Airframe Structures: Load Paths, Materials, and Fastening
Load paths
Every structures question begins by naming the stress: tension, compression, shear, bending, or torsion. In a wing, spars are the primary bending members, ribs hold the airfoil contour and pass loads into the spars, stringers stiffen the skin, and the skin carries shear plus part of the bending in a semi-monocoque design. In a fuselage, longerons run fore-and-aft, frames and formers give shape, bulkheads pick up concentrated loads, and the skin carries shear and the hoop tension of pressurization. Truss structures carry load as tension or compression in individual members; true monocoque construction relies on skin alone and is rare.
Materials and fasteners
Know 2024 (good fatigue strength, usually Alclad for corrosion protection), 7075 (highest strength, poor corrosion resistance, not readily weldable), and 6061 (weldable, good corrosion resistance). Tempers: -O annealed, -T6 solution treated and artificially aged, -H strain hardened. Rivet choices: 2117-T4 for general structure, 2024-T4 where shear strength is critical, 5056 for magnesium, Monel near steel and hot areas. Holes are drilled and deburred, never punched.
Traps
Edge distance is measured from the center of the hole to the edge of the sheet, commonly 2D minimum, with pitch of roughly 3D to 4D; the repair table governs. Expect corrosion items: exfoliation in clad sheet, galvanic attack where stainless fasteners meet aluminum, stress-corrosion cracking in 7075. Also expect bend allowance and setback math.
Worked example
Bend allowance for 0.040-in 2024-T3, inside radius 0.125 in, 90-degree bend: BA = (0.01743R + 0.0078T) x degrees = (0.01743 x 0.125 + 0.0078 x 0.040) x 90 = (0.00218 + 0.00031) x 90 = about 0.224 in. Setback for a 90-degree bend = R + T = 0.165 in. Flat pattern = sum of legs minus setbacks plus bend allowance.