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