Boeing and US Navy say aircraft's transonic wing-drop problems resolved and flight-control software upgraded

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 flight- control 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 gun-tracking 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 wing-drop 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 1g 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, 30°/s "pirouette" turn resembling the "helicopter gun" turn perfected by the thrust-vectoring X-31.


Source: Flight International