Aircraft General Knowledge — Airframes & Systems

Fuselage, Wings and Stabilizing Surfaces

An aircraft is subject to various forces which act on the structure both on the ground and in flight.

During flight the wings produce lift which tends to bend the wing upwards, as a result there will be compression on the upper surface and tension on the lower. Lift also causes a torsional force which twists the wing.

Definitions, Loads Applied to Aircraft Structures

Tension: A tension, or tensile load is one which tends to stretch a structural member. Components designed to resist tensile loads are known as ties.

Compression: Compressive loads are the opposite of tensile loads and tend to shorten structural members. Components designed to resist compressive loads are known as struts.

Shear: A force which tends to slide one face of the material over an adjacent face. Riveted joints are designed to resist shear forces.

Bending: Involves tension (outer edge stretches), compression (inner edge squeezes), and shear across the structure.

Torsion: Twisting forces produce tension at the outer edge, compression in the centre and shear across the structure.

Stress: Internal force per unit area inside a structural part. Measured in N/mm2 or MN/m2.

Strain: Deformation caused by the action of stress on a material, expressed as a percentage change.

Fatigue: A structure subject to cyclic loads will fail at a load less than for a steadily applied load.

Design Limit Load (DLL): Maximum load expected in service. Transport Aircraft: +2.5 / -1.0g. Utility: 4.4g. Aerobatic: 6g.

Design Ultimate Load (DUL): DLL × safety factor (minimum 1.5). Structure must withstand DUL without collapse.

Design Philosophies

Safe Life: Minimum life during which no catastrophic damage should occur. Items replaced after elapsed life-count.

Fail-safe / Damage Tolerant: If one structural member fails, an adjacent part carries loads until next inspection.

Damage Tolerant: Spreads loading over larger area; damage detectable during normal inspection cycles before failure occurs.

Fuselage Design Types

Rectangular: Non-pressurized, easy to construct, high weight-to-strength ratio.

Circular: Ideal for pressurised aircraft, hoop stresses spread evenly.

Double Bubble: Effective use of space for passengers and cargo.

Fuselage Construction

Framework: Truss type structure using longerons, frames and struts.

Monacoque: Skin carries all loads, no internal framework.

Semi-monocoque: Skin plus internal formers and stringers share loads - most common in modern aircraft.

Axial Stress: Longitudinal stresses that tend to elongate the fuselage under pressurization.

Hoop Stress: Radial stresses that tend to expand the fuselage cross-section. Up to 65.5 kN/m2 (9.5 psi).

Wing Structure: Spars (main structural members), ribs (maintain aerofoil shape), stringers and stressed skin.

Flutter: Oscillation of control surface due to bending/twisting under load. Prevented by mass balancing.

Materials: Aluminium alloys (most common), titanium (high strength/heat areas), steel, composites.

Corrosion: Deterioration due to chemical action. Types: galvanic, intergranular, stress corrosion, fretting.

Station Numbers: Fuselage Station (FS) measured in inches from datum. Wing Station (WS) from centreline. Water Line (WL) from horizontal datum.