Weight estimates 35% over target led to efforts to improve structural efficiency

After an intensive eight-week nose-to-tail rework to improve the structural efficiency of the F-35 Joint Strike Fighter, Lockheed Martin expects the preliminary design review (PDR) to be closed out on 26 June.

Completion of the PDR, which was delayed from March by concerns over the maturity of airframe design, keeps the programme on track for a first flight in late 2005, according to programme general manager Tom Burbage.

The airframe PDR was kept open after weight estimates raised concerns about the maturity of the design. "The first bottom-up roll up of weight came out higher than expected," says Burbage. Typically initial weight estimates are 20-25% over target, but the F-35 came out 35% over. Following the rework, weight is now within 10% of target. "That's about where we should be. Now we can go get the last 10%," he says.

Burbage says certain design features - in the aircraft for affordability reasons - were driving the weight up, including quick-mate joints incorporated to speed final assembly. A panel-by-panel review of the structure revealed that an autosizing routine used to design the wing carry through structure was focusing the loads on one frame, which came out too heavy. "Now the loads are stretched over all four frames," which are thinner, he says.

Most of the design issues related to packaging. "We are trying to shrink-wrap the aircraft around a lot of stuff," says Burbage. The F-35 is the first fighter to have power-by-wire flight controls, and routing of wiring for the electro-hydraulic actuators has been a challenge. The design rework "has been a huge benefit to the programme", says Burbage.

The challenge now is to re-integrate the airframe design, which has been offline for eight weeks, with that for the rest of weapon system, says Burbage. Lockheed Martin and partners Northrop Grumman and BAE Systems must now complete detail design and release build-to packages for the first aircraft. "The programme is massively parallel," he says. "We had four months of schedule reserve to first flight, but we've eaten most of that up."

In the run up to next year's critical design review, the F-35 team must progressively close the design. To that end, Burbage has reorganised the programme structure to create an office of the chief engineer responsible for design closure. Lockheed Martin is aiming for 80-85% drawing release by CDR. After the critical design review, all changes to the design must be incorporated into production blocks.

Source: Flight International