Propellers: Theory, Constant-Speed and Icing
Theory
Know blade angle, pitch, chord, and the difference between geometric and effective pitch. Understand that a propeller converts engine power into thrust, and that blade twist keeps the angle of attack roughly constant along the span. Traps include the difference between a fixed-pitch and a constant-speed propeller, and the meaning of propeller slip.
Constant-speed units
A constant-speed unit uses a governor with flyweights and a pilot valve to control oil flow to the hub, changing blade angle to hold RPM. Know that an increase in selected RPM increases oil flow to decrease blade angle. Traps include the direction of blade angle change with RPM selection, and the effect of oil pressure loss, which usually drives the blades to fine pitch or feather depending on design.
Feathering, synchronising and icing
Feathering rotates blades to the flight position to reduce drag on a failed engine. Synchronising matches RPM of multiple engines; synchrophasing matches blade position. Propeller icing adds weight and disrupts thrust, and is handled by electrical heating or fluid. Traps include the fact that feathering is not the same as a windmilling propeller.
Worked example
If RPM falls below the selected value, the governor increases oil flow to reduce blade angle and restore RPM. Rule of thumb: more RPM selected means finer blade angle; feathering is for a failed engine, not for normal shutdown.