Spatial Characteristics of F/a-18 Vertical Tail Buffet Pressures Measured in Flight

Buffeting is an aeroelastic phenomenon which plagues high performance aircraft, especially those with twin vertical tails, at high angles of attack. Previous wind-tunnel and flight tests were conducted to characterize the buffet loads on the vertical tails by measuring surface pressures, bending moments, and accelerations. Following these tests, buffeting estimates were computed using the measured buffet pressures and compared to the measured responses. The estimates did not match the measured data because the assumed spatial correlation of the buffet pressures was not correct. A better understanding of the partial (spatial) correlation of the differential buffet pressures on the tail was necessary to improve the buffeting estimates. Several wind-tunnel investigations were conducted for this purpose. When combined and compared, the results of these tests show that the partial correlation depends on and scales with flight conditions. One of the remaining questions is whether the windtunnel data is consistent with flight data. Presented herein, cross-spectra and coherence functions calculated from pressures that were measured on the high alpha research vehicle (HARV) indicate that the partial correlation of the buffet pressures in flight agrees with the partial correlation observed in the wind tunnel. ________________ Copyright © 1998 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. Introduction For high performance aircraft at high angles of attack, vortices emanating from wing/fuselage leading edge extensions (LEX) often burst, immersing the vertical tails in their wake, as shown in Figure 1 for the F/A-18. The resulting buffet loads on the vertical tails are a concern from airframe fatigue and maintenance points of view. Previous wind-tunnel and flight tests have been conducted to quantify the buffet loads on the vertical tails of the F/A-18. Figure 1. Flow Visualization of Leading Edge Extension (LEX) Vortex Burst, 30 Degrees Angle of Attack The spectral nature of the unsteady differential pressures on the F/A-18 vertical tail caused by a burst LEX vortex are well documented. As illustrated in Reference 1, the power spectral densities and root mean square (rms) values of the differential pressures vary with flight speed, angle of attack, dynamic pressure, and tail position. The