowed more stringent standards. ICAO calls these standards “chapters,” while the U.S. calls them “stages.”

While the early 707s, DC-8s and others generated approximately 115dB under the Chapter 1 category, next-generation types such as the DC-9 and early 727s, 737s and 747s had to stay below 105dB to meet the succeeding Chapter 2 limit. In 1990, Chapter 3 lowered the bar even further to 95dB, and most aircraft flying today meet this standard. Consequently, most nations progressively banned the use of Chapter 1 and 2 aircraft.

Chapter 4 on Approach

Now, Chapter 4 is coming, with all new-design aircraft having a gross weight greater than 75,000 pounds certified after Jan. 1, 2006, required to be quieter than 85dB. However, this new limit was anticipated well in advance of its ICAO adoption, so most newer airliners already comply with it.

Nevertheless, 85 EPNdB is still quite loud, and experts see little near-term likelihood of cutting engine noise by a further 10dB to meet a possible future Chapter 5 demand. Accordingly, manufacturers are now turning their attention to other parts of the aircraft, since it has been discovered that air turbulence from the undercarriage, flaps, wings, tail and other protuberances on a large, Chapter 4 compliant jet airliner can, at low landing weights, generate enough noise to drown out the engines. As a result, studies are now under way to reduce the noise produced by the aircraft’s actual shape.

A Revolution in Quiet

The next major step in aircraft design could therefore be revolutionary, rather than evolutionary, since it is aimed at defining a truly silent aircraft. Is such a machine possible? Researchers at the UK’s Cambridge University and the U.S. Massachusetts Institute of Technology are jointly investigating its feasibility. While optimistic, they caution that it remains a long-term prospect.

The Silent Aircraft Initiative research group, which includes specialists from airframe and engine manufacturers, the acoustic industry, airlines, airports, government agencies and other disciplines, has turned to the proverbial blank sheet of paper to conceptualize the ultimate silent aircraft. The $7 million project was launched in November 2003 and is due to run until November 2006. Partners include Rolls-Royce, Marshall Aerospace, the UK Civil Aviation Authority, British Airways and UK airport operator BAA.

Aerodynamically Optimized

The optimum layout that has emerged is a blended fuselage/wing design with tall winglets and engines partly buried above the rear fuselage. The planform is aimed at maximizing lift, thereby reducing thrust requirements, while the embedded engines shield noise from the ground below–a technique already used in some military stealth aircraft. Besides noise shielding, the engines would also employ boundary-layer ingestion to suck the air off the upper surface of the wing, thereby further reducing power-stealing airframe drag.

The fuselage would be devoid of today’s turbulence-creating bumps and protrusions, and its exterior would use new slippery skin manufacturing methods. But the gear and flaps pose major challenges in reducing noise created by air turbulence. Clearly, both are essential, not just for the landing but for increasing aerodynamic drag, thereby reducing the aircraft’s speed during the approach to landing.

The laws of physics dictate that the lower the speed, the less the turbulence and the lower the resulting noise. Conceivably, this could see the return of the covered wheels and streamlined gear legs of the 1930s. It might also result in the application of current U.S. research into actual warping of the wing’s leading and trailing edges in flight to substitute for today’s mechanical flaps and slats. A wing whose cross section (or camber) could be warped seamlessly in this way could have an almost flat camber for high-speed cruise but could then be curved for maximum lift and drag for a low-speed approach and landing. (Wing warping is not entirely new, having been developed by the Wright brothers to control their first flying machines.)

The silent aircraft’s landing approach would also be unique, with a continuous descent path–curving as necessary to avoid noise sensitive areas–onto a steep final approach at significantly lower speed, with the ear lowered only near the runway. Today’s long, shallow approaches with gear, flaps and slats extended would be no more.

How far away is the silent aircraft? Certainly, very many years and possibly decades. But its benefits should not be regarded as simply providing quiet neighborhoods around airports. To the airlines, such an aircraft would offer tremendous advantages by allowing unlimited, 24-hour worldwide aircraft operations, freed of curfews and noise abatement restrictions. All that will then remain is to convince passengers that a 4 a.m. arrival or departure would be beneficial to them, too.