Contributions to Active Buffeting Alleviation Programs by the Nasa Langley Research Center

Buffeting is an aeroelastic phenomenon which plagues high performance aircraft, especially those with twin vertical tails like the F/A-18, at high angles of attack. This buffeting is a concern from fatigue and inspection points of view. By means of wind-tunnel and flight tests, this phenomenon is well studied to the point that buffet loads can be estimated and fatigue life can be increased by structural enhancements to the airframe. In more recent years, buffeting alleviation through active control of smart materials has been highly researched in wind-tunnel proof-of-concept demonstrations and full-scale ground tests using the F/A-18 as a test bed. Because the F/A-18 resides in fleets outside as well as inside the United States, these tests have evolved into international collaborative research activities with Australia and Canada, coordinated by the Air Force Research Laboratory (AFRL) and conducted under the auspices of The Technical Cooperation Program (TTCP). With the recent successes and advances in smart materials, the main focus of these buffeting alleviation tests has also evolved to a new level: utilize the F/A-18 as a prototype to mature smart materials for suppressing vibrations of aerospace structures. The role of the NASA Langley Research Center (LaRC) in these programs is presented. Introduction The Problem For high performance aircraft, such as the F/A-18, at high angles of attack, vortices emanating from wing leading edge extensions (LEX) often burst, immersing ______________________________________________ Copyright © 1999 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U. S. Code. The U. S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. the vertical tails in their wake (Figure 1). Although these vortices increase lift, the resulting buffet loads on the vertical tails are a concern from airframe fatigue and maintenance points of view. Figure 1. Flow Visualization of Vortex from the LEX Bursting ahead of the Vertical Tail (Photograph Courtesy of the NASA Dryden Flight Research Center) Previous Investigations Previous wind-tunnel and flight tests have been conducted to quantify the buffet loads on the vertical tails of the F/A-18. These tests were designed to characterize the flow mechanism and to quantify the unsteady differential pressures acting on the vertical tails during high-angle-of-attack maneuvers. The major findings of these tests were: 1) that the buffet pressures vary with flight conditions; 2) that the buffeting (response of the tail) varies with flight conditions; and 3) that the frequency spectra scale with Strouhal number. Later comparisons among pressure data from reduced-scale wind-tunnel, full-scale windtunnel, and flight tests revealed that the (spatial) correlation time delays as well as the frequency spectra scale with Strouhal number.