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F22 Air Dominance for the 21st Century

The industry team of Lockheed Martin and Boeing is working with the U.S. Air Force and Pratt & Whitney to develop the F-22 to replace the current F-15 beginning in 2004. First flight was September 7,1997

Lockheed Martin Aeronautical Systems
Marietta, Georgia

The F-22 Raptor is an air dominance fighter with much higher capability than current U.S. Air Force aircraft. Compared with the F-15 it is designed to replace, the aircraft has higher speed and longer range, greater agility, enhanced offensive and defensive avionics, and reduced observability. Furthermore, both the air vehicle and the engine designs emphasize reliability and maintainability of systems, and are the result of a team approach called Integrated Product Development.

The F-22 can carry medium and short-range air-to-air missiles in internal bays, and has an in internal 20-mm cannon and provisions for carrying precision ground attack weapons. Pilots will have a "first-look, first-shot, first-kill" capability because of the Raptor's stealth properties and advanced sensors. In addition, the avionics suite is a highly integrated system that will allow the pilot to concentrate on the mission, rather than on managing the sensors as in current fighters.

Much of the increased capability is based on the materials in the engine, the structure, and the skin. Efficient processing methods also contribute to higher reliability, lower costs, and simplified maintenance. This article describes some of those materials and processes.

Raptor materials

Traditional aircraft materials such as aluminum and steel make up about 20% of the F-22 structure by weight. Its high-performance capabilities require significant amounts of titanium (42% of all structural materials by weight) and composite materials (24% by weight). These are stronger and lighter than traditional materials, and offer better protection against corrosion. Titanium also offers tolerance to higher temperatures. In fact, titanium accounts for a larger percentage of the structural weight on the F-22 than any other current U.S. fighter.

• The forward fuselage is just over 5.2 in (17 ft) long, slightly more than 1.5 m (5 ft) wide at its widest point, 1.7 m (5 ft 8 in.) tall, and weighs about 770 kg (1700 lb). Built up in two sections, the forward fuselage is joined together by two long and relatively wide side beams and two longerons that run the length of the assembly. The beams, which are made of composite materials, also provide an attachment point for the "chine," the fuselage edge that provides smooth aerodynamic blending into the intakes and wings. The 5.2 m (17 ft.) long aluminum longerons form the sills of the cockpit, and the canopy rests on them.
• The canopy is about 356 cm long, 114 cm wide, 70 cm tall (140 x 45 x 27 in.), and weighs about 160 kg (360 lb). it is the largest piece of monolithic polycarbonate material being formed today. It is made of two 0.9 cm (0.375 in.) thick sheets that are heated and fusion-bonded, then drape-forged. It has no canopy bow, and offers superior optics throughout, as well as the requisite stealth features.
• The mid-fuselage is also about 5.2 m (17 ft long), 2 m (6 ft) high, and weighs about 3900 kg (8500 lb). Almost all systems pass though this section, including hydraulic, electrical, environmental control, fuel, and auxiliary power systems. It also includes three fuel tanks, four internal weapons bays, and the 20-mm cannon. Only 35% of the mid-fuselage structure is aluminum. Composites make up 23.5%, and titanium is nearly 35%. The lower keel chord is a Ti6-22-22 alloy forging that weighs about 18 kg (40 lb). The four bulkheads are made of titanium Ti6-4; one of these is the largest single titanium part ever used on an aircraft.
• The aft fuselage is 67% titanium, 22% aluminum, and 11 % composite by weight. It measures 5.8 in long by 3.6 m wide (19 x 12 ft), and weighs 2270 kg (5000 lb). About 25% by weight of the aft fuselage is comprised of large electronbeam-welded titanium forward and aft booms. The largest is the forward boom, which is more than 3 m (10 ft) long and weighs about 300 kg (650 lb). The welded booms reduce the need for traditional fasteners by about 75%.
• The wings are composed of 42% titanium, 35% composites (including the skin), and 23% aluminum, steel, and other materials in the form of fasteners, clips, and other miscellaneous parts. Each wing weighs approximately 900 kg (2000 lb) and measures 4.8 in (16 ft) on the side-of-body, by 5.5 m (18 ft) along the leading edge. After analyzing the results of live-fire tests that simulated severe combat damage, engineers chose to reinforce the wing by replacing every fourth composite spar with one made of titanium. This reinforcement ensures that the F-22 will be even more survivable in combat situations.
• The empennage consists of the vertical and horizontal tails. The verticals are a multi-spar configuration internally, and have a HIP'ed cast rudder actuator housing. The edges and rudder are made of composites, and have embedded VHF antennas. The horizontal surfaces, known as stabilators, are made of honeycomb materials with composite edges. They are movable assemblies, and are deflected by the composite pivot shaft described below.
• The main landing gear is made of Carpenter Technology's Airmet 100 steel alloy. It is one of the first applications of a steel that has been specially heat treated to provide greater corrosion protection to the main gear piston axle.

