Active Control of Helicopter Gearbox Supports and Effects on Cabin Acoustic Field

In the framework of the European project FRIENDCOPTER, the Department of Aerospace Engineering of Politecnico di Milano is involved in developing active control systems aimed at reducing the vibration transmission from the gearbox to the cabin in medium-size civil helicopters. An AgustaWestland A109MKII mock-up to perform vibroacoustic measurements and control tests. The selected active configuration consists of two smart rear struts with 4 piezoceramic actuators pairs each and a smart anti-torque plate with 6 piezo pacthes on each side. Four independent narrowband FXLMS vibration controllers are designed and implemented to control each beam and the two sides of the plate. From open-loop vibration and acoustic analysis, a set of annoying tones in a frequency band up to 4 kHz are targeted to be reduced. A movable rack of microphones is placed inside the cabin to monitor the acoustic field over a grid spreading the whole cabin width in an area close to the passenger back seats. Vibration reductions up to 20 dB of the target tones are obtained on the accelerometer error sensors. Acceptable noise reductions of a few tones are achieved over certain zones, but the result cannot be extended to the whole cabin. An optimization algorithm to find the best positioning of the error sensors has been also tested to improve acoustic closed-loop performances.