Summary

In High Performance Twin-Tail Aircraft (HPTTA), buffet induced tail vibrations occur when unsteady pressures associated with separated flow, or when vortices, excite the vibration modes of the vertical fin structural assemblies. At high angles of attack, flow separates at the leading edge of the wings, and vortices are generated at different locations such as the wing fuselage interface or the leading edge extensions. These vortices are convected by the geometry of the wing-fuselage interface toward the twin vertical tails. This phenomenon, along with the aeroelastic coupling of the tail structural assembly, results in vibrations that can shorten the fatigue life of the empennage assembly and limit the flight envelope due to the large amplitude of the fin vibrations.

The main goal of this research work is to develop an active buffet alleviation system for high performance twin-tail aircraft using piezoceramic stack based actuators in combination with Acceleration Feedback Control (AFC) theory. In order to complete this task, the work is divided into three main research areas.

For the acceleration feedback control work, new methods for the design of the controller parameters are presented for generalized single degree of freedom systems. These new designs are based on pure active damping and quadratic performance criterions, which are based on structural generalized coordinates, minimization. Then, non-collocated acceleration feedback multi-mode controller design methods are developed for a single sensor and a small number of actuator arrays (for Multi-Input Single-Output systems).

Then, a new type of moment inducing actuator, the Offset Piezoceramic Stack Actuator (OPSA), which is based on the use of piezoceramic stacks, is developed to provide the needed control authority for buffet alleviation. This actuator is also designed to satisfy high reliability and maintainability requirements. In addition, a technique is developed to analytically model the actuator on the basis of the modal expansion of the offset piezoceramic stack actuator driving a benchmark structure. The results of this analysis are used to create a low frequency approximation of the offset piezoceramic stack actuator as well as to optimize its offset distance and its placement.

Because of the non-availability of reliable mathematical or numerical models for both the controlled structure and the buffet induced loads, a control system design method, which is based solely on the use of experimental data, is first developed. Then, two sets of experiments are conducted to show the feasibility of controlling the buffet induced vibrations during high angle of attack operations of a selected HPTTA. The first experiment validates both the effectiveness and the robustness of the developed active buffet alleviation system on an aeroelastically scaled model in wind tunnel studies. The second experiment shows that the combination of Offset Piezoceramic Stack Actuators and Acceleration Feedback Control can suppress vibrations in a full-scale vertical tail sub-assembly.