Definition
A ring-shaped chamber inside a turboprop engine that collects compressed air from the compressor and distributes it evenly around the combustion section. In a split-shaft/free turbine engine, the circular plenum surrounds the combustion chamber so that compressed air can flow into it from all sides at a steady, controlled pressure.
Plain English
A round, donut-shaped space inside the engine that holds high-pressure air and feeds it evenly into the area where fuel is burned.
Context Anchor
Seen in descriptions of split-shaft or free-turbine turboprop engine airflow, especially when explaining how gas moves from one engine section to another.
Derivation
From Latin plenum, meaning 'full' or 'a full space.' In engineering, a plenum is any chamber held at a steady pressure that feeds air into something else. 'Circular' simply describes its shape — it wraps around the combustion section like a ring, so air can enter from any side.
Why Pilots Care
Uniform airflow from the plenum helps prevent compressor stalls and maintains smooth engine operation during all phases of flight.
Analogy
Think of water entering a round holding space before flowing out through several openings. The space helps the flow settle and spread instead of rushing unevenly to one side.
Intuition Check
Do not think of a circular plenum as just a round part or cover. In this context, it is a round internal space that manages the flow of air or gas.
Example Sentence 1
Compressed air from the compressor enters the circular plenum, which feeds it evenly into the combustion chamber.
Example Sentence 2
At higher airspeeds the circular plenum improves ram recovery by channeling air smoothly into the engine inlet.