Boeing has revealed the 747-8 could be the first commercial aircraft to fly with variable-area, or “smart” engine chevrons, following lessons learned from the recently completed Boeing Quiet Technology Demonstrator (QTD) 2 flight research effort in Glasgow, Montana. The -8 requires noise reduction advances to guarantee maximum flexibility under London Heathrow’s stringent “QC” noise rules from 2009.
The recommendation for potential use of the advanced chevrons comes with reassurances from the QTD2 demonstration that baseline fixed chevrons will also provide low internal noise levels in future aircraft, particularly the 787. For QTD2, the 777-300ER test aircraft’s starboard GE90-115B had chevrons on the fan and primary exhaust nozzles.
The chevrons, or serrations, were asymmetrical in this installation, and helped mix the fan and core exhaust streams and bypass flow, thereby reducing shear and noise. The Goodrich-developed one-piece inlet liner produced a cut of up to 15dB in “fan-related tones”, says Belur Shivashankara, Boeing QTD2 project manager. 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.
The “smart” chevrons, tested in the latter stages of QTD2, were made of a shape-memory nickel titanium alloy material called Nitinol. The alloy can be “trained” to be in two positions – straight for cruise and bent inwards by around 1in (2.5cm) for take-off – using Nitinol, which reacts to ambient temperatures. When the engine starts and the aircraft taxies, 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 to cut drag.
GUY NORRIS/LOS ANGELES
Source: Flight International
