Aircraft and Aerospace Applications: Part Two

---

Transport Aircraft Structures and Materials

Transport aircraft operated by commercial airlines, corporations, and military organizations typically employ semi-monocoque and sheet-stringer aluminum construction. The primary alloys used today include 2024-T4 and higher-strength variants such as 2014-T6, 7075-T6, 7079-T6, and 7178-T6. When sheet materials are required, alclad forms are preferred for enhanced corrosion protection.

Upper wing skins and spar caps frequently utilize 7075-T6 and 7178-T6 alloys due to their exceptional compressive strength in structures where tension loading and fatigue are less critical concerns. For wing tension members, shear webs, and ribs, alloys 2014-T6, 2024-T4, and 7075-T6 are extensively employed. These applications demand an optimal balance of fatigue performance and fracture toughness combined with high strength. Although 7075-T6 offers greater strength than 2024-T3 or 2024-T4, it exhibits higher notch sensitivity and faster fatigue-crack propagation rates. Nevertheless, 7075-T6 structures typically achieve weight reductions compared to equivalent 2024-T3 or 2024-T4 designs.

Wing skins commonly incorporate rolled sheet and plate ranging from 0.040 to approximately 0.375 inches thick. Manufacturers prefer using wider and fewer pieces where possible. Fail-safe design principles are implemented through numerous separate stiffeners, which may be formed from sheet, milled from standard extrusions, or machined from stepped extrusions to accommodate integral end fittings.

Alclad sheet and plate are the preferred materials for wing skins to ensure superior corrosion resistance. Roll-tapered alclad sheet and plate create structurally efficient skins without extensive machining while allowing for optimized stiffener spacing and design. Some manufacturers employ adhesive bonding rather than riveting to attach doublers and stiffeners to skin sheets.

Modern airline transport and executive aircraft feature pressurized fuselages, where pressurization cycles and safety requirements dictate design parameters for high-load, fatigue-resistant, and fracture-resistant structures. While design is paramount in achieving desired performance, the fracture toughness of the alloy significantly impacts structural weight. Alloys with favorable combinations of static strength, fracture toughness, and corrosion resistance perform best in these applications. Typical materials include alclad sheet 0.040 to 0.187 inches thick in 2014-T6, 2024-T3, 7075-T6, and 7079-T6. Alclad 2219-T81 and 2219-T87 demonstrate good fracture toughness but lower room-temperature tensile strengths compared to other options.

Components such as trim tabs, servo tabs, control surfaces, flaps, and non-load-carrying access doors require thin skins in both skin-rib and sandwich-type construction. Primary material choices include alclad 2024-T3, alclad 7075-T6, and alloy 6061-T6. Aluminum honeycomb cores typically use 3003-H19, 5052-H19, or 5356-H19 foil. For applications requiring extended service at high temperatures, 2024-T81 foil offers advantages.

Landing gear structural components for heavy aircraft are frequently manufactured as aluminum alloy forgings. Main cylinders are produced on hydraulic presses as conventional closed-die forgings with a central parting plane. While alloy 2014-T6 was historically common, more recent designs utilize alloy 7079-T6 or T611. Alloy 7075-T73 and X7080-T7 merit consideration due to their excellent stress-corrosion cracking resistance, with X7080-T7 offering additional benefits in thick sections (over 3 inches) including favorable properties and low quenching stresses. Other landing gear components connected to the main cylinders are also produced as aluminum forgings, including structural elements in the fuselage and wings for load distribution and parts for retraction mechanisms.

Wheels for heavy civilian or military aircraft typically follow safe-life design principles, with replacement occurring at scheduled intervals during the aircraft's service life. This approach enables lighter-weight designs compared to those required for long-term fatigue resistance.

High-Performance Military Aircraft Applications

Military high-performance aircraft are engineered to withstand 9 to 12g loads (9 to 12 times greater than those experienced in unaccelerated flight). These maximum loads occur infrequently and may never be encountered in some aircraft. Since 1-g stresses predominate during most flight operations and aircraft service life in flying hours is generally limited, high-cycle fatigue is not a primary concern. However, the occasional high stresses experienced during maneuvers necessitate careful consideration of the structural materials' high-stress fatigue characteristics. These aircraft also feature high wing loadings requiring thick wing skins, typically 0.5 to 1.5 inches at the root.

Since approximately 1945, high-performance military aircraft have been constructed using the highest-strength aluminum alloys approved for military service. Alloy 7075-T6 has been the predominant choice, supplemented in specialized applications by 2014-T6, 2024 in both naturally and artificially aged tempers, 7079-T6, and 7178-T6. One large Navy carrier aircraft employs 2020-T651 plate for wing and tail surfaces to leverage its low density and high modulus of elasticity (11.4 million psi). The notch sensitivity of 2020-T6 requires careful design and fabrication to minimize stress concentrations and maximize the alloy's structural potential.

Extrusions 1 to 5 inches thick in alloys 7075-T6 or 7079-T6 serve as machining stock for spar caps, which in some designs run continuously from one wing side to the other. Significant sweepback and dihedral angles create forming challenges for continuous spars, leading some swept-wing aircraft designs to use stepped extrusions as machining blanks for spar caps with integral attachment fittings. These connect to carry-through members machined from thick plate, hand forgings, or die forgings, primarily using alloys 7075-T6, 7075-T73, and 7079-T6.

The main disadvantage of machined-plate skin is the absence of an alclad exterior surface for enhanced corrosion resistance, necessitating effective coating systems for adequate protection. Military services typically approve systems involving conversion coating, one or two zinc chromate primer coats, and one or two high-quality organic coating layers. If coating systems fail or sustain damage, aircraft operating in severe or tropical salt environments may experience exfoliation corrosion on top surfaces of 7075-T6 and 7178-T6. Alloy 7075-T73 and artificially aged tempers of 2xxx series alloys resist exfoliation but offer lower yield strengths than 7xxx series alloys in T6 temper. A more recent development, 7178-T76, approaches 7075-T6's structural capability while providing exfoliation resistance similar to 7075-T73.

Premium-strength aluminum alloy castings find applications in high-performance aircraft structural components including canopy supports and frames, fuselage members, and heavily loaded pylons supporting external loads. Alloys 354-T6 and A357-T6 are typically specified for these premium-strength castings. Emerging 2xxx series alloys, though not yet in production, demonstrate potential for 20% increased mechanical properties in simple shapes.

Supersonic Aircraft Materials and Design

Supersonic aircraft designed to withstand aerodynamic heating up to 250°F for over 100 cumulative service hours primarily utilize 2xxx series alloys in artificially aged tempers for skin sheet applications. Alloys 2024-T81 and T86 are most widely used, while 2014-T6 and 2024-T62 or T81 serve for extruded members. Heat-affected areas employ forged products in alloys 2014-T6 and 2618-T61, with alloy 2024 also suitable for similar applications. Alloy 2219 has seen limited use in engine pods as sheet, rivets, and forgings.

One supersonic bomber design extensively incorporates honeycomb core sandwich construction for wing panels, creating a stiff structure resistant to buckling when subjected to compressive stresses approaching the material's yield strength. These sandwich panels utilize 5052 aluminum foil honeycomb, except where fiberglass provides additional insulation between fuel and aerodynamic heating. Honeycomb panel frames are predominantly machined from 7075-T6 plate to eliminate corner joints. Aluminum honeycomb also reinforces beaded areas of skin doublers to enhance fuselage skin stiffness. For elevated temperature applications, 2024-T81 foil delivers higher strength than work-hardened alloys such as 5052-H39 and 5056-H39.

The Anglo-French supersonic transport primarily employs alclad and bare 2618-T6 throughout its structure. This alloy, with a long history in forged engine components, is available in various wrought forms. Alloy 2219-T81 or T87 offers similar tensile strength for design purposes as 2618-T6, though limited data indicates 2618-T6 provides superior creep strength.

Helicopter Rotor and Structure Applications

Helicopters present unique structural requirements for rotor blades. Alloys 2014-T6, 2024-T3, and 6061-T6 in extruded or drawn hollow shapes are extensively used for main spar members. Blade skins, typically 0.020 to 0.040 inches thick, primarily utilize alclad 2024-T3 and 6061-T6.

Some blade designs incorporate alloy 3003-H19 or 5052-H39 honeycomb core, while others rely on ribs and stringers spaced 5 to 12 inches apart to prevent excessive buckling or canning of thin trailing edge skins. Adhesive bonding represents the most common joining method for these components.

Helicopter cabin and fuselage structures generally follow conventional aircraft design principles, featuring formed sheet bulkheads, extruded or rolled sheet stringers, and doubled or chemically milled skins.