Boeing Commercial Airplanes and Rolls-Royce Aero Engines have completed a noise reduction flight test program that promises to make quiet jets quieter.
Known as the Quiet Technology Demonstrator (QTD), a Rolls Royce Trent 800 engine was modified with a package of noise reduction technologies developed collaboratively by the two aerospace companies.
Using a 777-200ER, the three week flight-test demonstrated noise levels significantly below those of a standard 777, which is known as one of the quietest aeroplanes in service. Takeoff jet exhaust noise was reduced by up to four decibels and inlet fan noise was reduced by up to 13 decibels.
Engineers used saw-tooth-shaped aerodynamic devices at the rear of the nacelle and on the exhaust nozzle to control the mixing of the hot jet exhaust, the bypass stream and the ambient air. The shape of the devices was determined by computational fluid dynamics modelling and verified in wind tunnel tests using scale models.
Fan noise also was also said to be reduced with acoustic improvements to the redesigned engine nacelle inlet. A new technology called Amax (area maximisation) increased by 30 percent the area of acoustic treatment in the inlet casing. A new lining design was used that reduces objectionable ‘buzz saw’ noise passengers often hear during takeoff and climb.
The flight tests reportedly verified the computer and laboratory results. Some 200 microphones were placed on the ground along the flight path, and 100 microphones were affixed to the 777.
Teamed with computers, the microphones became an ‘acoustic camera’ that pinpointed high-frequency noise sources on the aeroplane as it took off, flew the flight test pattern and landed again.
Although the purpose of the QTD program was to reduce noise heard on the ground, levels within the cabin were also analysed. Nearly 100 microphones placed along the entire length of the cabin registered a reduction of forward cabin buzz-saw noise by seven decibels.
The culmination of years of work, these successful tests are said to mark one of the final stages before QTD noise reduction technology is implemented in service.