Keywords: aeroelasticity, flutter, sandwich, cork, piezoelectric, control.

Several passive solutions such as strengthened materials and mass balancing have been developed to prevent the hazardous phenomenon of flutter [1]. It is believed that cork based materials may have a contribution in flutter suppression due to its natural damping characteristics [2]. When changes in the structure or in the aerodynamics are not viable for flutter prevention, the use of active materials becomes a good option. Recent investigations [3,4,5,6] confirm the advantages of using adaptive structures for this purpose, the most part combining high performance composite materials with bonded smart actuators, such as piezoelectric materials. When a command signal is applied to the piezoelectric actuators, these will exert control over the damping and stiffness properties of the component.

In this work both passive and active flutter suppression concepts for aeronautical components were studied. The feasibility of using a cork agglomerate combined with carbon reinforced plastics in a sandwich structure in order to increase structural eigenvalues and, consequently, critical flutter speed and frequency was evaluated by numerically comparing the passive flutter behaviour of a cork sandwich plate with conventional carbon-epoxy and aluminium plates. It was shown that the cork sandwich plate can provide the same flight envelope for a lighter structure. This is due to the fact that the second moment of area is increased by a thicker structure without compromising the weight of the structure.

An active solution was also studied using ANSYSRtransient analysis. A macro was built in order to actuate the piezoelectric element with a fixed voltage but out of phase depending on the oscillation of the plate. This actuation causes a decrease in the oscillation amplitude as if the structure became stiffer. Thus, considering an equivalent stiffness plate, the data was exported to ZAERORin order to compute the increased flutter speed due to actuation. A 20% increase in flutter speed was achieved using a voltage of 250 V, the maximum allowed by the piezoelectric patches used in this study.

1

J. Heeg, "Analytical and Experimental Investigation of Flutter Suppression by Piezoelectric Actuation", NASA Technical Paper 3241, March 1993.

2

L. Gil, "A cortiça como material de construção - Manual Técnico", APCOR, 2007.

3

T. Costa, "Estudo numérico de uma asa com controle active de flutter por realimentação da pressão medida num ponto", PhD Thesis, São Paulo University, São Carlos, 2007.

4

A. Suleman, A.P. Costa, "Adaptive control of an aeroelastic flight vehicle using piezoelectric actuators", Computers and Structures, 82(17-19), Computational Mechanics in Portugal, 2004. doi:10.1016/j.compstruc.2004.03.027

5

F. Canfield, D. Morgenstern, D. Kunz, "Alleviation of buffet induced vibration using piezoelectric actuators", Computers and Structures, 86, 281-291, 2008. doi:10.1016/j.compstruc.2007.02.027

6

C. Nam, T.A. Weisshaar, Y. Kim, "Optimal Sizing and Placement of Piezo Actuators for Active Flutter Suppression", '95 SPIE Conference on Smart Structures & Materials, San Diego, CA, Feb. 1995. doi:10.1117/12.208282

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