Structural fatigue and damage due to buffeting is a common problem in high performance aircraft and over the years numerous methods to alleviate vibrations have been studied. The F-16 ventral fin was one such structure prone to inflight failures from excessive vibrations attributed to buffeting. Historically, structural modifications are made to provide passive vibration control by altering the design and/or incorporating different materials. While such solutions have been successfully applied, they typically result in an increase in structural mass and cost.
For the F-16 ventral fin, the implemented fix was to redesign the ventral fin and add material to passively suppress the buffeting. However, the use of active control techniques offer the promise of reducing the vibration levels without significantly increasing structural mass and cost. To achieve this, the Department of Aeronautics and Astronautics is conducting research to reduce buffet induced vibrations through the use of active structural control. The research involves embedding a combination of piezo-electric actuators and sensors into the ventral fin to facilitate active vibration control.
During the initial phase of the project called ACTIVE FIN, AFIT in conjunction with the Air Force Test Pilot School, designed and flew an experimental ventral fin to obtain in-flight data. Using the flight test results, the current ffort seeks to improve upon the initial design by increasing the number of actuator layers to increase control authority, and by incorporating multi-input/multi-output (MIMO) control algorithms. The research involved experimental modal identification of the ventral fin along with determination of the principle strain directions, followed by selection of system components, determination of a mathematical plant model, and design and test of candidate control algorithms.
AFIT’s research resulted in a control system hardware and software design suitable for flight, with the ability to test several different control algorithms to include single-input/single-output (SISO) positive position feedback (PPF), multivariable PPF, two-input two-output linear quadratic Gaussian (LQG), and two input four-output LQG. To date, the laboratory results of this study show significant reductions in buffeting at the first four structural modes of the ventral fin using the MIMO LQG controller. These bench-level laboratory results will be validated during wind tunnel testing conducted byAFRL at Wright-Patterson AFB, followed by flight tests on the F-16 at Edwards AFB in California.