Empirical Prediction vs. Test Data

The reduced frequency of the flow spectral peaks has been seen to scale with model size and freestream velocity.  Thus, the empirical reduced frequency found from 1/32-scale F-15 tests is used to predict the peak frequencies for the 4.7% F-15.  The 4.7% F-15 test tunnel dynamic pressure was 56 psf and angle of attack was variable.  Using the 4.7% model length scale and the given tunnel speed, the frequency of the flow fluctuations is predicted.  This is shown in Figure 6.  The peak in the 4.7% 1st torsion RMS occurs at AOA of 24° and at 190 Hz.  The predicted frequencies show that between 22° and 24° AOA the flow would be tuned to this 1st torsion mode.  The 1st bending mode (49 Hz) is much lower than the flow excitations, and the 2nd bending mode (229 Hz) is higher than the flow excitations.  The 2nd bending does come near the flow excitation frequency at low AOA, but the intensity of the fluctuations is very low in this region.  The fact that the reported 4.7% data is near the predicted frequency values is encouraging, but also shows that the given test conditions do not exactly "tune" to the structural mode.  A higher freestream velocity or a intermediate AOA would have precisely tuned the flow to the tail mode and would have generated a higher buffeting response.  These empirical predictions could have been used to better determine the wind tunnel test conditions for investigating the maximum buffeting cases.

 F-15 Tail Buffet Alleviation Results
F-15 Velocity Spectra    Frequency vs. Velocity    Reduced Frequency
4.7% F-15 Torsion    4.7% F-15 Bending    Empirical Prediction vs. Test Data