Knowledge

As the twentieth century progressed, aluminum became an essential metal in aircraft. The cylinder block of the engine that powered the Wright brothers' plane at Kitty Hawk in 1903 was a one-piece casting in an aluminum alloy containing 8% copper; aluminum propeller blades appeared as early as 1907; and aluminum covers, seats, cowlings, cast brackets, and similar parts were common by the beginning of the First World War.

In 1916, L. Brequet designed a reconnaissance bomber that marked the initial use of aluminum in the working structure of an airplane. By war’s end, the Allies and Germany employed aluminum alloys for the structural framework of fuselage and wing assemblies.

Alloys for Airframe Components

The aircraft airframe has been the most demanding application for aluminum alloys; to chronicle the development of the high-strength alloys is also to record the development of airframes. Duralumin, the first high-strength, heat treatable aluminum alloy, was employed initially for the framework of rigid airships, by Germany and the Allies during World War I. Duralumin was an aluminum-copper-magnesium alloy; it was originated in Germany and developed in the United States as alloy 17S-T (2017-T4). It was utilized primarily as sheet and plate.

Alloy 7075-T6 (70,000-psi yield strength), an Al-Zn-Mg-Cu alloy, was introduced in 1943. Since then, most aircraft structures have been specified in alloys of this type. The first aircraft designed in 7075-T6 was the Navy’s P2V patrol bomber. A higher-strength alloy in the same series, 7178-T6 (78,000-psi yield strength), was developed in 1951; it has not generally displaced 7075-T6, which has superior fracture toughness. Alloy 7178-T6 is used primarily in structural members where performance is critical under compressive loading.

Alloy 7079-T6 was introduced in the United States in 1954. In forged sections over 3 in. thick, it provides higher strength and greater transverse ductility than 7075-T6. It now is available in sheet, plate, extrusions, and forgings.

Alloy X7080-T7, with higher resistance to stress corrosion than 7079-T6, is being developed for thick parts. Because it is relatively insensitive to quenching rate, good strengths with low quenching stresses can be produced in thick sections.

Cladding of aluminum alloys was developed initially to increase the corrosion resistance of 2017-T4 sheet and thus to reduce aluminum aircraft maintenance requirements. The coating on 2017 sheet - and later on 2024-T3 - consisted of commercial-purity aluminum metallurgically bonded to one or both surfaces of the sheet.

Electrolytic protection, present under wet or moist conditions, is based on the appreciably higher electrode potential of commercial-purity aluminum compared to alloy 2017 or 2024 in the T3 or T4 temper. When 7075-T6 and other Al-Zn-Mg-Cu alloys appeared, an aluminum-zinc cladding alloy 7072 was developed to provide a relative electrode potential sufficient to protect the new strong alloys.

However, the high-performance aircraft designed since 1945 have made extensive use of skin structures machined from thick plate and extrusions, precluding the use of alclad exterior skins. Maintenance requirements increased as a result, and these stimulated research and development programs seeking higher-strength alloys with improved resistance to corrosion without cladding.

Aluminum alloy castings traditionally have been used in nonstructural airplane hardware, such as pulley brackets, quadrants, doublers, clips, ducts, and wave guides. They also have been employed extensively in complex valve bodies of hydraulic control systems. The philosophy of some aircraft manufacturers still is to specify castings only in places where failure of the part cannot cause loss of the airplane. Redundancy in cable and hydraulic control systems "permits" the use of castings.

Casting technology has made great advances in the last decade. Time-honored alloys such as 355 and 356 have been modified to produce higher levels of strength and ductility. New alloys such as 354, A356, A357, 359 and Tens 50 were developed for premium-strength castings. The high strength is accompanied by enhanced structural integrity and performance reliability.

Electric resistance spot and seam welding are used to join secondary structures, such as fairings, engine cowls, and doublers, to bulkheads and skins. Difficulties in quality control have resulted in low utilization of electric resistance welding for primary structure.

Ultrasonic welding offers some economic and quality-control advantages for production joining, particularly for thin sheet. However, the method has not yet been developed extensively in the aerospace industry.

