motivation for this work developed from the tail buffeting problems experienced
by a number of today's twin-tailed fighter aircraft. The two most notable
cases are the F/A-18 and F-15. There is currently a vast data set
available on the subject of vortex burst and its fatigue effects on tail
surfaces. On the F/A-18 at angle of attack, the flow field is dominated
by strong vortices generated over the wing leading edge extensions (LEX).
These strong vortices undergo a rapid expansion and violent break down
at certain flight conditions such that the tails are engulfed in intense
broad-band turbulent velocity fluctuations. The tails are driven
to buffet by this intense fluctuating flow energy. On the contrary,
the F-15 flow field does not have a strong leading edge vortex present,
and the vortex motion present is referred to as wake-like (with axial velocities
in vortex center a fraction of the freestream). In this flow field
where the vortex burst phenomenon is not the dominant feature, nearly periodic
velocity fluctuations are present at the top of the vertical tails.
This much lower intensity flow excitation, due to its frequency variation
with angle of attack and velocity, is presumed to at some flight conditions
"tune" to the tail structural modes. This is seen to be the driver
of tail buffet on the F-15. Methods of predicting these flow fluctuation
frequencies and methods of attenuating their energy are presented
in this work.