Airframe Structures and Loads

26.1 Types of Structural Loads

• An airframe is subjected to five fundamental types of stress:
• 1. Tension (Tensile Load): A stretching or pulling force. Components designed to resist
• tension are called Ties.
• 2. Compression: A squeezing or crushing force. Components designed to resist compression
• are called Struts.

• 3. Shear: A force that tries to slide one part of a structure over another. Riveted joints are
• primarily designed to resist shear.
• 4. Bending: A combination of tension and compression. The top of a bending beam is in
• compression, while the bottom is in tension.
• 5. Torsion: A twisting force.

26.2 Structural Concepts

•  Stress: The internal force within a material that resists an external load. (Stress = Force /
• Area).
•  Strain: The deformation or change in dimension of a material when a load is applied. (Strain
• = Change in Length / Original Length).
•  Elastic Limit: The maximum load a material can withstand and still return to its original
• shape. Loads beyond this limit cause permanent deformation.
•  Design Loads:
• Design Limit Load (DLL): The maximum load the designer expects the airframe to
• encounter in service.
• Design Ultimate Load (DUL): The DLL multiplied by a safety factor (typically 1.5). The
• structure must withstand the DUL without catastrophic failure.

26.3 Structural Design Philosophies

•  Safe Life: A philosophy where a component is designed to be removed from service after a
• specific number of hours or cycles, well before a fatigue failure is expected to occur.
•  Fail-Safe: A design philosophy that uses redundancy, such as multiple load paths or backup
• systems. If one component fails, others are there to carry the load, preventing a catastrophic
• failure.
•  Damage Tolerant: A more modern philosophy that assumes cracks or damage will
• eventually occur. The structure is designed to sustain these damages and allow them to grow
• slowly enough to be detected during regular inspections before they become critical.

26.4 Fuselage Construction

•  Truss/Framework: An underlying rigid framework of tubes (usually steel) that carries the
• primary loads. The structure is then covered by a lightweight, non-load-bearing skin (fabric
• r aluminum). Common on light, unpressurized aircraft.
•  Monocoque: A design where the outer skin carries almost all of the loads. It is very strong
• but susceptible to failure from dents or damage. It requires a very strong, heavy skin.
•  Semi-Monocoque: The most common method of construction for modern aircraft. The outer
• skin is the primary load-bearing structure, but it is reinforced and stiffened by an internal
• framework of:
• Formers/Frames: Give the fuselage its shape.
• Stringers: Run lengthwise to stiffen the skin and prevent buckling.
• Bulkheads: Solid partitions that provide significant structural strength, especially the
• pressure bulkhead at the rear of the cabin.

26.5 Wing Construction

• Modern aircraft use a cantilever monoplane design, where the wing is self-supporting
• without external bracing. The primary structural component is the Torsion Box (or wing
• box), which is designed to resist bending and twisting loads. It is composed of:
•  Spars: The main span-wise structural members of the wing. There is typically a front spar
• and a rear spar.
•  Ribs: Run from the leading edge to the trailing edge, giving the wing its aerofoil shape and
• supporting the skin.
•  Stringers: Run span-wise along the wing, stiffening the skin.
•  Skin: The outer covering of the wing, which carries a significant portion of the flight loads.