Redesigning the F-35 to significantly remove weight changed how the aircraft goes together. The biggest change affects the mating of wing to fuselage.
In the original design, of which aircraft AA-1 is the only example, the Lockheed Martin-built wing fitted into a notch in the Northrop Grumman-assembled centre fuselage, resulting in a horizontal join called a “waterline” mate.
The STOVL weight attack team changed the mating concept. Beginning with aircraft BF-1, the notch has gone and a new centre wing section runs full depth from bulkhead to bulkhead, resulting in vertical “buttline” splices with Northrop’s revised centre fuselage and BAE Systems’ aft fuselage.
The original mating concept was the result of significant risk-reduction work during the JSF concept demonstration phase, where the focus was on commonality and affordability. “The waterline mate carried a lot of inside weight in the joint. The buttline mate is lighter, and easier to assemble,” says Tom Burbage, general manager F-35 programme integration.
“With the waterline mate, we thought we were making an improvement in cycle time. But when we thought it through it did not have the benefit expected,” says Bobby Williams, air vehicle team lead. “The change affected where the aircraft mated, and how work was distributed. The lower part of the centre fuselage became part of the Lockheed Martin Fort Worth work.”
Edward Linhart, Lockheed vice-president F-35 production operations, adds: “It is a lot cleaner sequence of events. It saves time in mating, but added time in the wing. The centre fuselage used to be the long pole, now it is the wing.”
To save weight, the original concept of “hard” joins between subassemblies gave way to “soft” splices, where structure interleaves during mating.
“The aft/centre mate was an almost solid interface. We went to a softer interface, which is more difficult to manufacture but cuts down weight,” says Tom Fillingham, BAE F-35 programme manager.
The wing is no longer built as a single piece, but the outer wingboxes are spliced into the centre section during assembly. “It’s a soft splice. They’re not bolted on (as in the F-16) – you can’t undo them,” says Linhart.
The single-piece composite upper wing skin was one of the signature advanced manufacturing features of the original F-35 design.
Now it has no fewer than seven pieces. “It is now much more traditional,” says Williams. “But is also improves supportability.”