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Copyright © 1999, MCB UP Limited
The challenge of reducing aircraft noise
The challenge of reducing aircraft noise
Keywords Airbus Industrie, Aircraft, Noise levels
While air traffic is increasing, the industry has still to meet noise levels imposed by the ICAO (International Civil Aviation Organisation). Making aircraft as quiet as possible is of key importance to manufacturers.
Airbus Industrie is conducting various studies to reduce aircraft noise, especially to reduce aerodynamic drag. Products such as "riblets" plastic films similar in texture to sharks' skin are fitted to the aerodynamic surfaces of the aircraft, helping airflow. The engineers at Airbus Industrie are also investigating ways of reducing drag by keeping the airflow smooth over large parts of the aircraft's profile. This can be achieved by sucking in the air through microperforations in the leading edge of the wing surface.
Other sources of noise being studied include the landing gear and the flaps. Tests have shown that placing fairings around a full-scale A320 main landing gear reduces aerodynamic noise by five to ten decibels. Landing flap noise is currently being tested on a full-scale model of an A321, and the first application of the results will be incorporated on the A340-500/600.
ONERA (French Office for Aerospace Study and Research)is researching aerodynamic aircraft noise with Aérospatiale. The research is based in part on an experimental approach that is carried out in a wind tunnel with a 1:11 scale model of an A320/321. Acoustic imaging is being used to study noise of aircraft passing overhead. Alain Julian of ONERA explains: "This technique is based on the use of a cross-shaped network of microphones placed in the wind tunnel or on the ground during fly-bys. One can then see an acoustic image of the wing surface of the model in landing configuration (high-lift flaps extended). This result, obtained for a frequency f° = 2500Hz, confirms the complex structure of the noise zone. It shows, in particular, that there are three sources of noise, located at the root of the leading edge of the wing and in the region of the leading edge slats and the flaps."
The main cause of noise in turbojets comes from the fan. Ways of reducing fan noise include modifying the source of noise itself, and absorbing the sound waves as they pass through the nacelle. The noise spectrum generated by a fan consists of spikes at the harmonic frequencies of the movement of the blades. The acoustic waves are created by the rotor and its interactions with the stator.
Tests are being performed on models within the framework of the European RESOUND programme. "ONERA's objective", explains Serge Lewy, manager of rotating machinery acoustics, "is to evaluate a new way of reducing noise by active flow control. We carry out a basic experiment which involves creating an acoustic wave with a phase opposite to that from the interaction between the rotor and stator." The research effort is currently being extended within the framework of the European RAIN (Reduction of Airframe and Installation Noise) programme which also supports research and development into noise reduction devices.
Hurel-Dubois, an aerospace equipment supplier, is a European leader in nacelles/ thrust reverser systems for commercial aircraft, and is actively pursuing a policy of technological research in three main areas: safety, the environment and performance.
Christophe Kauffmann of Hurel-Dubois says: "Our engineers have developed absorbent cladding for the internal walls of nacelles, which reduces engine noise by ten decibels. This cladding consists of a perforated skin that is positioned in front of one or two honeycomb structures. We can control the number and diameter of the holes these perforated skins can contain up to 50,000 holes per square metre. We considered several techniques, and our final choice was drilling with a cutting tool. We therefore developed an NC drilling machine specially designed for this operation. This equipment is today used extensively on the whole range of our products fitted to Airbus aircraft (A320 and A340 powered by CFM engines, and A330 powered by Rolls-Royce), Boeing (747 and 767 powered by Rolls-Royce), Gulfstream V and Bombardier Express Global (powered by BMW Rolls-Royce)."
Thanks to its expertise in the design and integration of sound-reducing aircraft structures, Hurel-Dubois was chosen by Quiet Technology Venture, a US company, to design and manufacture conversion kits for the DC8-50/61S. These kits, which replace certain parts of existing nacelles, make it possible to reduce engine noise levels, bringing them in line with the regulations.
Socata, part of the Aérospatiale group, has been working on noise reduction of tourist aircraft for several years. The acoustic spectrum of a light aircraft during a fly-by contains peaks emerging from a broadband range. According to Philippe Matharan, pre-production research programmes manager, "The different sound sources of this aircraft are: the propeller which produces a spiky sound spectrum due to rotation and the number of blades; the exhaust which also generates a spiky sound spectrum due to the number of cylinders; and lastly the aerodynamic noise which is broadband, caused by the interaction of the airflow with the fuselage and by the vortex created at the trailing edges of the blades. Of these three sources the propeller and the exhaust make the major contribution."
The helicopter manufacturer, Eurocopter, is working hard to reduce the noise levels from its helicopters. Today, Eurocopter's products have a noise level much lower than the ICAO standard (by up to 10dB). Particular effort has been made to make the "Fenestron" ducted tail rotors silent. The first stage was to reduce sound while changing the distribution of the blades in the rotor, and to reduce the circumferential speeds and the amount of thin-blade stators. These designs have already been successfully incorporated in the EC 120, the EC 135 (winner of the 1996 Golden Decibel prize awarded by the French Ministry of the Environment) and also the EC 155.
For more information, please contact Dan Ray at the French Technology Press Bureau.