Adhesive bonding is a common method of joining in both primary and secondary structures. Its selection is dependent on the design philosophy of the aircraft manufacturer. It has proven satisfactory in attaching stiffeners, such as hat sections to sheet, and face sheets to honeycomb cores. Also, adhesive bonding has withstood adverse exposures such as sea-water immersion and atmospheres.

Fusion welded aluminum primary structures in airplanes are virtually nonexistent, because the high-strength alloys utilized have low weldability and low weld-joint efficiencies. Some of the alloys, such as 2024-T4, also have their corrosion resistance lowered in the heat-affected zone if left in the as-welded condition.

The improved welding processes and higher-strength weldable alloys developed during the past decade offer new possibilities for welded primary structures. For example, the weldability and strength of alloys 2219 and 7039, and the brazeability and strength of X7005, open new avenues for design and manufacture of aircraft structures.

Light Aircraft

Light aircraft have airframes primarily of all-aluminum semi-monocoque construction, however, a few light planes have tubular truss load-carrying construction with fabric or aluminum skin, or both.

Aluminum skin is normally of the minimum practical thickness: 0.015 to 0.025 in. Although design strength requirements are relatively low, the skin needs moderately high yield strength and hardness to minimize ground damage from stones, debris, mechanics’ tools, and general handling. Other primary factors involved in selecting an alloy for this application are corrosion resistance, cost, and appearance. Alloys 6061-T6 and alclad 2024-T3 are the primary choices.

Skin sheet on light airplanes of recent design and construction generally is alclad 2024-T3. The internal structure comprises stringers, spars, bulkheads, chord members, and various attaching fittings made of aluminum extrusions, formed sheet, forgings, and castings.

The alloys most used for extruded members are 2024-T4 for sections less than 0.125 in. thick and for general application, and 2014-T6 for thicker, more highly stressed sections. Alloy 6061-T6 has considerable application for extrusions requiring thin sections and excellent corrosion resistance. Alloy 2014-T6 is the primary forging alloy, especially for landing gear and hydraulic cylinders. Alloy 6061-T6 and its forging counterpart 6151-T6 often are utilized in miscellaneous fittings for reasons of economy and increased corrosion performance, when the parts are not highly stressed.

Alloys 356-T6 and A356-T6 are the primary casting alloys employed for brackets, bellcranks, pulleys, and various fittings. Wheels are produced in these alloys as permanent mold or sand castings. Die castings in alloy A380 also are satisfactory for wheels for light aircraft.

For low-stressed structure in light aircraft, alloys 3003-H12, H14, and H16; 5052-O, H32, H34, and H36; and 6061-T4 and T6 are sometimes employed. These alloys are also primary selections for fuel, lubricating oil, and hydraulic oil tanks, piping, and instrument tubing and brackets, especially where welding is required. Alloys 3003, 6061, and 6951 are utilized extensively in brazed heat exchangers and hydraulic accessories. Recently developed alloys, such as 5086, 5454, 5456, 6070, and the new weldable aluminum-magnesium-zinc alloys, offer strength advantages over those previously mentioned.

Sheet assembly of light aircraft is accomplished predominantly with rivets of alloys 2017-T4, 2117-T4, or 2024-T4. Self-tapping sheet metal screws are available in aluminum alloys, but cadmium-plated steel screws are employed more commonly to obtain higher shear strength and driveability. Alloy 2024-T4 with an anodic coating is standard for aluminum screws, bolts, and nuts made to military specifications. Alloy 6262-T9, however, is superior for nuts, because of its virtual immunity to stress-corrosion cracking.

Transport Aircraft

Transport aircraft of the types operated by commercial airlines, by corporations for executive travel, and by the military, including Concorde craft, are generally of semi-monocoque and sheet-stringer aluminum construction.

The alloys primarily utilized today are 2024-T4 and the alloys having still higher strength (2014-T6, 7075-T6, 7079-T6 and 7178-T6). Where sheet is used, the alclad form is preferred. The upper skins and spar caps of wings often are of 7075-T6 and 7178-T6, because the critical requirement is high compressive strength, and the structure generally is not critical in tension loading or fatigue.

