The quest to silence airliners with sophisticated new noise monitoring and attenuating technology is showing results during tests
Against the sigh of the warm easterly breeze, the distant chatter of birds and the hum of insects, the approaching Boeing 777-300ER, a goliath of an aircraft and the world’s largest twin, seems at first to be virtually inaudible. Soon however, the rushing sound of the airframe and the familiar low whistle of powerful high bypass ratio turbofans fills the air.
The gathered test engineers look pleased. Noise, after all, is what the Quiet Technology Demonstrator (QTD) 2 is all about, and the clearer the recorded sound, the clearer the picture becomes of just where it all comes from. The 777, in the blue and white colours of QTD2 partner All Nippon Airways, is overflying the 4,120m (13,500ft)-long runway at Glasgow, in north-east Montana.
The remote site, 30km (19 miles) from the nearest town, is owned by Boeing, but was once home to Strategic Air Command, on constant alert during the Cold War. The bombers have long gone, leaving Glasgow an isolated island in the midst of an ocean of empty prairie. This also makes it the perfect place to probe the exact noise signature of a large airliner and experiment with ways to silence it.
In a climbing attitude at around 1,250ft, the 777’s broad wingspan sweeps its shadow over an array of more than 600 microphones arranged in a spiral cluster pattern across the 100m width of the end of Glasgow’s runway 10. This pattern is the “acoustic camera” with which Boeing and its QTD2 partners, ANA, General Electric, Goodrich’s Aerostructures Division and NASA, hope to pinpoint the source of every sound at all frequencies. The phased array is complemented by a series of microphones located close to the threshold in the standard aircraft noise certification positions, but the main focus is on the acoustic camera.
“We can convert the noise of the aircraft into a picture, just like a thermal image,” says Belur Shivashankara, leading Boeing’s QTD effort. “We had only 187 microphones in 2001 [when the first noise research effort, QTD1 was undertaken], but now we can look at low-frequency noise sources. We couldn’t tell if it was jet exhaust or the landing gear making noise at certain frequencies, but with this we can.” Data at the rate of 100,000 samples per channel per second are collected by the microphones and processed by “Linux clusters”, or 10 parallel computers each with dual processors that “act like a mini supercomputer” adds Shivashankara. During a typical fly-over, the acoustic camera collects around 2,800Mbits of data in 16s.
Acoustically smooth
The phased array roves all over the aircraft for sound generation, finding noise “hot spots” everywhere, from leading edges and flap sections to main and nose gear. During QTD1, for example, the array successfully identified relatively tiny wing anti-icing exhaust holes in the outboard leading edge as the source of a very intense 2,000Hz tone which peaked at 95dB. The noise was eliminated by altering the shape of the holes to slots, and has since been introduced as a change on the production line.
Most attention in QTD2, however, is focused on the aircraft’s starboard GE90-115B, which is fitted with a set of chevrons on both the fan and primary exhaust nozzles. The chevrons, or serrations, are asymmetrical in this installation, and promote additional mixing between the fan and core exhaust stream and bypass flow, thereby reducing shear and therefore noise. The inlet is also a one-piece inlet liner structure developed for the demonstration by Goodrich, and which is ultimately destined for general production.
“The inside of the nacelle is acoustically smooth with no disruptions or discontinuities,” says Boeing noise engineering assistant technical fellow Eric Nesbitt. “We found this to be incredibly important in QTD1. In a standard nacelle, the sound energy comes to a splice and scatters out of the inlet. But with no discontinuity it stays in the nacelle for longer and gets more of a chance to be treated.”
Ahead of the one-piece inlet liner is another new feature especially developed by Goodrich. The “porous” inlet lip incorporates a low-power electric de-icing system that allows the lip to be de-iced, but still have acoustic treatment. To produce the right “porosity”, the liner is made of a wire mesh material, but would probably be made from a perforated titanium in a production configuration.
The inlet lip absorbs most of the high frequency tones and broadband noise generated by the low-pressure compressor. The one-piece liner attenuates the distinctive “buzz saw” noise made by supersonic shocks from the fan blades on take-off and at high power settings, as well as other tones emanating from the fan. The liner also helps attenuate the noise generated by the fan flow interactions with the outlet guide vanes aft of the fan set.
Having completed initial tests on the fixed, asymmetric chevrons around mid-August, the QTD2 effort moved on to tests of a more ambitious variable geometry nacelle which incorporated “smart” chevron made of a shaped memory nickel titanium alloy material called Nitinol. “The idea is to get more noise reduction during take-off, and to minimise any losses in cruise,” says Nesbitt. “The alloy can be ‘trained’ to be in two positions – straight for cruise and bent inwards [by around 1 in (2.5cm)] for take-off.” The material reacts to ambient temperatures and, when the engine starts up and the aircraft taxies for take-off, the crystalline structure in the alloy alters and bends the chevron inwards slightly. After take-off, in the climb and cruise, the super-cooling effect of the high altitudes will again alter the shape of the alloy which will then align itself with the rest of the nacelle line.
“On QTD1 we had indications of erosion, so the less time they spend out of the slipstream the less erosion we’d expect,” he adds. “We have not done a lot of fatigue testing on them and to be honest we don’t really plan to because we are not actually straining beyond its limitations.”
The second half of the programme will also evaluate the noise reduction effectiveness of two large fairings, dubbed “toboggans” that will fit beneath the main truck of the undercarriage between the wheels. The fore and aft fairing are simply attached and will envelope the exposed piping and wiring around the strut and measure 3.3m in length by around 0.86m wide.
Further tests
The results of the tests, which are expected to finish at the end of August after almost 200 flights, will be fed into several programmes including the 787, which will feature chevrons on the nacelle trailing edge. In the case of the 787, which will have the highest bypass ratio engines in commercial service (more than 9:1 at take-off), the effects of the noise treatment are more important to reducing sound in the cabin than affecting community noise. The silvery tracery of transducers on the fuselage side of the 777 is evidence of this aspect.
Shivashankara says after QTD2 “we are thinking about QTD3, and we will be looking for something significant beyond what we’re doing today”. Provisionally timed for 2008-09, the QTD3 effort is widely expected to support efforts for the planned 737 successor in the next decade, though Shivashankara says “nothing has yet been decided”.
GUY NORRIS/MONTANA
Source: Flight International
