Polymer composites make up about 18 percent of an upgraded U.S. Navy fighter's airframe, and should help the F/A-18E/F fly longer missions with heavier loads than the current model. ``We needed to get weight out of the aircraft in a cost-effective manner,'' Doug Jaspering, a group manager of engineering at McDonnell Douglas Corp. in St. Louis, said in a telephone interview. ``We designed-in proven, low-risk production technology to get the results we wanted.''
Designers incorporated some fiber placement techniques and did risk reduction on the related manufacturing processes, Jaspering said. Also, use of composites in difficult-to-reach internal surfaces, generally involving spars or bulkheads, was minimized.
Jaspering sees composites as ``excellent materials because of strength-to-weight ratio,'' but they are ``not a panacea.'' Metal remains the aircraft's dominant structural material.
The 18 percent translates into about 2,890 pounds of polymer composites in the multimission F/A-18E/F Hornet's 15,941-pound airframe structure, as compared with 10 percent, or 1,251 pounds, of composites in the smaller C/D. Beyond the airframe, the E/F's out-er engine cases contain 150 pounds of high-temperature-resistant polyimides.
Improvements are numerous. A toughened epoxy resin system offers 25 percent better compression strength after impact and is expected to reduce incidental damage. Stiffened prepreg allows wing skins with 40 percent fewer plies, inlet ducts with composite-substructure string-ers and a lighter vertical tail. Structural changes should increase internal fuel capacity by about 33 percent and extend the mission range by nearly 40 percent.
Engineering and manufacturing development work proceeds under a seven- to 10-year, $4.8 billion Navy contract that calls for seven aircraft for flight testing and three for structural testing on the ground. The first 12 combat-ready E/F models are slated for delivery in 1999, and the overall plan calls for production of 1,000 aircraft through 2015.
Prime contractor McDonnell Douglas makes the forward fuselage section and wings, and does final assembly in St. Louis.
Principal subcontractor Northrop Grumman Corp. produces the center and aft fuselage sections and vertical tails in El Segundo, Calif., and could generate sales of $10 billion for E/F development and production.
Among the composite suppliers, Hercules Inc. makes AS4 and IM7 carbon fibers in Magna, Utah, and ICI Fiberite uses the fibers and its toughened 977-3 epoxy resin to produce prepreg in Greenville, Texas.
Compared with the C/D, theE/F's use of composites saves 303 pounds in the center and aft fuselage, 68 pounds in control surfaces and 30 pounds in inlet ducts.
The fuselage savings includes 128 pounds from substitution of AS4/977-3 for aluminum and 175 pounds from elimination of nine frames, made possible by the use of composites.
Use of IM7/977-3 saved 150 pounds on wing and horizontal and vertical tail skins. The wing platform on the E/F is 25 percent, or 100 square feet, larger than the C/D's, which has control surfaces made primarily of metal and composite skins of AS4 fiber and 3501-6 resin bonded to aluminum honeycomb core.
``This type of structure, although extremely efficient for strength and weight, is susceptible to damage as compared to other types,'' said Linda Bergstrom, a McDonnell Douglas material and process engineer. ``Stiffened composite structure on the E/F replaces most of the honeycomb structures on the C/D'' and reduces the likelihood of incidental damage.
A.M. Rubin of McDonnell Douglas and D.R. Perl of Naval Aviation Depot North Island described repair concepts for E/F composite structures in a technical paper at the Society for the Advancement of Material and Process Engineering's 1995 symposium.
They said products from Hysol Corp. of Pittsburg, Calif., have been selected as field repair materials and are being qualified for E/F bonded repairs.
Designers saved 11/2 pounds in the ribs, and 5 7/10 pounds in the spars, of the vertical tail.
``Getting weight out of the tip of the tail improves dynamics,'' Jaspering said.
Laminate thicknesses range from 0.075 inch on aft fuselage skins to 0.62 inch on wing skins and are tailored to survive strain, buckling or lightning strikes.
Facesheets over aluminum honeycomb core measure 0.025-0.05 inch on avionics doors and spoiler doors.
Composite applications vary. Quartz/RS3 cyanate ester is used for an antenna, and S-2/LRF-0507 glass epoxy for the 480-gallon fuel tank. C/D continuations include the use of Kevlar/CE9000 aramid fiber and E-glass/CE9000 epoxy for nonstructural fairings and environmental-control system ducts and S-2-LRF-XXX glass epoxy for the radome.
Northrop Grumman engineered the inlet ducts to increase airflow for two GE Aircraft Engines' F414 turbofans, which generate total thrust of 44,000 pounds, up 35 percent from the C/D's F404 engines.
``We will build the entire inlet ducts out of composites'' using automated tow placement, said Marion E. McHugh, vice president of manufacturing, product assurance and logistics.
Previously, the ducts were made of aluminum.
``With automated tow placement, we can generate more acute angles and achieve much more process control,'' McHugh said.
Technicians create a mandrel in the form of the duct, and a machine applies threads of composite fiber, spinning and nudging, tapering and varying thickness.
Northrop Grumman has ordered an automated tow placement machine from Cincinnati Milacron Inc. and, meanwhile, subcontracted initial systems to Alliant Techsystems of Magna, Utah.
``We will build our part in the El Segundo division next to the F-18 line,'' McHugh said.
Northrop Grumman shipped its first E/F section to St. Louis on April 19.
In a related development, Northrop Grumman projects use of polyimides on the aft decks and exhaust nozzles of a next-generation attack airplane as planned in the joint advanced-strike technology program. JAST, a 2-year-old effort, seeks commonality among Air Force, Navy and Marine Corps versions of a yet-to-be-named aircraft.
``We see use of composites where the exhaust comes across before getting away from the aircraft,'' McHugh said. ``We made the transition for cost savings, reducing labor and cycle times.''
Cycle improvement benefits the E/F, he said.
``When we cure different parts under pressure in an autoclave, we tend to get very precise about how long and at what temperature,'' McHugh said. ``We're trying to streamline the number of cycles and get full through-put in the autoclave. We work with design people to reduce 50 percent of the cure cycles.''
McDonnell Douglas and North-rop agreed in 1974 to develop a Navy fighter based on North-rop's YF-17 design and, since 1977, have produced 1,267 Hornets.
Only the Hornet is versatile enough for the F/A, or fighter/attack, designation. The aircraft also is used for reconnaissance missions and, most often, flies from carriers.
E is the single-seat model; F has two seats.
Existing versions of the Hornet fly in the military inventories of the Navy and Marine Corps and the air forces of Kuwait, Spain, Australia and Canada. Air forces in Finland and Switzerland take C/D deliveries this year; Malaysia will take C/D deliveries in 1997.
Naval Air Systems Command, McDonnell Douglas and North-rop conducted a 1987 study to define potential Hornet upgrades.
The 18E/F remains on schedule for its inaugural flight in December at St. Louis' Lambert Airport.