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Technology update
Aerodynamic noise reduction
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Airbus is using riblets, high durability plastic films similar in texture to sharks' skin, to reduce aerodynamic noise.
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Because of increased air traffic worldwide, particular focus has been given to reducing aircraft noise. Several companies based in Europe have already begun studies into aircraft noise reduction. Some of these have resulted in design changes and retrofitting of existing aircraft, while other solutions have remained under development.
Airbus Industrie has recently launched various studies aimed at reducing noise and aerodynamic drag on its aircraft. High durability plastic films that are similar in texture and function to sharks' skin have been developed to help airflow over aircraft surfaces. Another study by the company involves drag reduction by "smoothing" the airflow, or keeping it laminar over larger parts of the aircraft's surface profile. This is accomplished by airflow suction through microperforations in the leading edge of the wing
surface (see Aerospace Engineering, Jan/Feb 1999).
Other areas of the aircraft are being studied for their contribution to aerodynamic noise, particularly the landing gear and flaps. Among these tests is the installation of fairings around a full-scale A320 main landing gear. The results of these experiments show that aerodynamic noise from the main landing gear can be reduced by 5-10 dB. Flap noise during landing is also being studied using a full-scale model of an A321. Results from the study have potential applications on the A340-500/-600.
A fast propeller model is tested in the S1 wind tunnel at ONERA's Modane-Arieux Center.
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Because of the attention being given to aerodynamic noise, ONERA, a cooperation between France's National Office for Aerospace Study and Research and Aerospatiale, is conducting its own study using a 1:11 scale model of an A320/321 in the CEPRA 19 anechoic wind tunnel. Acoustic imaging, which was initially planned for the wind tunnel tests, has recently been extended to the study of noise sources during aircraft fly-bys.
"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," explained Alain Julienne, head of the flow-generated noise unit at ONERA. "One can then see an acoustic image of the wing surface of the model in a landing configuration, that is with high-lift flaps extended. This result, obtained for a frequency of 2500 Hz, 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, in the region of the leading edge slats, and in the flaps."
According to ONERA, the primary cause of noise in turbojets with high bypass ratios comes from the fan. Acoustic research and the means to reduce noise fall into two main categories—modifying the noise sources themselves, and the absorption of sound waves as they propagate through the nacelle. The noise spectrum generated by a fan essentially consists of spikes, which makes it particularly obtrusive, at the harmonic frequencies of the movement of the blades. The acoustic waves are created by the rotor and its interactions with the stator.
Advacements made in computational fluid dynamics are making it possible to accurately calculate the aerodynamic field at and around the blades, which contains the noise sources. ONERA is currently investigating the possibilities offered by the Kirchhoff method of noise prediction, which consists of substituting the real sources with a distribution of virtual sources on a closed surface surrounding the volume of the real sources.
Tests are being performed on models within the framework of the European RESOUND program, which began in early 1998. "ONERA's objective is to evaluate a new way of reducing noise by active flow control," said Serge Lewy, Manager of Rotating Machinery Acoustics. "It means carrying out a basic experiment, which involves creating, with the help of rods, an acoustic wave with a phase opposite to that from the interaction between the rotor and stator."
An aerospace equipment supplier located in France and the United Kingdom, Hurel-Dubois, is studying methods of noise reduction in aircraft. "Our engineers wanted to develop absorbent cladding for the internal walls of nacelles, making it possible to obtain, at the lowest cost, a reduction of about 10 dB in engine noise," said Christophe Kauffmann, head of the Technology Department at Hurel-Dubois. "This cladding is generally made of composites and consists of a perforated skin positioned in front of one or two honeycomb structures. Their acoustic attenuation operates on the principle of Helmholtz resonators. The main parameters that are possible to control are the number and diameter of holes, bearing in mind that these perforated skins can contain up to 50,000 holes/m2.
"This research led to the development of software for calculating acoustic attenuation, making it possible to study the relative influences of the different parameters. 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, which includes numerous innovations. This equipment is used today extensively on the whole range of our products, which are fitted to Airbus (A320, A330, and A340), Boeing (747 and 767), Gulfstream V, and Bombardier Express Global aircraft." Powerplants used by these aircraft include CFM (A320 and A340), Rolls-Royce (A330, B-747, and B-767), and BMW Rolls-Royce engines (Express Global). Quiet Technology Venture, and American company based in Miami, Florida, has chosen Hurel-Dubois to design and manufacture conversion kits for the DC8-50/61 aircraft.
Socata's efforts in noise reduction have been aimed at sources such as propellers and exhaust. The TRINIDAD TP 20 aircraft's exhaust silencer is one example of this solution.
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Socata, part of the Aerospatiale group, has for several years been studying methods of noise reduction for tourist aircraft. A light aircraft's acoustic spectrum during a fly-by contains spikes emerging from a broadband component. According to Philippe Matharan, Pre-production Research Programs Manager, this type of spectrum can be explained by different sound sources such as the propeller, which produces a spiky sound spectrum with frequencies that are characteristic of the rotation regime and the number of blades; the exhaust, which also generates a spiky sound spectrum that has frequencies characteristic of the regime and the number of cylinders; and lastly the aerodynamic noise, which is broadband and caused by the interaction of the airflow with the fuselage, and by the vortex created at the trailing edges of the blades. Of these sources, the major contribution of noise comes from the propeller and exhaust.
Eurocopter has incorporated several design changes into the EC 120 helicopter for noise reduction.
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Eurocopter is searching for ways to reduce aircraft noise, particularly in the "Fenestron" tail rotors. The first stage involves reducing the high-level pure sound while introducing into the rotor design a change in the blade distribution, a reduction in the circumferential speeds, and thin-blade stators. These changes were incorporated into the EC 120, EC 135, and EC 155.
Frank Bokulich
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