Comparison with Flight Tests

Colvin, et. al. report maximum tail buffet at 22° angle of attack for full-scale flight tests and for a scale model with elastically scaled tails.  Comparing the GT model's maximum spectral peak intensity with angle of attack, we get the axis on the right in the figure.  At angles of attack less than 22°, this comparison suggests that low intensity but narrow-band fluctuations drive the tails to their maximum buffet.  Also, buffet is actually reduced as spectral intensity dramatically increases at higher angles of attack.  This illustrates the need to measure the velocity fluctuations present in the vertical tail flow field on an F-15.  Tail accelerometers measurements are dominated by the response motion of the structure.  Tail fluctuating pressure data also are dominated by the response tail motion.  Off body velocity measurements will show the fluctuations that are the primary drivers of tail buffet.

 F-15 Tail Buffet Alleviation Results
Introduction:    Spectral Amplification:    Centrifugal Instabilities:
Reduced Frequency Scaling:    Linear Peak Frequency/Velocity Slopes:    Models Tested:
Empirical Peak Frequency Data:    Surface Fence Tests:    Reduction in Spectral Intensity:
Reductions Over all Angles:    Comparison with Flight Tests:    Summary:
Future F-15 Wind Tunnel Tests: