Preliminary Testing of Cabin Noise Reduction in Helicopters Using Active Gearbox Struts

The paper presents the preliminary testing results of an active gearbox strut control system on a helicopter mock-up, with the goal of improving tonal vibration reduction, under real operative conditions. The gearbox is driven by two electric motors at the nominal rotating speed and a suitable aerodynamic brake is attached to the rotor must to reproduce the load effect of the rotor blades. The normalised FXLMS adaptive control algorithm was implemented to control the rear struts, each equipped with three pairs of piezoelectric patches. The reference signal was supplied by accelerometers positioned on the upper gearbox attachment. Error signals were derived by accelerometers positioned both outside the cabin, on the lower struts attachment devices, and inside the cabin. Appropriate band-pass digital filters were designed and used to limit the required control voltages. Different control configurations, in terms of number and position of actuators and sensors, were tested. Advantages and limitations were discussed, as well as power actuation requirements and convergence rate. In these preliminary tests, only one tone at a time has been considered; results showed that it is possible to reject large-Amplitude high frequency tonal vibration disturbances.