For wing tension members, shear webs, and ribs, alloys 2014-T6, 2024-T4, and 7075-T6 are used extensively. For these applications, fatigue performance and fracture toughness, combined with high strength, are the alloy characteristics of chief concern. Although 7075-T6 is stronger than 2024-T3 or 2024-T4, it is more sensitive to notches and has a higher fatigue-crack propagation rate. However, structures designed and fabricated in 7075-T6 have somewhat less weight than is possible in a 2024-T3 or 2024-T4 structure for equivalent performance.

Rolled sheet and plate 0.040 to approximately 0.375 in. thick are employed for wing skins by other manufacturers who prefer as wide and as few pieces as possible. Fail-safe design in this type of construction is achieved by many separate stiffeners, formed from sheet or milled from standard extrusions, or machined from stepped extrusions to accommodate integral end fittings.

Alclad sheet and plate are preferred for wing skins to obtain good corrosion resistance. Roll-tapered alclad sheet and plate provide skins that are structurally efficient without extensive machining. Also, optimum spacing and design of stiffeners are practicable with this approach. Adhesive bonding, instead of riveting, is employed by some designers for attaching doublers and stiffeners to the skin sheet.

Fuselages on virtually all modern airline transports and executive aircraft are pressurized. The pressurization cycles and safety requirements dictate the design parameters of high-load, fatigue-resistant and fracture-resistant structures for this application. Although the design is the most important consideration in achieving a desired performance, the fracture toughness of the alloy probably has the most influence on the weight of the structure. Alloys and tempers with good combinations of static strength, fracture toughness, and corrosion resistance are the best for this application. Alclad sheet 0.040 to 0.187 in. thick in 2014-T6, 2024-T3, 7075-T6, and 7079-T6 is utilized. Alclad 2219-T81 and 2219-T87 have good fracture toughness, but their tensile strengths at room temperature are lower than those of the other alloys.

Thin skins are required for such components as trim tabs, servo tabs, control surfaces, flaps, and non-load-carrying access doors; they are applied in both skin-rib and sandwich-type construction. Alclad 2024-T3, alclad 7075-T6, and alloy 6061-T6 are the primary selections. Aluminum honeycomb core generally is made from 3003-H19, 5052-H19, or 5356-H19 foil. Foil of 2024-T81 is produced and used advantageously for core for long service at high temperatures.

Landing gear structural parts for heavy airplanes are often produced as aluminum alloy forgings. The main cylinders are made on hydraulic presses as conventional closed-die forgings, with the parting plane at the center of the cylinder. In the past, alloy 2014-T6 was employed extensively, but in recent years alloy 7079-T6 or T611 has been used. Alloy 7075 in the new T73 temper and alloy X7080-T7 also should be considered, because of their good resistance to stress-corrosion cracking, and in the case of X7080-T7, its good properties and low quenching stresses in thick (over 3 in.) sections. Other landing gear members, attached to the main cylinders, also are produced as aluminum forgings, including structural forgings in the fuselage and wings, which distribute the landing gear loads into other structures, and forged parts for the retracting mechanism.

Wheels for heavy civilian or military airplanes generally are designed on a safe-life basis. They are replaced at regular intervals during the life of an airplane, allowing use of lighter-weight designs than are required for long-time fatigue resistance.

High-Performance Aircraft

High-performance aircraft required by the military services are designed to withstand 9 to 12g loads (9 to 12 times greater than those imposed by unaccelerated flight).

The maximum loads are infrequent, and on some aircraft may never be encountered. Since the l-g stresses prevalent during most of a flight period are low, and the life of the aircraft in terms of flying hours is also generally low, high-cycle fatigue is not a major problem. However, the high stresses that occasionally may be imposed in maneuvers demand consideration of the high-stress fatigue characteristics of the structure material. Another characteristic of this type of aircraft is high wing loadings, which dictate thick wing skins, typically 0.5 to 1.5 in. at the root. Design requirements resulting from aerodynamic heating at high speeds are discussed subsequently, under supersonic aircraft.

Since about 1945, all high-performance aircraft have been manufactured of the highest-strength aluminum alloys approved by the military services. Alloy 7075-T6 has been the workhorse, complemented in specialized applications by 2014-T6, 2024 in both naturally and artificially aged tempers, 7079-T6, and 7178-T6. In one large Navy carrier aircraft, 2020-T651 plate is used for wing and tail surfaces to obtain the advantages of its low density and high modulus of elasticity (11.4 million psi). The notch sensitivity of 2020-T6 requires care in design and fabrication to minimize stress concentrations and to realize the full structural capabilities of the alloy.

