Empirical Peak Frequency Data

An empirical reduced peak frequency ( n=(df/dV)*s ) is defined for each model over the range of angles of attack which spectral peaks were present.  Data for three aircraft models and four delta wing models are given.  Moderately swept configurations (<60°) show a decrease in reduced frequency with increase in angle of attack.  The unique faceted features of the F-117, as well as the high wing sweep, lead to the varied results seen in the figure.  It is interesting to note that spectral peaks exist even when vortex burst location (on 59.3° delta wing) moves from the trailing edge toward the apex.  On the moderately swept wings, no "unburst" strong vortices are present in the flow field, and yet spectral peaks dominate the flow field.  With this empirical data set, the spectral peak frequency can be predicted on larger scale models at higher velocities.  Scaling to full scale appears successful based on F-15 tail buffet data.

 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: