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Coming together

In a hangar in Marietta, Georgia, the prototype Lockheed Martin/Boeing F-22 air-superiority fighter stands amid an impressive array of sample parts and prototype components ranging from avionics connectors to fuselage bulkheads.

"We were not talking viewgraphs," says F-22 programme general-manager Gary Riley, referring to the critical design-review (CDR), which cleared the way for fabrication and assembly of nine development aircraft.

The array of parts, including classified items hidden from view behind a curtain, was produced for a review process, which began in November 1993, with the first of 211 component and subsystem reviews, continued through 24 major subsystem CDRs and culminated with the week long air vehicle CDR completed at the end of February.

The review determined whether the team had fulfilled its promise, made when the $9.55 billion engineering- and manufacturing-development (EMD) contract was awarded in August 1991, to complete some 6,200 activities in the integrated master plan (IMP) - the blueprint for F-22 development. All but six were accomplished, Riley says.

The CDR's purpose was to ensure that all performance and functional requirements have been incorporated into the F-22 design; to verify that required development tasks involving detail design have been completed; and to confirm that the programme meets the criteria to proceed into fabrication and assembly of the EMD aircraft. Areas covered include aircraft configuration, structure, materials, manufacturing processes, propulsion, flight performance, supportability and survivability.

"Damned-near perfect" is how Riley describes the CDR. "Six IMP items were not ready for CDR, but they are not show stoppers and they do not detract from the [design-definition] maturity of the aircraft," he says. The open items range from qualification of a new battery-supplier to completion of a non-destructive-test report on electron-beam-welding of titanium.

"The F-22 is much better defined than any previous US fighter at this stage of development," Riley says. There is a cost associated with the unprecedented design fidelity, he admits, saying that "...we have spent more money by this stage than in any other programme, but we believe that it will pay dividends later".

Major challenges leading up to the CDR were aircraft weight, unit cost and radar cross-section. Late in 1994, the F-22 was around 300kg overweight and was missing several of its performance parameters by small margins (Flight International, 7-13 September, 1993). "Now we are in the box on performance," Riley says, noting that the F-22 design team works to performance, rather than weight, specifications, with excess weight surfacing as a performance shortfall.

Projected per-unit production cost is within 2% of the target and "...we are well, within the green band on development cost", he says, meaning that the cost is within 10% of the target. The project is also on schedule, he adds. Budget cuts have forced the first flight to be delayed by three months, to May 1997, and will extend EMD by six months, into 2002.

SIGNATURE PROBLEM

Radar signature emerged as a problem early in 1994, when a computer model incorporating results from full-scale component tests revealed a higher-than-expected radar cross-section. The aircraft underside has been reworked, to reduce the number of access panels, surface apexes and drain holes. "Signature is right back in the box, and we have not compromised maintainability or supportability," Riley says.

Results from full-scale range tests of the modified parts have been fed back into the computer and aft-body signature "...is doing well", he says. A full-size pole model of the F-22 is being built for range testing in mid-1996.

Much effort was devoted to confirming aircraft performance and proving manufacturing processes. Some of the many wind-tunnel models used in the 16,500h of testing completed so far (of a planned 17,500h) are displayed, including a highly detailed propulsion-integration model; a jet-effects model used to assess the control effectiveness of the thrust-vectoring nozzles; and several aero-elastic wing and tail models flutter-tested to destruction.

Multiple models were used for internal and external weapons-carriage tests. The F-22 will carry four AIM-120 advanced medium-range air-to-air missiles (AMRAAMs) in under-fuselage bays and two AIM-9 short-range air-to-air missiles in side bays. Alternatively, the aircraft can carry six compressed-carriage AIM-120s, or two AMRAAMs and two 450kg Joint Direct Attack Munitions in the under-fuselage bays, and four 2,300litre (600USgal) fuel tanks, or eight AMRAAMs on four under-wing pylons.

