Forghieri A., Dozio L., Ghiringhelli G., Cenedese F.

The paper presents the preliminary testing results of an active gearbox strut control system on a helicopter mock-up with the goal of improving the interior acoustic comfort. The experiments were performed on a full-scale mock-up consisting of a gearbox housing connected to a middle-size fuselage via two front and two rear struts and an anti torque plate. The mock-up is provided with two suitable electric motors that drive the gearbox at the nominal rotating speed. A suitable aerodynamic brake is attached to the rotor must in order to reproduce the load effect of the rotor blades. Peaks at the gear meshing frequencies reveal amplitudes very close to the ones acquired during in-flight measurements. After extensive laboratory tests carried out on an isolated active strut, the normalised FXLMS adaptive control algorithm was implemented directly on the helicopter mock-up rear struts. Each strut was equipped with three pairs of piezoelectric patches. The reference signal was supplied by accelerometers positioned on the upper attachment device between the gearbox and the struts. Error signals were derived by accelerometers both positioned outside the cabin in the lower struts attachment devices and inside the cabin. Appropriate band-pass digital filters were designed and used in order to limit the required control voltages. Different control configurations, in terms of number and position of actuators and sensors, were analysed. For each configuration advantages and limitations were discussed and comparison was made with respect to the power actuation requirements and convergence rate. In these preliminary tests only one tone at a time has been considered. Results of applied control revealed that it is possible to reject large-amplitude high frequency tonal vibration disturbances. Moreover the cabin noise was affected by the structural control action and satisfactory reductions in the zone where the head of passengers are assumed to be were observed.