PROVING THAT THE F-22 can be produced affordably is an increasingly important part of the engineering and manufacturing development (EMD) effort. "It's E-M-D, not E-D," emphasises Randy Simpson, director of production operations at Lockheed Martin Aeronautical Systems (LMAS). He is responsible for the F-22 assembly line, where the first aircraft came together with "unreal" precision, he says.
While externally similar to the YF-22, the F-22 is very different internally to the prototypes - the result of several design iterations to drive down weight and cost, with more changes in the pipeline to improve producibility and reduce production cost. The most marked changes have been in the use of materials: aluminium accounted for 32% of the YF-22's structure weight, titanium 27% and composites 21%; the F-22 is 16% aluminium, 39% titanium and 24% composite.
Titanium content was increased at the expense of aluminium because of loads, temperatures and damage-tolerance requirements. Composites content was once intended to be as high as 35%, but cost was a driver in reducing the use ofthermoplastics, which accounted for 11% of the YF-22's structure, to less than 1%.
HIP TITANIUM
Two titanium alloys are used: Ti 6-2-2-2-2 and Ti 6-4. The F-22 is the first aircraft to use Ti 6-4 hot isostatic pressed (HIP) castings in primary structure. Howmet produces HIP castings including aileron, flaperon and rudder actuator-housings, the "side-of-body" fittings which attach the wing to the fuselage, and the inlet frames - all of which carry high loads. The HIP process uses heat and pressure to close voids in titanium castings and increase their strength. A special heat-treatment process reduces variability between casting suppiliers.
The tail booms, which are subjected to high temperatures and torsion loads, are electron-beam (EB)-welded Ti 6-4. Aerojet welds the forward and aft sections in a vacuum chamber, in a computer-controlled process which produces a continuous weld around the boom. Previous EB-welded aircraft structures have involved only straight-line welds, F-22 engineers point out.
Every third wing intermediate spar was changed to titanium after live-fire testing revealed that the original all-composite design could not withstand a round exploding in the fuel tank. Titanium is also used for four of the seven mid-fuselage bulkheads (closed-die forgings produced by Wyman-Gordon). The titanium-honeycomb engine-bay doors are produced by Rohr using a liquid-interface diffusion-bonding pro-cess. Titanium vent screens, required to reduce radar cross-section, have thousands of precisely shaped and aligned holes cut by abrasive waterjet.
The materials content of the F-22 changes from mostly titanium in the aft fuselage to predominantly aluminium and composite in the forward fuselage. Only stress-corrosion-resistant tempers of aluminium are used. The sill longeron, which joins the forward and mid fuselage sections, is the most complex aluminium part, almost 5.5m long and with a constantly changing cross-section. A special heat-treatment was developed to prevent warping .
Simpson says that the avionics racks are the most demanding aluminium components to produce, with a machining tolerance of 0.05mm "-from one end to the other". The racks, which provide liquid-flow-through cooling to the avionics modules, "...can't leak at 9g," he says.
COMPOSITE CURE
Carbonfibre composites are used for the aircraft skin panels, wing intermediate spars, fuselage frames, doors and other components. Three main resin systems are used: bismaleimide (BMI), epoxy and thermoplastic. BMI is used for its strength at elevated temperatures and its toughness, and the skin panels are of co-cured BMI/honeycomb sandwich construction.
Composite layup is by hand, says Simpson, but using a laser-projection system to guide the placement of plies. Invar steel tooling is used, which is expected to last the life of the programme, and tools are designed to eliminate the possibility of operator error, he says.
Resin-transfer moulding (RTM) of composite parts is used where dimensional tolerance is critical, and where it is cheaper than producing a machined part of the same shape. Dow-UT produces RTM parts for the F-22. Examples include the wing sine-wave spars, frames in the forward-fuselage fuel tank and the inlet bypass doors. In the RTM process, dry-fibre layups are placed in a closed mould into which the resin is injected. Tooling is expensive, but the process is fast and "very repeatable", say F-22 engineers.
Thermoplastic composites are now used only where toughness is required, such as the landing-gear and weapons-bay doors. These are produced using a dual-resin bonding process in which the thermoplastic skins are bonded to RTM stiffeners.
A major composite component is the horizontal-stabiliser pivot shaft, which was titanium on the YF-22. The composite shaft costs more, but is lighter, and it is produced by Alliant Techsystems using automated fibre-placement. The cross-section changes from a circle to a square as the shaft sweeps and tapers along its almost-3m length. To produce the pivot shaft, carbonfibre tows are wound on to a mandrel - to a depth of 426 plies (almost 65mm) at the thickest point. At intervals during the winding, the shaft is part-cured to avoid cracking and wrinkling. It takes 60 days to produce the shaft, but a move to double-width tows could halve that time, F-22 engineers say,
Simpson says the F-22 is a "machined-part aircraft", and this has eliminated "hundreds" of steps in assembly. The decision to machine bulkheads and other components from plate, rather than forgings, has also reduced the time required to make design changes. It can take up to two years to qualify a new forging die, compared with two days to change a numerical-control (NC) machining tape, he says.
This came to the team's aid when it was discovered that inlet loads had been underestimated and the airframe would have to be strengthened. Parts for the first two aircraft had already been machined, but the team was able to retool parts for the third aircraft onwards - now referred to as 'Block 2' F-22s. Simpson regards the change as a success story, because the team had the analytical tools to detect the error before the aircraft had been flown. "In past programmes, we would have discovered the problem in flight test. Then it would have been tremendously expensive to change," he says.
REDUCING ERRORS
Use of the CATIA computer-aided design and manufacturing system has reduced errors in parts production: 78% of composite parts for were defect-free on the first attempt, and 90% were useable on the first aircraft, Simpson says. Metallic-part yields were not as high, because of the difficulty of predicting part deflection and machine variation during NC tape-proofing. About 27% of parts were defect-free on the first attempt, rising to 78% by the third try, he says.
Tight dimensional control of the F-22's external lines is required for stealth: "Outside mould-line is sacred," Simpson says. Using the same CATIA three-dimensional database for design of the part and the tool has made achieving the required tolerances possible. Electronic gauging has eliminated some 270 master gauges to reduce costs. Master gauges are physical tools which require "care and feeding" throughout the life of a programme, he explains.
Simpson says that a tolerance of 0.15mm is required to bring together the wing, produced in Seattle, with the forward fuselage assembled at Marietta, mid-fuselage built in Fort Worth and aft fuselage produced in Seattle. When the assemblies are mated at Marietta, electronic gauging, together with a computer-aided laser theodolite, is used to align each one with tooling points stored in the CATIA database. "It's incredible how good the fit is," he says.
Final-assembly tooling is supplied by the UK's Hyde Group, as part of an $18 million contract to provide a total of 665 F-22 tools, and NC programming for parts machining, to LMAS. Hyde is the largest tooling supplier to the programme.
The challenge now facing the F-22 manufacturing team is to incorporate producibility improvements to bring down the production cost of the aircraft. Simpson says that the team has the ability to predict production from a "huge" database of information on every part produced. This includes cost estimates on every iteration of a part and can be used to predict the average unit production cost of the F-22.
To keep the aircraft affordable, recurring costs will have to be reduced. Simpson says that this will involve converting some parts to forgings for production. This incurs the non-recurring expense of creating the die, but reduces the recurring cost of producing the part, resulting in net savings. Similarly, greater use is to be made of RTM composites, because once the expensive tool is created, "-you can push the parts out like cookie cutters", he says.
Source: Flight International
