The previous lesson treated the structure as an abstraction that carries stress. This one makes it physical: how the parts of an airframe are fastened together, what they are made of, where the wings and tail sit and why, which members inside a wing carry which loads, and the vibration problem (flutter) that shapes more design decisions than any other single hazard.
How parts are joined
An airframe is thousands of parts and hundreds of thousands of fasteners, and every joint is a place where loads must cross from one part into another. Six attachment methods divide the work:
Riveting — the classic method for metal skins and frames: a solid rivet is driven so it fills its hole and clamps the sheets, carrying load in shear. Cheap, reliable and inspectable by eye, which is why rivet lines are what a walkaround mostly looks at.
Bolting — for heavy, concentrated loads and for parts that must be removable: wing-to-fuselage attachments, engine mounts, gear fittings. A bolt carries tension as well as shear.
Screwing — for light removable panels and fittings: access panels, fairings, interior equipment.
Pinning — for joints that must rotate: control-surface hinges and landing-gear linkages turn on pins.
Welding — fuses the parts into one piece: steel-tube fuselage frames, exhaust systems, fittings. Only some alloys weld well; many aircraft aluminium alloys lose their strength when welded, which limits its use on primary aluminium structure.
Adhesives (bonding) — glues the parts together over a whole surface: no holes, so no stress concentrations at fastener holes, and the load spreads across the full joint area. The standard joining method for composite structures and honeycomb panels.
Joints are also where attachments fail, and a developing faulty attachment shows itself to a pilot during the pre-flight inspection: rivet or bolt heads that are popped, tilted or missing; the dark oxide trail of a fastener working loose in its hole (the smoking rivet the previous lesson introduced); cracked paint or sealant lines along a joint; panels that bulge, rattle or no longer sit flush; corrosion staining concentrated along a seam; and on bonded structure, bubbling or a lifting edge where the bond is letting go. Any of these means the load path through that joint is changing, and it is maintenance's problem before it is flight's.
Materials and composites
Several types of materials are used on an aircraft, chosen by the type of structure or component and the material properties required there:
Aluminium alloys — for most metal airframes: light, strong, affordable, but prone to the fatigue and corrosion of the previous lesson;
Steel — where great strength in a small volume matters (landing-gear members);
Titanium — where strength must survive heat;
Composites — increasingly, for whole primary structures.
A composite is two materials working as one: strong, stiff fibres embedded in a resin matrix. The fibres carry the loads along their own direction; the resin fixes them in position, transfers load between them, and gives the part its shape. The usual non-metallic ingredients:
carbon fibre — the stiffest and strongest per kilogram; the primary structure of modern transport aeroplanes;
glass fibre — cheaper and less stiff; fairings, radomes and light-aircraft structures;
Kevlar aramid — exceptionally tough against impact and abrasion; used where damage resistance matters;
resin or filler — the matrix that binds any of them, with fillers adjusting its properties.
Against metal alloys, the comparison the exam wants runs point by point:
Strength-to-weight ratio — better: the same strength is bought with less mass, which is the whole reason airliners moved to composite wings and fuselages.
Tailoring strength to the load direction — a composite's strength lies along its fibres, so the designer lays the fibres along the load paths and puts strength only where load actually goes. A metal sheet is equally strong in every direction, needed or not.
Stiffness — carbon composites can be made very stiff, and the stiffness can be tuned in chosen directions, which matters again when this lesson reaches flutter.
Electrical conductivity (lightning) — worse, and dangerously so: a metal skin conducts a lightning strike around the aircraft, while composite barely conducts at all, so composite skins carry an embedded metal mesh or foil to spread strike current.
Resistance to fatigue and corrosion — much better: composites do not corrode, and they resist the cyclic crack growth that limits metal life.
Cost — higher, in both raw material and manufacturing.
Discovering damage during a pre-flight inspection — harder: an impact that would leave an honest dent in metal can leave a composite looking nearly untouched while the layers beneath have separated (delamination). Barely visible impact damage is the reason any known impact on composite structure gets reported and inspected, however clean the surface looks.
Where the wing and tail go
Before the wing's insides, its position. Where the designer mounts the wing and the tailplane is a package of trade-offs the syllabus wants argued both ways:
High-mounted wing — the fuselage hangs low for easy loading, with door sills near ground height even on rough fields, and the wing and its engines or propellers sit clear of ground debris. A wing riding above the fuselage also gains a strong dihedral effect of its own from the sideslip crossflow over the body (the Principles of Flight course showed the mechanism), so little geometric dihedral is needed. Against it: the landing gear cannot go into the high wing, so it goes into the fuselage, with a narrow track or bulky external fairings, and the wing's carry-through structure crosses above the cabin.
Low-mounted wing — the gear attaches short and light into the wing structure with a wide track, and the wing box passes beneath the cabin floor where it steals no cabin space. Against it: less natural roll stability, so more dihedral is needed, and underwing engines run closer to debris on the ground.
Low- or mid-set tailplane — the conventional arrangement: the tailplane loads feed straight into the fuselage, keeping the fin light, and stall behaviour stays conventional. Against it: the tailplane flies in the wing's downwash and wake, so it works in disturbed air.
T-tail — the tailplane rides on top of the fin in air the wing's wake does not reach, so it works in clean flow (and clears the exhaust efflux of rear-fuselage-mounted engines). Against it: the fin must now carry and stiffen against the tailplane's loads, making it heavier, and at very high angles of attack the wing's wake can blanket the high tailplane: the deep stall the Principles of Flight course examined.