Chapter 26: 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.