MODIFICATION
Fixes boost Super Hornet agility
Boeing and US Navy say aircraft's transonic wing-drop problems resolved and flight-control software upgraded
http://www.flightglobal.com/pdfarchive/ ... 01843.html
Boeing and the US Navy have
developed what appears to be a
final fix for the F/A-18E/F Super
Hornet's long-running transonic
wing-drop problem and
have successfully flight
tested a new flightcontrol software package to
improve spin recovery and provide
more combat manoeuvrability.
The wing-drop problem cropped
up during Phase 1 testing as early
as July 1997 when test pilots experienced
uncommanded bank angle
changes during manoeuvring flight
at transonic speeds. This meant
pilots could not complete guntracking in air-to-air combat,
and was only 80% cured through
adjustments to the flight-control
system and rescheduling of the
leading-edge flaps.
After a variety of changes,
including single and multiple vortex generators,
vortilons and wing fences, the investigations focused
on the wingfold where a discontinuity
between the 11% thickness
ratio of the wing around the hinge
line and the 7% ratio of the wing
caused a flow disruption. Flight
tests later showed that the wingdrop
problem vanished when the
door covering the wingfold was
removed, leading to the adoption
of a "porous" door as a solution.
However, in service the porous
door has caused maintenance problems, with its many small holes
often becoming clogged with dirt
and paint, and the resulting aerodynamic benefits disappearing. The
door also generated problems of its
own, including early, heavier buffet
onset at lower angles of attack (AoA).
Flight tests to follow a more permanent fix began with a second
phase in late 2000 and culminated
with the development of a package
of improvements that have been in
flight test from August 2003 to the
present. Although some testing
continues, the baseline improvements are now being introduced to
the fleet as well as to new production aircraft.
They include a sawtooth leading-edge flap that
reduces the buffet problem, and
the resealing of the wingfold hinge
door. The most significant new
external feature is a 127mm (5in)-
high, full-chord wing fence at butt
line 152, which alters the flow and
prevents the wing drop as well as
reducing buffet onset and intensity.
"That's the biggest selling
point,"
says the US Navy test team,
which points out the fixes add an
extra 3° AoA and up to 3g additional
margin before buffet onset.
"The result is better for weapons, as
well as the lifetime of the aircraft,
which could see a 10 to 15 times
increase for some parts," it says.
Drag effects are minimal and are
compensated for by improvements
in trailing-edge flow separation.
Upgrades to the flight-control
software have also been introduced
with version 34-1 to cure some
"nagging issues" discovered during
fleet use. These have cropped up on
at least four known occasions
when pilots have inadvertently
entered inverted spins following
tail slides. The fix was considered
"high priority", says Boeing Super
Hornet test pilot Ricardo Traven,
who adds that the goal of the
programme was "
better inverted spin
resistance, faster spin recovery and
more manoeuvrability".
The cause was discovered to be a
"turbo trim" device added to speed
up lg recovery from manoeuvring
flight. "In hands-off mode, this
device was essentially pitching the
aircraft into a spin," says Traven.
The turbo trim feature was
removed and the spin recovery
logic altered, as well as several
other changes made. A bonus is
added manoeuvrability at high
AoA and a yawing, 30o/s "pirouette"
turn resembling the "helicopter gun" turn
perfected by the thrust-vectoring X-31.