Definition
The hydraulic and mechanical assembly inside a constant-speed or feathering propeller that rotates the blades around their long axis to change the blade angle. It typically uses engine oil pressure acting on a piston in the propeller hub, which moves the blades through a system of links or gears to a new pitch setting commanded by the governor or the pilot.
Plain English
It is the system inside the propeller that twists the blades to a steeper or flatter angle while the engine is running.
Context Anchor
Seen in discussions of constant-speed and feathering propellers, especially when explaining how the propeller blades move into or out of feather.
Derivation
Pitch here means the angle of the propeller blade, borrowed from the idea of how far a screw advances per turn. A pitch changing mechanism is simply the equipment that changes that angle in flight.
Why Pilots Care
It enables RPM control in normal flight and allows the propeller to feather, reducing drag after engine failure.
Analogy
Think of window blinds twisting open or closed. The slats stay in the same frame, but their angle changes; the pitch changing mechanism does something similar with the propeller blades.
Grounding Statement
The key idea is that the mechanism twists the propeller blades; it does not simply make the propeller spin faster or slower.
Intuition Check
Pitch here does not mean the airplane’s nose-up or nose-down attitude, and it does not mean a sound. It means the angle of a propeller blade as it meets the air.
Example Sentence 1
When the pilot pulled the propeller control back, the pitch changing mechanism rotated the blades to a higher angle and the engine RPM dropped.
Example Sentence 2
After the engine failed, the pitch changing mechanism moved the blades to the feathered position to stop windmilling.