Helicopter interior noise reduction by active vibration isolation with smart gearbox struts

The paper gives an overview about the development and tests of an active vibration isolation system for the reduction of helicopter interior noise. The interior noise of modem helicopters is still not satisfactory. The most important noise source in helicopter cabins is the gearbox. The rather annoying gear meshing frequencies are usually 10 to 20dB above the background noise level. The frequency range of gearbox noise is typically between 700Hz and 4kHz. Therefore it is difficult to apply active noise cancellation or active structural acoustic control. However, measurements have shown that the structure-born noise path via the gearbox struts is dominant. Therefore it should be sufficient to control the structure-born noise by applying additional control forces to the struts. Piezoceramic shells which are directly glued onto conventional gearbox struts are used as actuators. The main advantages of this smart strut concept are the low space requirements and the low additional weight introduced by this type of actuator. The controller applied uses a feedforward control concept based on the well known filtered-x LMS-algorithm. Accelerometers have been used as sensors. The optimization and verification of the active strut concept has been done by performing multi-axis laboratory tests with an EC135 prototype fuselage. Final tests have been done with a BK117 helicopter which has been fully equipped with smart gearbox struts.