Extrusions 1 to 5 in. thick in alloys 7075-T6 or 7079-T6 are utilized as machining stock for spar caps, which in some designs are continuous from one side of a wing to the other. Appreciable sweepback and dihedral angles present forming problems for continuous spars; therefore, in some swept-wing aircraft, stepped extrusions are employed as machining blanks for spar caps with integral attachment fittings. These are attached to carry-through members, machined from thick plate, hand forgings, or die forgings. Alloys 7075-T6, 7075-T73 and 7079-T6 predominate.

The primary disadvantage of the machined-plate skin is its elimination of the use of an alclad exterior surface for greater corrosion resistance, thus requiring effective coating systems for adequate corrosion protection.

In general, the military services approve systems involving a conversion coating, one or two coats of zinc chromate primer, and one or two coats of high-quality organic coating. If the coating fails or is damaged, aircraft operating in very severe and tropical salt atmospheres may encounter exfoliation corrosion on top surfaces of 7075-T6 and 7178-T6. Alloy 7075-T73 and the artificially aged tempers of the 2xxx series alloys do not exfoliate, but they have lower yield strengths than the 7xxx series alloys in the T6 temper. A more recent development is 7178-T76, which approaches the structural capability of 7075-T6 and the exfoliation resistance of 7075-T73.

Premium-strength aluminum alloy castings are used in some high-performance airplanes. They are employed in structural components such as canopy supports and frames, fuselage members, and heavily loaded pylons that support external loads. Alloys 354-T6 and A357-T6 are usually specified for these premium-strength castings. New alloys of the 2xxx series, not yet in production, show a capability of 20% increase in mechanical properties for simple shapes.

Supersonic Aircraft

Supersonic aircraft, designed to withstand aerodynamic heating to 250°F for over 100 hr (the time in service is accumulated in small increments), generally utilize the 2xxx series alloys in artificially aged tempers for skin sheet.

Alloys 2024-T81 and T86 are the most extensively employed; 2014-T6 and 2024-T62 or T81 are used for extruded members. Alloys 2014-T6 and 2618-T61 are employed for forged products located in heat-affected areas; alloy 2024, which can be forged, also can be considered for parts of this type. Alloy 2219 has had limited application in engine pods as sheet, rivets, and forgings.

The designers of one supersonic bomber have made extensive use of honeycomb core sandwich construction for wing panels, to achieve a stiff structure that does not buckle when stressed in compression near the yield strength of the material. The honeycomb in these sandwich panels is 5052 aluminum foil, except where fiber glass is applied to further insulate the fuel from aerodynamic heating.

Honeycomb panel frames are predominantly 7075-T6, machined from plate to eliminate corner joints. Aluminum honeycomb is also used in the beaded areas of skin doublers, to help stiffen the fuselage skin. At elevated temperatures, 2024-T81 foil provides higher strength than is obtained in work hardened alloys, such as 5052-H39 and 5056-H39.

The supersonic transport being developed by British and French interests makes general use of alclad and bare 2618-T6 for the structure. This alloy, which has served many years in forged engine parts, is available also in other wrought forms. Alloy 2219-T81 or T87 has approximately the same tensile strength for design purposes as 2618-T6; however, limited data show 2618-T6 has higher creep strength.

Helicopters

Helicopters have critical structural requirements for rotor blades. Alloys 2014-T6, 2024-T3, and 6061-T6, in extruded or drawn hollow shapes, are utilized extensively for the main spar member.

The blade skins, typically 0.020 to 0.040 in. thick, are primarily alclad 2024-T3 and 6061-T6.

Some blades have alloy 3003-H19 or 5052-H39 honeycomb core; others depend on ribs and stringers spaced 5 to 12 in. apart to prevent excessive buckling or canning of the thin trailing edge skins. Adhesive bonding is the most common joining method.

The cabin and fuselage structures of helicopters generally are of conventional aircraft design, utilizing formed sheet bulkheads, extruded or rolled sheet stringers, and doubled or chemically milled skins.