Part-66 Module 17: propeller theory, pitch control and maintenance

Module 17 is 32 multiple-choice questions in 40 minutes and is part of the B1.1 licence because many turbine aeroplanes are turboprops.

Fundamentals

• Blade angle: chord line to plane of rotation. Angle of attack: chord line to relative airflow. Helix angle: relative airflow to plane of rotation. Blade angle = angle of attack + helix angle.
• Blades are twisted (smaller angle at the tip) because the tip moves faster; this keeps the angle of attack similar along the blade.
• Geometric pitch is the theoretical advance per revolution; effective pitch is the actual advance; the difference is slip.
• Forces: centrifugal force (largest, pulls blades out), thrust bending (forwards), torque bending (against rotation), centrifugal twisting moment (towards fine pitch) and aerodynamic twisting moment (towards coarse pitch).
• At constant rpm, more forward speed means a smaller angle of attack, so fixed-pitch propellers are efficient at only one speed. Tip speeds near Mach 1 waste power, which is why turboprops use reduction gearboxes.

Construction

Fixed-pitch, ground-adjustable, controllable-pitch and constant-speed propellers. Blades are aluminium, composite or wood; composite blades carry metal erosion shields; wooden propellers need periodic hub bolt torque checks and horizontal storage. Single-acting hubs use oil one way and springs, counterweights or CTM the other; double-acting hubs use oil both ways.

Pitch control

A governor holds the selected rpm: flyweights against a speeder spring move a pilot valve. Overspeed throws the flyweights out and increases pitch; underspeed does the opposite. On a typical single-engine aircraft, loss of oil pressure sends the blades to fine pitch; on a typical light twin, springs or counterweights send them to feather, turning the blades edge-on to stop windmilling. Turboprops add beta (ground range with direct blade control and reverse), flight fine pitch stops, overspeed governors, pitch locks, feathering pumps and autofeather.

Synchronising and ice protection

A synchroniser matches slave rpm to the master; a synchrophaser also controls blade phase. Authority is limited and the system is off for take-off and landing. Electrical de-icing boots are fed through slip rings and brushes and cycled by a timer; fluid systems use a slinger ring and isopropyl alcohol.

Maintenance and storage

Check track (tips in the same plane) and balance (static on a stand, dynamic on the aircraft with an accelerometer and optical tachometer). Dress small nicks out within limits; reject cracked blades; never straighten a bent blade in the field; inspect the engine after a propeller strike. Treat a piston-engine propeller as live. Store propellers protected, preserved and, for wood, horizontal.