Tail buffet of the F/A-18 at high incidence with sideslip and roll

Abstract

In an earlier paper, the variations of the mean and rms normal force, bending and torsional moment coefficients on the vertical fin of an F/A18 air& for different orientations and Mach numbers were examined by integrating the measured pressure dilke~ across the fin’s surfaces. This paper examines statistically the fin loading with particular attention paid to the relationships between the normal force coefficient and the local surface pressure fluctuations. Space-time pressure correlation studies were conducted to estimate the average convection velocities and paths of the coherent turbulent structures over the two sur&es of the fin. Power spectra for the force and moments were also computed. Within ranges of small to nxxierate sideslip and roll angles, all spectra contained distinct peaks at the same nondimensional 6equency for a tixed Mach number and angle of attack Only results at an incidence of 30” are reported herein, for which this nondimensional frequency is shown to decrease as the Mach number increased from 0.25 to 0.80. At large positive roll angles (about 300). distinct peaks were no longer identitiable. The correlations and spectra. as well as coherence functions between the buffet load and the measured pressure fluctuations demonstrated that the unsnxtdy loading on the fin is due primarily to the passage of coherent urbulent eddies over the fin’s surfaces. The general conclusion of the present work is that asyumxtry in the aircraft’s orientation has a significant effect on buffet loads.