We consider the problem of active reduction of the structural vibrations induced by the sloshing of large masses of fuel inside a partly full tank. The proposed study focuses on an experimental device mimicking an aircraft wing made of an aluminium rectangular plate equipped with piezoelectric patches at the clamped end and with a cylindrical tip tank, more or less filled with liquid. After deriving a representative finite-dimensional model of the complete system, containing the first five structural modes of the plate and the first three liquid sloshing modes, a controller is computed. Since our main scope is to control the most energetic mode of the structure, a full state-feedback method coupled with an observer is used. Finally, the controller is also tested for different initial conditions/perturbations and the results are compared with the ones obtained with an H-infinity controller. Experimental results illustrate the relevance of the chosen strategy.