ThePointblank wrote:These are also present on A and B models as well and are called LEX fences:
They were installed in response to an issue that was causing premature cracks and fatigue in the vertical stabilizers. This structural damage limited the first batch of planes to a few hundred flight hours, as opposed to the several thousand flight hours the Navy required for the service life of its aircraft. The cause of the structural cracks was eventually traced back to the LEX vortices impacting on the vertical tails and creating loads the tails weren't designed to handle.
Specifically, the issue was due to a phenomenon called vortex bursting. As a vortex travels downstream, it enlarges and becomes weaker. If the rotational velocity of the vortex drops low enough, the increasing pressure within the vortex causes it to lose its tornado-like structure and break apart, and it was found that this was occurring just forward of the vertical stabilizers. This was causing extreme buffeting on the vertical stabilizers, and it just happened that frequency of loads induced by vortex bursting coincided with the natural frequency of bending in the vertical tail.
It took a team from NASA using the F/A-18 High Alpha Research Vehicle (HARV) to figure out what was going on; NASA conducted a series of test flights using their HARV and injecting smoke into LEX vortices to observe the vortices in flight, which identified the issue.
To deal with the the vortices, McDonnell Douglas implemented a number of design changes to the F/A-18. The first is most visible; a pair of LEX fences were installed on the LERX, which creates a second unsteady vortex that interacts with the vortex created by the leading edge extension. This interaction strengthens the rotation of the main vortex so that vortex bursting is eliminated in the vicinity of the vertical stabilizers and actually improved the Hornet's performance under high angles of attack.
The second big change were a series of L brackets installed on the vertical stabilizers where they meet the fuselage to provide increased structural strength.
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