Thursday, April 26, 2007

Silent Aircraft Initiative

The Silent Aircraft Initiative is a study being undertaken by the Cambridge-MIT Institute to discover ways to reduce aircraft noise dramatically, to the point where it would be virtually unnoticeable to people outside the airport perimeter.

Launched in November 2003, the program has the support of Rolls-Royce, Boeing and Marshall Aerospace.Part of the study is examining if a steeper continuous descent into an airport will reduce approach noise levels.



Theoretical analysis will be backed-up by field trials using real jetliners.A major part of the study is developing jetliner/engine designs that can meet the SAI objectives. The favoured configuration is a blended wing design, with the engines located on the upper surface of the wing, to shield ground observers from the engine noise. A very low specific thrust (i.e. very high bypass ratio) turbofan is proposed for the aircraft.



Engine handling and low jet noise are facilitated by the use of a variable area final nozzle, to rematch the fan. Acoustic treatment in the intake and exhaust ducting minimizes turbomachinery noise. Because the blended wing is too shallow to accommodate twin engines, a four engine configuration is proposed.


Problems that need to be overcome are:


1. Serious lack of pitch stability, because of higly destabilizing protruding nose and lack of horizontal tail. So airfoils are to be reflexed (loss of efficiency), and the wing has to be twisted (washout). High lift devices are prohibited because strong negatives pitching moments cannot be balanced. The claimed advantage "efficient high-lift wings" may be discussed...

2. Difficulty in pitch and yaw control, owing to the lack of a tail

3. Greater strength needed to maintain internal pressure, compared to tube-shaped body

4. Due to the majority of passengers' location far from the roll axis of the aircraft, passengers will be far more affected as a result of a steep turn than they would be in a conventional tube-with-wings airliner, where all the passengers are located immediately next to the roll axis

5. Most of the aircraft's occupants will not be able to see a window for looking outside the aircraft at their immediate disposal; a system of 'false-windows', such as LCD displays at each seat or each group of seats that would simulate the presence of a window through use of an externally mounted camera. Emergency evacuation of a passenger aircraft may also pose a challenge.

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