Researchers investigate potential for ‘silent aircraft’

 - January 30, 2007, 10:58 AM

Even though noise wasn’t a factor in the accident, February’s Challenger overrun at Teterboro has inevitably resurrected local residents’ complaints about aircraft noise. It doesn’t take much, as we all know, to reinvigorate the anti-noise folks.

Yet while Teterboro has no current noise bans–Stage 2, 3 and 4 aircraft can operate there– AIN understands that most corporate jets using the field are Stage 4 compliant. Still, based on the protesters’ theory that any noise from an aircraft is bad, is there a solution?

There is, according to researchers at the UK’s Cambridge University and the Massachusetts Institute of Technology who are investigating the possibility of a truly silent aircraft. But they caution that it’s a long-term prospect.

As most of us have observed, jets that meet ICAO’s Stage 4 standards are quieter than their Stage 2 and 3 predecessors–but they still make a fair amount of noise. Yet it turns out that while developments in engine technology have produced the noise reductions achieved from Stage 2 through Stage 4, the law of diminishing returns dictates that not much more can be done in that direction. There won’t, for example, be a Stage 5 engine for many years.

Aircraft Design
The effort now, according to the researchers, is to reduce aerodynamic noise, since it turns out that during many approaches by Stage 4 aircraft the noise from the rush of air around gear, flaps, slats and the overall shape of the aircraft actually drowns out the engine noise. So the research group, which includes experts from airframe and engine manufacturers, airlines, airports and government agencies, has turned to the proverbial blank sheet of paper to devise a silent aircraft.

The optimum layout that has been proposed is a striking-looking blended and all-lifting delta fuselage/high-aspect wing arrangement with tall winglets and engines partly buried in the upper rear of the fuselage. The layout is aimed at maximizing lift, thereby reducing thrust requirements, while the embedded location of the engines above the fuselage shields noise from the ground. The engines will also employ a boundary layer ingestion technique that essentially sucks the air off the upper surface, reducing drag even further.

The research group has also performed studies on multi-engine installations where smaller, lower- noise engines are used for takeoff and landing and larger engines are used for cruise, but the dead weight of the unused engines and the associated transmission and gearing required could negate the concept’s benefit.

The airframe will be devoid of the turbulence-creating excrescences of today’s designs and will undoubtedly build on the “slippery skin” techniques Boeing is developing for its 787. But the gear and flaps will pose a challenge for reducing the turbulence noise. This is a double-edged sword, since they also increase drag, which is essential for speed reduction, and as speed decreases, so does aerodynamic noise.

Conceivably, this exercise could prompt the return of the spatted wheels and streamlined undercarriage legs of the 1930s and might also foster a revival of the USAF’s development work at Wright Patterson in the 1970s, where researchers investigated internal “warping” of the wing’s trailing edge as a substitute for mechanical flaps, which would also provide a variable-camber wing for different flight regimes.

Approach techniques would also change, with continuous descents–often curving to avoid noise-sensitive areas–onto much steeper glideslopes at significantly lower approach speeds. Long, ILS-like, three-degree final approach segments with gear and flaps down would be no more, with the likelihood that pilots would lower the gear much later in the approach.

So who besides the noise protesters on the ground would benefit? Actually, everyone would.

No night curfews, no demanding noise-abatement procedures and no other noise-related restrictions, allowing pilots to come and go as they please.