Composite parts

On the F-22, the number of parts made from thermoset composites is approximately a 50 / 50 split between epoxy resin parts and bismaleimide (BMI) parts. Exterior skins are all BMI, which offers high strength and high-temperature resistance.

Thermoplastic composites are also highly durable materials but, unlike thermosets, thermoplastics can be reheated and re-formed. However, thermo-plastics proved to be more expensive and more difficult to incorporate in the F-22 than had been hoped in the early days of the program. As a result, although thermoset composites comprise about 24% of airframe structural materials, thermoplastics are only about 1%. They -ire being applied on the F-22 for items such as doors for the landing gear and weapons bay, where tolerance to impact damage is required from things such as small rocks that may be kicked up from the runway.

Resin transfer molding

The F-22 is the first aircraft to take advantage of resin transfer molding (RTM) of composite parts. RTM is a method of composite parts fabrication well suited to economically fabricating complex-shaped details repeatedly to tight dimensional tolerances.

Large composite parts traditionally are formed by applying and pressurizing hundreds of layers of fabric that contain a pre-embedded resin, and curing them in an autoclave. This is a very time-consuming and labor-intensive process.

In the RTM process, fibrous preforms are first shaped under vacuum from stacks of fabric, and then placed in metal tooling that matches the shape of the part. The tool is then injected with heated resin under pressure. The benefit of matching the metal tooling to RTM is a high level of part reproducibility, consistency in assembly operations, and economies of scale.

RTM is used to fabricate more than 400 parts of the F-22 structure, ranging from inlet lip edges to load-bearing sine-wave spars in the fighter wings. At Boeing, RTM has reduced the cost of wing spars by 20%, and has cut in half the number of reinforcement parts needed for in stalling the spars in the wings. Both bismaleimide and epoxy parts are fabricated by RTM.

Composite pivot shaft

The composite pivot shaft is an application of automated fiber placement (AFP) technology, which was combined with unique tooling approaches to produce a lightweight composite structure that replaced titanium in a flight-critical application - the F-22 horizontal stabilizers.

AFP technology makes possible the exact fiber positioning required to achieve the complex geometry of the pivot shaft. It has a 25-cm (10in.) diameter cylinder at one end, a rectangular spar about 10 cm (4 in.) wide at the other, and an offset in the transition area. Its shape can be likened to that of an oversized hockey stick.

It is composed of more than 400 plies of composite tow tapes ranging from 3 to 12 mm (1 / 8 to 1 / 2 in.) wide. The shaft is cured in stages to prevent internal cracking and eliminate wrinkles, as no allowance is made for voids in the shaft. After layup, the shafts are nondestructively inspected and tested.

Production requires up to 60 days, but weight is reduced by 40 kg (90 lb) per shipset (two shafts) over titanium, which is an extremely large amount of weight to take out of an aircraft at one time. Thicker tow tapes are planned for future pivot shafts, which should greatly reduce production time.

Hot isostatic pressing

Hot isostatic pressing (HIP) is a process in which metallic castings are subjected to very high temperatures in a static pressure environment of >70 MPa (10 ksi). On the F-22, structural titanium castings are HIP'ed to collapse internal shrinkage cavities and diffusion-bond the walls of the cavities. Six large structures on the F-22 are HIP'ed: the rudder actuator housing (one for each rudder); the canopy deck; the wing side-of-body forward and aft fittings (four total, two for each wing); the aileron strongback (one for each aileron, two total); and the inlet canted frame (one each for the left and right inlets). The canted frame was originally a four-piece assembly. By switching to a casting, mechanical joints were eliminated and machining was minimized.

