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It is interesting to note that there are recent instances in flight testing where two flight tests
with nearly identical configurations (except for slight aerodynamic differences with the
underwing missiles) display a dramatic disparity in LCO response characteristics. It appears that
the differing LCO responses can be attributed to subtle aerodynamic differences in the
underwing missiles. These unexpected flight test results have lead to the accidental discovery of
a previously unknown LCO sensitivity.
Previous LCO Research Techniques
It is widely accepted in the flutter engineering community that LCO is triggered by the
same mechanisms that initiate flutter and that the nonlinear interaction of the structural and
aerodynamic forces acting on the aircraft structure produce finite amplitude oscillations, rather
than the unbounded oscillations predicted by dynamically linear flutter theory. However, there is
substantial disagreement as to which of these sources is the major contributing factor to the
bounding mechanism. 27 There are many approaches to investigating the cause of LCO. Most of
them will be covered in this section.
GVT
As of yet, there is no direct evidence for any structural nonlinearities in the F-16. That is,
no ground vibration test (GVT) is reportedly conducted at various excitation levels to test for
stiffness or damping nonlinearities. However, it is known that the connection between the store
and pylon is inherently nonlinear. Northington 28,29 is currently performing nonlinear structural
deflection testing in order to explore structural nonlinearities and hysteretic behaviors
contributing to the LCO mechanism.
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