Experimental test of semi-active shunt damping on a helicopter trim panel

Abstract

The increased need for lightweight structures in modern aerospace transport due to lower fuel consumption and increased transport capacity reveals an acoustic challenge. Because of the high stiffness and low mass the coincidence frequencies of lightweight structures are low compared to classical aluminum structures. That is the reason for a weak transmission loss of lightweight structures made of glass or carbon fiber reinforced plastics. Therefore, passive solutions are widely investigated to increase the transmission loss of lightweight helicopter trim panels. For example, in the European GARTEUR AG 20 "Cabin internal noise: simulation and experimental methods for new solutions for internal noise reduction", two helicopter trim panels are studied which are structurally optimized for an increased TL. In this paper the applicability of a semi-active shunt damping system to this structurally optimized helicopter trim panel is investigated experimentally. Prior to this, the semi-active shunt damping performance is studied on a classical steel plate and afterwards the damped helicopter trim panel is investigated in a transmission loss facility. Two experiments are conducted. First of all, the trim panel is excited with a point force and the surface vibrations are measured with a laser scanning vibrometer. In the second test the trim panel is excited with a diffuse sound field. It can be seen that the negative capacitance shunt is less effective on a higher damped structure. Global vibration reductions of only $3-4\usk\deci\bel$ are achieved during the shaker test. Concerning the transmission loss, the results are comparable to the vibration reduction. The main reason for this is supposed to be a lower piezoelectric coupling to the sandwich panel.