17.3 and 17.4 Propeller pitch control, feathering, reverse, overspeed protection and synchronising

Scope

• 17.3 Pitch control — A level 1, B1 level 2: speed control and pitch change — mechanical and electrical/electronic; feathering and reverse pitch; overspeed protection.
• 17.4 Synchronising — B1 only, level 2: synchronising and synchrophasing equipment.

The constant-speed principle

The pilot selects an rpm with the propeller lever, which sets the tension of the governor speeder spring. The governor then changes blade angle so that the propeller absorbs exactly the power delivered:

• engine power increased or aircraft dives → rpm tends to rise → governor increases pitch (coarser) → more load → rpm returns;
• power reduced or aircraft climbs → rpm tends to fall → governor decreases pitch (finer).

Hydromechanical governor

Part	Function
Governor gear pump	Boosts engine oil pressure to the higher pressure needed to move the blades
Flyweights	Driven by the engine; centrifugal force moves them outward as rpm rises
Speeder spring	Opposes the flyweights; its compression (set by the propeller lever) determines the rpm selected
Pilot valve	Moved by the flyweights/spring balance; directs oil to or from the propeller pitch-change piston
Relief valve	Limits governor oil pressure

Condition	Flyweights	Pilot valve	Result
On-speed	Balanced against the spring (upright)	Centred — oil trapped	Blade angle held
Overspeed	Fly out	Moves to change pitch towards high pitch	rpm falls back
Underspeed	Move in	Moves to change pitch towards low pitch	rpm rises back

Oil reaches the propeller through a transfer bearing/sleeve and the hollow crankshaft (or propeller shaft).