Aggressive timescale, evolving design and need for weight savings are tough challenges for the F-35 programme

Lockheed Martin engineers powered up the 270V electrical system of the nearly-assembled first F-35 Joint Strike Fighter in Fort Worth this month. A day later in Los Angeles, JSF supplier Northrop Grum­man loaded a set of bifurcated inlet ducts into a tool-jig – the first step in assembling the centre fuselage section for aircraft number two.

Both may seem like arcane production milestones, but are in fact a major source of pride for the JSF development team. The fact that both events occurred on schedule offers the latest evidence that the JSF programme can defy the doubters and keep up with an extraordinarily fast-paced schedule for system development and demonstration.

After a two-year delay caused by unexpected weight growth, Lockheed’s team is now racing to build and deliver all 24 test aircraft within the next 30 months, while incorporating a massive redesign that is still being defined.

Some in Congress have already said it cannot be done. The House appropriations committee says in the fiscal year 2006 defence spending bill: “It remains to be seen whether the JSF programme will be able to keep to the aggressive schedule. Previous aircraft production programmes have all faced significant schedule delays in delivering test aircraft, which have in turn consistently forced delays in the delivery of early production-representative aircraft.”

The criticism came with a sting. The panel voted to eliminate the programme’s request for $150 million in long-lead production funding for fiscal year 2006. Programme officials counter that they have not taken such a big gamble after all. The fact that the JSF is the first modern US military jet to be designed digitally should dramatically reduce the risk of further delays, says the team.

“We didn’t use the design toolsets in legacy programmes that we have on the F-35,” says Northrop’s F-35 vice-president and general manager Janis Pamiljans. He notes that the greatest risk of delays in the past came during the static and fatigue test phase. But Catia design tools have allowed F-35 engineers to see trouble spots before the aircraft is assembled.

Meanwhile, Lockheed officials point to the brisk pace of the JSF manufacturing line as another reason to play down the risks of the aggressive schedule. The first aircraft will never meet its performance targets because of later design changes, but its assembly has been a valuable proving ground for some of the programme’s far-reaching production ideas.

For example, the experience made it easier for the programme to abandon the F-35’s single-wing structural concept to save weight. The process of assembling the huge composite skin for the wing proved far too difficult anyway, says Edward G Linhart, Lockheed’s vice-president for the JSF production operations integrated product team. The JSF schedule “re-plan”, endorsed by the US Department of Defense’s acquisition chiefs in June, mandates the sequence of events.

The team has abandoned other advanced manufacturing ideas because of impracticality and the focus on saving weight. One example, says Pamiljans, is that they originally wanted to be the first aircraft programme to co-cure the composite frames and the inlet ducts in the autoclave, but this was problematic and it was found that switching to an aluminium frame would save 12kg (26lb). The JSF team is striving to completely offset a 1,360kg weight gain for the F-35B short take-off and vertical landing (STOVL) variant through weight reduction, aerodynamic and thrust improvements and tweaks to mission requirements.

Even as Northrop starts to piece together the centre fuselage for the first weight-optimised STOVL aircraft, engineers are working to shed the final 113-136kg of excess weight, says Dan Crowley, Lockheed’s vice-president and general manager for the F-35. The team has commissioned a special harness and wiring attack team (HWAT) to find weight savings in the electrical conduits snaking through the F-35’s structure.

The HWAT is complementing the STOVL weight attack team (SWAT), which has come up with most of the weight-saving ideas so far. Simple changes that yield huge weight offsets have all been done. Some of the highest-yielding changes are reconfiguring the weapons bay, opening the mouth of the engine inlets by 5° and improving engine thrust by 3%.

Northrop’s Pamiljans says the weight of the centre fuselage alone has dropped by between 270kg and 450kg. The first weight-optimised centre fuselage is currently over its target weight by 7.7kg, he says, but that will be reduced gradually as each of the development aircraft enters assembly. The final push to find weight offsets is likely to require scores of small-yielding changes to meet the team’s target. The final design is due to be locked in place early in 2006.


Source: Flight International