17B (B3) Propeller fundamentals and construction for light piston aeroplanes

Who takes Module 17B?

Module 17B is the propeller module for the B3 licence (piston-engine, non-pressurised aeroplanes of 2,000 kg MTOM and below). The syllabus headings are the same as 17A (17.1–17.7) at level 2 (level 3 for maintenance), but the aircraft you will work on mostly have fixed-pitch or single-engine constant-speed propellers, sometimes ground-adjustable or in-flight adjustable propellers on light-sport types. EASA: 32 questions (UK: 28).

How a propeller makes thrust

A propeller blade is a rotating aerofoil. Each section of the blade (blade element) moves on a helical path through the air; its lift component along the axis is thrust, and its drag produces a torque that the engine must overcome. Adding up the elements along the blade gives total thrust and torque — this is blade element theory.

Angles

• Blade angle — between the chord line and the plane of rotation.
• Angle of attack — between the chord line and the relative airflow (the helical flow seen by the blade element).
• Blade angle is largest near the hub and smallest at the tip (blade twist), because the tip travels much faster; this keeps the angle of attack roughly constant along the blade.
• Low (fine) pitch = small blade angle; high (coarse) pitch = large blade angle.

Speed effects on a fixed-pitch propeller

• Faster forward speed (same rpm) → relative airflow comes more from ahead → angle of attack falls → thrust and torque fall → rpm rises (risk of overspeed in a dive).
• On the take-off run (low forward speed) the angle of attack is high → a coarse "cruise" propeller gives poor static rpm and take-off performance; a fine "climb" propeller gives good take-off but limits cruise speed (rpm red line reached early).

Pitch and slip

Geometric pitch = distance a propeller would advance in one revolution with no slip: GP = 2 × π × R × tan(blade angle), R and blade angle taken at the 75 % station (FAA).

Effective pitch = actual distance advanced per revolution. Slip = geometric − effective pitch.

Worked example: A light-aircraft propeller is described as "72 × 54" — diameter 72 in and geometric pitch 54 in. At 2,400 rpm and 100 kt (≈ 121,520 in/min): effective pitch = 121,520 ÷ 2,400 ≈ 50.6 in. Slip = 54 − 50.6 = 3.4 in ≈ 6 % of geometric pitch. (100 kt × 6,076 ft/NM × 12 in/ft ÷ 60 min/h ≈ 121,520 in/min.)

Worked example — blade angle from pitch: GP = 54 in at a 75 % radius of 27 in (72 in diameter → radius 36 in × 0.75): tan θ = 54 ÷ (2π × 27) = 54 ÷ 169.6 = 0.318 → θ ≈ 17.7°.

FAA: propeller efficiency (thrust power ÷ shaft power) is typically 50–87 %, depending on slip.

Forces on the blades

Force	Effect
Centrifugal (largest)	Pulls blades outward — tension in blade and hub retention
Thrust bending	Bends tips forward
Torque bending	Bends blades against the direction of rotation
Aerodynamic twisting moment	Tends to increase blade angle
Centrifugal twisting moment	Tends to decrease blade angle — stronger than ATM; used by many light constant-speed propellers to move blades to fine pitch

Other effects pilots and engineers meet: torque reaction (roll), slipstream on the fin (yaw), gyroscopic precession, P-factor (down-going blade produces more thrust at high angle of attack). Propeller-engine combinations can have resonance at certain rpm — the tachometer then shows a restricted range (e.g. a red arc) where continuous operation is not allowed.

Blade and hub terminology

• Hub, blade shank (thick, round section near the hub), blade butt/root, tip, leading/trailing edge.
• Blade back = cambered side (forward-facing); blade face = flat side (facing the pilot).
• Blade stations measured outward from the hub centre — used for blade-angle checks and damage location.

Materials and construction

Type	Construction	Key maintenance points
Wood	5–9 laminations of birch (or maple, ash) glued and varnished; fabric/plastic sheath on outer blade; metal tipping (brass/Monel) on leading edge with drain holes	Re-torque hub bolts periodically and after humidity changes; check for cracks, delamination, loose tipping, elongated bolt holes, charring at hub; store horizontally
Aluminium alloy	Usually one-piece forged (fixed-pitch) or separate blades in a hub (constant-speed)	Dress nicks within limits, check for cracks and corrosion; never straighten in the field
Composite	Carbon/glass fibre shell over foam core with erosion shield (nickel/stainless)	Coin-tap for delamination, erosion shield bonding, lightning damage

Propeller types on B3 aeroplanes

Type	Notes
Fixed-pitch	Single blade angle; climb (finer) or cruise (coarser) versions; static rpm check on run-up confirms correct propeller/engine power
Ground-adjustable	Blade angle set on the ground by loosening clamp bolts and turning blades (common on light-sport/microlight composite propellers); all blades must be set equal with a protractor
In-flight adjustable	Electric or hydraulic adjustment by the pilot without a governor
Constant-speed	Governor holds selected rpm; pilot has a throttle (manifold pressure) and a propeller lever (rpm)

Installation

• Most B3 engines have a flanged crankshaft: propeller (with spacer/extension and spinner bulkhead) bolted to the flange; bushings/dowels index the propeller.
• Torque bolts in stages and a cross pattern to the specified value; safety with lockwire or self-locking nuts as specified; wood propellers often need re-torquing after the first hours and at intervals.
• Spinner: front and rear bulkheads; check for cracks around screw holes and correct alignment; a cracked spinner can cause vibration.
• After installation: track check, ground run, static rpm, vibration check.

Exam traps

• Blade angle reference is the plane of rotation.
• "72 × 54" = diameter × geometric pitch (inches).
• Faster airspeed with a fixed-pitch propeller → lower angle of attack, rpm rises.
• Wooden propellers: drain holes open, bolts re-torqued, store horizontal.
• Station numbers are measured from the hub centre.