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LCO amplitude but not the mechanism. The LCO amplitude typically becomes better as tank
fuel is burned and if the AIM-9’s at stations 2/8 are not present.
The second primary type of LCO is for configurations with long stores at stations 3/7 and
empty wingtip launchers. This mechanism is less significant operationally since it only occurs
with empty wingtip launchers. Not all of the loadings are restricted but many experience LCO to
some degree. The characteristics of the mechanism are only antisymmetric and occur around 7-
9Hz. The driving LCO mode is a result of the coupling of store pitch and antisymmetric first
wing bending modes. The presence of other stores affects the LCO amplitude but not the actual
mechanism. The primary parameter is pitch inertia of station 3/7 stores, which controls the
antisymmetric store pitch frequency. Proximity to the antisymmetric wing bending frequency is
critical.
Current LCO Analysis Techniques
Classical flutter analyses are predominantly used to calculate LCO onset for all the
permutations of a given store configuration. 15-19 These analyses combine 14 a linear structural
solver with a doublet lattice 20 (subsonic) or ZONA51 21 (supersonic) aerodynamic method
creating a completely linear method for predicting an inherently nonlinear LCO phenomenon.
This methodology is often good in predicting frequency, but can be off by as much as 200 knots
in predicting onset speed. 7,12,13 These large errors force flutter engineers to rely more on
experience and interpolation from similar configurations to design flight test parameters. This
uncertainty requires vast resources for each flight test to ensure adequate safety margins. 22 This
results in a large number of test points required to clear the margin of safety expanded
operational flight envelope, extending the flight flutter testing time, and therefore elevating the
associated costs.
25