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 identical to the proof-of-concept X-35 that earned Lockheed Martin victory in the JSF fly-off in 2001, the first production representative F-35 is virtually a new aircraft beneath the skin.

"From a geometry perspective it is the same," says F-35 programme general manager Tom Burbage, "but the demonstrator did not have weapons bays, mission systems or an integrated vehicle system. It was to show we could develop a family of aircraft for all three services."

The task of meeting the original UK and US navy deck limitations has been an exercise in tight packaging, and it has proved a major challenge to fit an array of production configuration systems into the relatively 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 vehicle 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 demonstrator aircraft and the production configuration is the choke point between the densely packed forward fuselage and the remainder of the aircraft.

It is already crowded around the junction of the bifurcated inlet ducts to the F135 engine, and overcoming the limitations 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 subsequent 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, compared 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 10% of target and falling gradually.

To work out predicted versus actual weight, Lockheed Martin is converting initial weight estimates derived from parametric analysis to more closely defined numbers produced via the bottom-up review.

The parametric system is based on historical data from 14 previous fighter programmes. "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 integrated 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 "optimisation design pass." This means completing the initial Batch 0 test aircraft according to the latest design release standard, and using the overweight aircraft for aerodynamics, 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 partsdidn'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 1,000lb [450kg]," says Burbage.

The bad news, however, is the alternative 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 has brought back about half the span time," says Burbage, adding that further improvements are being sought.

From a general standpoint, the JSF team is also ensuring the F-35 avoids the pitfalls of earlier designs such as the F/A-18E/F and F/A-22. To avoid the lateral directional instability problems experienced by the F/A-18, with its wing drop phenomenon, all variants of the F-35 design will feature a simple, single leading-edge flap.

Similarly, the design team has spent a long time looking at high angle-of-attack (alpha) flow characteristics to see if the F-35 might be susceptible to the vertical tail buffet issues encountered by the F/A-18 and F/A-22. "It's a good thing we did that," says Burbage, adding that the windtunnel tests show the F-35 chine does indeed generate a strong vortex at high alpha, and that flight tests would have revealed a distinct tail buffet. Structural reinforcement is being designed into the aircraft's F-35 vertical fins as a result of the tests says Burbage. He adds: "We are bound to have enough of our own problems without repeating those of others."

Flying testbed

Avionics and sensor development and integration is being undertaken with two flying testbeds. One of Northrop Grumman's three BAC One-Eleven testbed aircraft is dedicated to F-35 systems testing and has been converted into a co-operative avionics testbed, or CAT Bird.

The aircraft will be used to give sensor fusion demonstrations to potential non-US JSF customer nations in a series of flights planned to take place from NAS Patuxent River, Maryland in January.

Although not in the original plan, Lockheed Martin has also opted to modify a Boeing 737-300 in Mojave, California for use as the prime F-35 CAT Bird. The aircraft is expected to begin flight tests with the full mission system on board in May 2005.

Sensors will include the nose-mounted Northrop Grumman active electronically scanned array (AESA) radar, the electro-optical distributed aperture system and electro-optical targeting system. A non-aerodynamic, fuselage-mounted canard will be used to replicate the positioning of sensor apertures and embedded antennas in relation to the AESA. The configuration will strongly resemble the 757-based airborne testbed developed for the F/A-22 programme.

Looking beyond the initial three main variants, Lockheed Martin is also preparing to tailor the F-35 more specifically to the requirements of the eight international industrial partners.

As Flight International closed for press, the company was due to begin work under a newly signed "delta system" development and demonstration (SDD) contract designed to "incorporate the requirements from the international partners", says Burbage.

Apart from these delta SDD studies, conducted with the help of a Lockheed Martin Skunk Works advanced developments projects group, further customer-specific studies are also under way.

Although on a smaller scale than the mainstream delta SDD studies, the additional concept evaluations are expected to be important to the overall breadth and capability of the eventual F-35 family. One of these could be a two-seat design which Lockheed Martin says "was looked at in a preliminary way early on", and which is reportedly attracting interest from Israel.

Other longer-term variants studied briefly by Lockheed Martin, and expected to attract more interest, include a big-wing CTOL version with extended range and payload capability, and a conceptual electronic-warfare Wild Weasel variant concept for the US Marine Corps.

Source: Flight International