Electron beam welding

Electron beam (EB) welding is helping Boeing and Aerojet, its supplier, build lighter-weight titanium assemblies for the aft fuselage. Parts are EB Welded in a vacuum chamber to prevent exposure to oxygen, which can create a deleterious brittle surface. Compared with other methods, electron beam welding enables much more reliable joints when welding titanium parts more than an inch thick.

The process also reduces the need for fasteners in some fuselage components by up to 75%, which decreases weight, simplifies the assembly process, and avoids the costs associated with fasteners. The reduction in the number of fasteners also means fewer openings for possible fuel leaks.

Engine design

The product of more than 40 years of research into high-speed propulsion systems, the Pratt & Whitney F119 is proof that high-technology does not have to be complicated. A balanced approach to the design process led to an engine as innovative in its reliability and support as in its performance. Assemblers and flight line mechanics participated in the F119's design from its inception. The result is that ease of assembly, maintenance, and repair are designed into the engine. For example, the F119 cuts requirements for support equipment and labor by half, which also saves precious space in airlifters during combat zone deployments. The engine will require 75% fewer ship visits for routine maintenance than its predecessors.

The F119 has 40% fewer major parts than current fighter engines, and each part is more durable and does its job more efficiently. Computational fluid dynamics-airflow analyzed through advanced computers-led to the design of engine turbomachinery of unprecedented efficiency, giving the F119 more thrust with fewer turbine stages.

In fact, the F119-PW-100 engine develops more than twice the thrust of current engines under supersonic conditions, and more thrust without afterburner than conventional engines with afterburner. Each F-22 will be powered by two of these 35,000-pound-thrust-class engines. By comparison, the engines powering the Air Force F-15 and F-16 fighters have thrust ratings ranging from 23,000 to 29,000 pounds.

Jet engines deliver additional thrust by directly injecting fuel at the engine exhaust. The process, called afterburner, gives the aircraft a rocket-like boost as the fuel ignites in the exhaust chamber. The tradeoff is higher fuel consumption, a greater amount of heat, and consequently, greater visibility to the enemy. However, the F119 engine can push the F-22 to supersonic speeds above Mach 1.4 even without firing the afterburner, which gives the fighter a greater operating range and allows for stealthier flight operation.

These are some of the significant F119-PW-100 engine features:

• Integrally bladed rotors: In most stages, disks and blades are made from a single piece of metal for better performance and less air leakage.
• Long chord, shroudless fan blades: Wider, stronger fan blades eliminate the need for the shroud, a ring of metal around most jet engine fans. Both the wider blades and shroudless design contribute to engine efficiency.
• Low-aspect, high-stage-load compressor blades: Once again, wider blades offer greater strength and efficiency.
• Alloy C high-strength burn-resistant titanium compressor stators: Pratt & Whitney's innovative titanium alloy exhibits high elevated-temperature strength and a markedly improved resistance to sustained combustion. It increases stator durability, allowing the engine to run hotter and faster for greater thrust and efficiency.
• Alloy C in augmentor and nozzle: The same heat-resistant titanium alloy protects aft components, permitting greater thrust and durability.
• Floatwall combustor: Thermally isolated panels of an oxidation-resistant, high-cobalt alloy make the combustion chamber more durable, which helps reduce scheduled maintenance.

Thrust vectoring nozzle

The Fll9 engine nozzle for the F-22 is the world's first full production vectoring nozzle, fully integrated into the aircraft/ engine combination as original equipment. The two-dimensional nozzle vectors thrust 20 degrees up and down for improved aircraft agility. This vectoring increases the roll rate of the aircraft by 50%, and has features that contribute to the aircraft stealth requirements.

Heat-resistant components give the nozzles the durability needed to vector thrust, even in afterburner conditions. With precision digital controls, the nozzles work like another aircraft flight control surface. Thrust vectoring is an integrated part of the F-22's flight control system, which allows for seamless integration of all components working in response to pilot commands.

The following information was extracted from the May 1998 issue of the technical magazine,
ADVANCED MATERIALS & PROCESSES. 
Permission was granted to ALLSTAR by the magazine to use the preceding materials. 
For more information about this article, please contact Margaret W. Hunt, the magazine's editor

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Updated: February 23, 1999