Reduced Frequency

A reduced frequency is defined to represent the linear peak frequency/velocity relationship for each angle of attack.  Reduced frequency is defined as the slope of this linear relationship multiplied by a representative model length scale.  The fictitious model centerline chord (with wing leading and trailing edges extended to the model centerline) is used as this length scale.  This is chosen based on comparisons between F-15 scale models and F-15 planform, flat-plate delta wings.  This dimension takes into account wing sweep and wing span variations between configurations.  F-15 reduced frequency data is shown in Figure 3.  Reduced frequency is seen to decrease with angle of attack.  These results show that at some certain flight conditions, the flow nearly periodic excitation frequency will be "tuned" to a given structural mode.  There are numerous variations in freesteam speed and angle of attack in which tuning can occur.  Thus, these results can be used to empirically predict the flight conditions where the flow excitations are tuned with a given structural mode frequency.       4.7% F-15 Torsion

 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