Details discernible from the weapons-carriage models include the pop-up deflector added to the forward edge of each under-fuselage bay to direct airflow away from the briefly exposed well; and the angled, seeker-first, deployment of the side-bay AIM-9s, designed to minimise exposure.

Titanium is used extensively in the F-22 structure (see diagram), the backbone of which is formed by four major centre-fuselage bulkheads. The largest of these, the "583" bulkhead, begins life as a 2,770kg forging and weighs just 122kg when machining is complete.

In an example of teamwork, the numerical-control program to machine the forging was produced by Lockheed Martin's Fort Worth division and checked by the Marietta division, which used the program to machine the prototype bulkhead from aluminium. The highly loaded aft booms, to which the horizontal and vertical stabilisers are attached, consist of titanium castings, which are joined using electron-beam (EB) welding.

Carbonfibre composites account for about 25% of F-22 structure measured by weight. All external skins are bismaleimide (BMI), a toughened composite, and all doors are thermoplastic, for damage tolerance. Manufacturing techniques demonstrated for the CDR include precise machining of the signature reducing "chevron", or saw-tooth, edges on doors and panels.

Numerous composite parts, are produced by resin transfer moulding (RTM), including the auxiliary "sine-wave" spars in the wing. In RTM, dry carbonfibre laminates are laid up in a tool into which resin is pumped. The quality of initial RTM parts produced by subcontractor Dow UT is extremely good, says Riley.

A "scale-up" model of the F-22 wing produced for the CDR shows the arrangement of titanium main spars and composite auxiliary spars, which resulted from ballistic-survivability tests begun in 1992. These live-fire tests identified shortcomings in the original all-composite wing and demonstrated the benefits of a hybrid metallic-composite structure.

STEALTH EDGES

Edges are critical for stealth and Lockheed Martin has developed a "taco shell" edge-forming process using V-shaped tooling. The composite skin is pushed into the tool, using a shaped honeycomb core, which then forms the "filling" when the resulting V-shaped "sandwich" is cured. Precise machining of the honeycomb is critical, as voids between the skin and core are "a signature issue", says Riley.

Composite parts produced for the CDR range from a cockpit-display chassis to a horizontal stabiliser. The most challenging, Riley says, was the horizontal-stabiliser pivot shaft. This 3.65m-long component, which changes from round to square in cross-section and consists of more than 450 carbonfibre layers at its thickest, saves some 45kg over the original metal design. The shaft is produced by subcontractor Hercules, using fibre-placement technology.

Other manufacturing processes demonstrated for the CDR include laser drilling of the metal screens which prevent radar energy entering airframe apertures. The holes in these curved screens must be drilled precisely, to achieve the required reduction in signature, Riley indicates.

A major portion of the Marietta display is given over to avionics components produced for the CDR. These include line-replaceable modules (LRMs), which plug into the F-22's two common integrated processors (CIPs) - the aircraft's "brains". Riley says that getting all avionics suppliers to agree to use a common "bristle-brush" LRM connector was a major programme achievement. Hardware produced for the CDR includes modules with liquid flow-through cooling, used where air-cooling is not sufficient.

Fibre-optic databus components - and even a repair kit - have been produced, and a cockpit mock-up has been furnished with the actual controls, displays, seat and even the pilot's ensemble planned for the F-22. Conformal antennae for the integrated communication/ navigation/ identification (CNI) and electronic-warfare (EW) systems have been produced and are being tested on a pole model. The CNI suite includes intra-flight datalink and satellite-communications.

Transmit/receive modules for the F-22's Westinghouse/Texas Instruments APG-77 electronically scanned active-array radar were produced for the CDR. These have been reduced significantly in size, while cost, says Riley, has been reduced from around $30,000 for initial units to "$400-600" for production modules.

"The current state of the F-22's design is in excellent shape," says US Air Force programme director Maj. Gen. Robert Raggio. Assembly of the first production-standard aircraft will begin in the third quarter of this year, leading to a rollout early in 1997.

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