Researchers at German Aerospace Center, Deutsches Zentrum für Luft-und Raumfahrt (DLR), have conducted a first acoustic measurement campaign in order to obtain noise measurements inside a helicopter engine.
As part of the study, researchers from the division of engine acoustics at the DLR Institute of Propulsion Technology in Berlin have used new hot gas microphone probes particularly designed for studying the processes that contribute to noise generation.
Earlier, the helicopter noise reduction primarily focused on the attributes, configuration and number of rotor blades.
As relevant measurement technology did not exist previously, studying the sources of noise inside the engine has not been possible.
DLR Institute of Propulsion Technology engine acoustics department head Lars Enghardt said the first major challenge faced by the researchers was to develop measurement technology that would withstand the conditions inside an engine.
The conditions inside an engine include high pressures and temperatures, heavy variations in temperature and limited space for installing microphones.
"Using the probes and temperature sensors we have developed, we have been able to carry out research in the hot gas area of a turbine for the first time, and thus taken acoustic measurements of a helicopter engine," Enghardt added.
Research was completed in January this year as part of the EU-funded turboshaft engine exhaust noise identification (TEENI) project, which was held at the premises of engine manufacturer Turbomeca in Bordes, France.
It involved installing a series of microphone probes inside the engine and across the exhaust area and simultaneously recording their signals.
When the signals received by the internal and external sensors are linked to each other, the structure formed enables conclusions to be drawn with respect to the sound sources within the engine, and on the intensity levels of the noise being emitted into the open air.
The measurement allows researchers to study on noise-reduction steps such as low-noise design, or to make structural alterations to the engine.
The follow-up project has already started, the EU research on core noise reduction (RECORD) project is being coordinated by the engine acoustics department.
This involves experiments to study noise generation mechanisms in the engine core, and also develop prediction methods and noise reduction techniques.
Some 19 European partners, including eight European engine manufacturers and seven universities, will be working on the project for three years.
Image: In order to measure the noise inside the helicopter engine, the researchers used hot gas microphone probes. Photo: courtesy of DLR.