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Aviation History
2003
2003 - 2774.PDF
$*~ Weighty matters GUY NORRIS / LOS ANGELES Cramming everything into the F-35 and keeping the aircraft in trim has been a major problem for the programme Although externally almost identi cal to the proof-of-concept X-35 that earned Lockheed Martin vic tory in the JSF fly-off in 2001, the first production representative F-35 is vir tually a new aircraft beneath the skin. "From a geometry perspective it is the same," says F-35 programme general man ager Tom Burbage, "but the demonstrator did not have weapons bays, mission sys tems or an integrated vehicle system. It was to show we could develop a family of air craft for all three services." The task of meeting the original UK and US navy deck limitations has been an exer cise in tight packaging, and it has proved a major challenge to fit an array of produc tion configuration systems into the rela tively small F-35 without affecting the outer mould lines, says Burbage. Describing the process as akin to "shrink wrapping the structure around the engines and weapons bays", Burbage says: "We've found we get a complex routeing and vehi cle integration challenge. We don't have as much hydraulics, but there is a lot more electrical activity. For example, we have 270V cabling, and figuring out the bend radius of electrical wiring has been tough." One of the chief areas of difference between the design of the concept demon strator aircraft and the production configu ration is the choke point between the densely packed forward fuselage and the remainder of the aircraft. It is already crowded around the junc tion of the bifurcated inlet ducts to the F135 engine, and overcoming the limita tions of the choke point is critical for the short take-off and vertical landing (STOVL) and conventional take-off variants. The area aft of the liftfan (or fuel tank in non- STOVL-versions) also houses additional fuel tanks and the auxiliary inlet doors for the STOVL version. "There is a lot of piping and wiring to run through there, and we're trying to do that while not drying up too much fuel [taking up excessive tank space]," says Burbage. "It's been tougher than we thought to get that done." Frozen lines The outer mould lines were frozen at the preliminary design review (PDR) stage in March, with the internal design details gradually being finalised during a subse quent series of nose-to-tail reviews. There are also weight as well as volume problems. These have become perhaps the biggest single concern of the project, and could threaten the long-term schedule and success of the venture. Even before the PDR, it became obvious the initial weight estimate for the F-35 was around 35% over target, com pared with typical estimates of 20-25% over for previous programmes. To combat the problem, Lockheed Martin and its main partners Northrop Grumman and BAE Systems instituted a five-phase bottom-up rolling review of structural weight. Coming out of PDR, the focus was on the conventional take-off and landing (CTOL) version - the first to be built - and the start of the fourth bottom- up review. Weight by this stage was within The F-35 looks the same as the aircraft that won the 2001 fly-off, but is very different beneath the skin 10% of target and falling gradually. To work out predicted versus actual weight, Lockheed Martin is converting ini tial weight estimates derived from paramet ric analysis to more closely defined num bers produced via the bottom-up review. The parametric system is based on his torical data from 14 previous fighter pro grammes. "However, we've found out the database legacy is not as good as we thought for this job because this aircraft is significantly different from anything before it," says Burbage. "For example, it is 40% composite by weight, has big weapons bays and has much more inte grated systems than even the F/A-22." By October, around 15% (by weight) of the design for the CTOL aircraft had been released for manufacture, with an internal target of between 80-85% design release set for the first of three air system critical design reviews (CDRs) scheduled for around April or May next year. The second and third air system CDRs are targeted for the fourth quarter of 2004 and late second quarter of 2005, respectively. Assessment focus is on the CTOL version first, with the carrier and heavier STOVL variants "falling out" of the weight review. "Ideally, we want the bottom-up weight to converge on the parametric estimates as we go along," says Burbage, who adds that the process is on course for a 60% design release target by early December. "By then, we will have a good idea of where we are." If the difference between the predicted and actual weight is not closed, or seen to be closing sufficiently fast, Lockheed Martin has the option for what it calls an "optimi sation design pass." This means complet ing the initial Batch 0 test aircraft accord ing to the latest design release standard, and using the overweight aircraft for aero dynamics, flying qualities, static and dynamic loads tests. By continuing to release parts against the initial operating capability parametric target, Burbage says, "we will know specifically what parts didn't meet target". Meanwhile, Lockheed Martin is working through the implications of design changes already made to combat weight. One of the most significant was abandonment of the rapid-mate production joints in the wing carry-though structure, originally designed to reduce production time. "It led to some inherent inefficiency in structural weight, and by cutting that we saved around 1,0001b [450kg]," says Burbage. The bad news, however, is the alterna tive design adds 26 days to the overall assembly time, which adds extra cost. "To offset the increase in assembly time, we are going to a moving assembly line, which xiv 25 NOVEMBER - 1 DECEMBER 2003 FLIGHT INTERNATIONAL SUPPLEMENT www.flightinternational.com
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