The European Union’s research program on noise reduction, Silence(r), officially ended in June with promising results. It explored all noise sources, from engines to landing gear and flaps. However, although it achieved a reduction of five decibels in aircraft noise, several more leads need to be developed to reach the ambitious target of cutting a full 10 dB from average noise levels by 2020.
Silence(r) brought together 51 companies–including Airbus, Rolls-Royce, MTU Aero Engines and Snecma–as well as various research centers and universities. The project’s overall budget was ?112 million ($150 million), about half of which the European Commission funded. Snecma’s Eugene Kors was the program coordinator.
The engine nacelle has been the focus of a number of studies and prototypes. For example, the study established that a micro-drilled liner conventionally used in the air intake–basically, the cylinder upstream from the fan– can be extended to the nacelle’s lip. Kors told AIN that this design was more difficult to achieve than imagined because of the double curvature. Although test results have not been completely satisfactory, he expressed optimism about this technology.
One idea that did prove successful was the “negatively scarfed inlet.” The concept is intended to change the directional pattern of the radiated engine noise so that more noise is directed upward and less noise downward. “Such a design is not more expensive,” Kors pointed out. However, although it looks like a straightforward plug replacement on a nacelle, it cannot be retrofitted and a negatively scarfed inlet calls for the design of a completely new nacelle.
The research group also evaluated a zero-splice nacelle section, with the results of those tests being so encouraging that Airbus has speeded up the development and the new nacelles are to enter into service on the A380.
At the rear of the engine, researchers have long been exploring new ways to mix the cold and hot gases that come from the fan and the turbine exhaust, respectively. There is a tradeoff to be established because such mixers often translate into some performance-sapping drag at cruise.
The Silence(r) program also tested a so-called squid nozzle, with an asymmetrical, sawtooth-shaped rear lip on the turbine exhaust nozzle. The shape is relatively different from the chevrons that are in service on Aeroflot’s A321s and the ones that will fly on the Boeing 787. “We did not get down to zero performance loss, but this was a success, nevertheless,” Kors told AIN.
For a long time, noise specialists have understood the engine is no longer the main contributor to noise during the approach phase of flights. In fact, the airframe is now the main culprit, along with the devices pilots extend to prepare for landing–flaps and landing gear. This has prompted the idea of fitting landing gear with fairings that change the aerodynamics to create a quieter air flow. For example, one aim of these fairings is to suppress whistle effects.
These units, which fold to avoid interference with the complex movement of the extending or retracting gear, have been tested on an Airbus A340 and, according to Kors, can be easily produced.
At the beginning of the Silence(r) program, there was some concern these noise-reduction devices would reduce landing gear drag, which pilots use to slow speeds during approach. But Kors insisted that tests eventually proved that, “from the pilot’s point of view, there is very little impact on drag.”
In total, the Silence(r) programs carried out tests on some 35 prototype units to check 10 different noise-reduction technology concepts.
The tests allowed long-proposed active-noise technologies to be studied, but many positive results have proved quite challenging to implement. One problem is the weight of the active-noise actuators (in short, the speakers). Another is that the engine noise itself is loud, requiring powerful piezo-electric actuators to counter it (piezoelectricity is the ability of some materials to generate an electric potential in response to applied mechanical stress). Moreover, engine vibrations and high temperatures are tough on electronic components.
Nonetheless, Silence(r) researchers have tested a hybrid noise absorber. For high frequencies, it works like a conventional passive unit, but it also has active-noise speakers mounted in it for low frequencies. The main benefit is the reduced thickness of the resulting liner.
Another active-noise technology the study investigated related to the tone generated by an engine’s blade-passing frequency. It employed some 120 error-measuring microphones to close the noise feedback loop and integrated these in a tape to ease installation inside the fan case.