📖 The lesson
Propeller nomenclature
- Hub — the central part that mounts on the crankshaft or reduction-gear shaft and holds the blades. The hole through it is the hub bore; on wooden propellers a steel hub (flange) assembly carries the bolts.
- Blade shank — the thick, rounded section near the hub, designed for strength rather than lift. It may be covered by a cuff (an aerofoil fairing) to improve cooling airflow.
- Butt (root) — the end of the blade held in the hub, often with a flange or shoulders for the retention bearings.
- Blade face — the flat (pressure) side, seen from the cockpit. Blade back — the cambered (suction) side, like the top of a wing.
- Leading edge — the thick front edge that meets the air first. Trailing edge — the thin rear edge. Tip — the outer end, often square, elliptical or swept.
- Blade stations — reference distances measured outward from the centre of the hub, in inches or millimetres. They locate blade-angle reference points, damage and repairs (e.g. "a nick at station 30").
- Plane of rotation — the plane in which the blades rotate, perpendicular to the crankshaft. Pitch-change axis — the axis about which a variable-pitch blade turns.
Wooden propellers
- Built from at least five (often seven or more) laminations of hardwood such as birch, sugar maple, black cherry, ash or mahogany, each about 19 mm (3/4 in) thick. Layers are glued with waterproof resin glue, so the grain alternates slightly, reducing warping.
- The laminated blank is roughed out, shaped to templates, finished and balanced. The outer blade may be covered with cotton fabric (sheathing) to strengthen thin sections and resist splitting.
- Metal tipping (brass, Monel or stainless steel) is screwed and riveted to the leading edge and tip for erosion protection. Small drain holes drilled in the tip let moisture and wood sap escape and must be kept open.
- The finish is clear varnish or urethane to keep moisture out; moisture changes in the wood change balance and dimensions.
- Wood shrinks with time, so hub bolts lose torque. They must be re-torqued after the first run and at intervals (and with seasonal humidity changes), in the specified sequence, then safetied.
Aluminium alloy and steel propellers
- Most fixed- and variable-pitch propellers on light and medium aircraft use forged aluminium alloy blades (e.g. 2025-T6 or 7076-T61 types). The forging is machined to the aerofoil, heat-treated, ground and polished, then often shot-peened to improve fatigue resistance. They are anodised and painted.
- Aluminium alloy blades are thinner and more efficient than wood, need little maintenance and can be dressed to remove small nicks. They are, however, sensitive to fatigue from surface damage and to corrosion.
- Fixed-pitch metal propellers are one-piece forgings; blade angle is fixed at manufacture (small corrections by an approved repair station only).
- Steel blades (older, large propellers) were hollow, made from two welded halves with a rib or foam filling. They are heavy but very robust; few remain in service.
- Hubs of variable-pitch propellers are machined aluminium or steel forgings with blade retention bearings and the pitch-change mechanism.
Composite propellers
- Modern turboprop and many light-aircraft blades are composite: a foam or honeycomb core around a carbon-fibre (or metal) spar, with outer shells of carbon, glass or aramid (Kevlar) fibres in epoxy resin.
- A metal ferrule/shank at the root carries the retention loads. A nickel or stainless steel erosion sheath protects the leading edge, and a polyurethane coating protects the surface. Conductive mesh or strips give lightning protection.
- Advantages: low weight (less centrifugal load and lower hub loads), excellent fatigue resistance and vibration damping, and freedom to make complex swept, thin aerofoils that are quieter and more efficient.
- Disadvantages: impact damage may be hidden — delamination and disbonds are found by coin tap or instrumented tap testing. Repairs need approved materials, cure cycles and facilities.
- Natural-fibre composites (e.g. wood-composite blades with glass or carbon skins) combine the light weight of wood with the durability of composite shells.