F-16 Flutter Suppression System Investigation Feasibility Study and Wind Tunnel Tests

A study was conducted to determine the feasibility of employing active controls on the F-16 to suppress wing- store flutter for several external store configurations. It was determined that the existing flaperons, with modifications to the integrated servoactuators, were effective in suppressing flutter. The F-16 flutter model was tested with active flaperons. Open-loop frequency response functions (FRF's) were successfully measured in the wind tunnel environment both with the feedback loop physically opened and with the loop closed. These measurements provided guidance in the selection of sensor locations and feedback control laws to suppress flutter. Control law variations were made to obtain the desired FRF characteristics. A 100% increase in dynamic pressure above the flutter dynamic pressure was demonstrated. IGHTER aircraft are required to carry a very large number of external store configurations. The probability is high that at least a small subset of these configurations will flutter within the desired operational envelope of the airplane. This probability is further increased as new stores are added to the operational airplane inventory. When wing-store flutter problems occur, the solution requires a modification of the airplane (usually a change in stiffness and weight) and/or a speed restriction which reduces the operational envelope of the airplane. Flutter suppression with active controls is another solution which has been investigated in recent years. Several approaches have been taken to design these sys- tems.1-3 One of the more promising approaches to the design of calibrated active control systems is the frequency response method. This approach is attractive because of the state of development of methods for computing oscillatory aerodynamic pressure distributions, because frequency response functions (FRF's) are experimentally measurable, and because the military specification for the stability of aeroservoelastic systems is based on the Nyquist criteria. This method was applied in a flutter suppression system (FSS) feasibility study4 for the F-4. The results of this study indicate that the milder flutter modes and the lower flutter frequencies associated with wing-store configurations could be relatively easily suppressed with active controls. The initial design approach for the active control system for the YF-17 semispan flutter model was the root locus method.5 However, a modified Nyquist approach was subsequently used.6 The Nyquist stability criteria has been applied extensively by Turner.7 The Nyquist criteria was employed as the principal design tool in the F-16 FSS investigations. For the system with negative feedback shown in Fig. 1, the transfer function for the closed loop system is