Definition
An air duct in a turbine engine that first narrows (converges) and then widens (diverges) along its length. This shape is used to control the speed and pressure of airflow passing through it. In a converging section, subsonic air speeds up and its pressure drops; in the diverging section that follows, the behavior depends on whether the flow has reached the speed of sound. Con-di ducts are commonly seen in supersonic engine inlets and in the exhaust nozzles of high-performance jet engines, where they are used to accelerate exhaust gases to supersonic speeds.
Plain English
A pipe-like passage for air inside a jet engine that gets narrower in the middle and then wider again. The shape is deliberate — it changes how fast the air moves and how much pressure it has, which is important for engines that need to handle very high-speed airflow.
Context Anchor
Seen in aircraft powerplant discussions, especially when studying airflow or exhaust flow through turbine engine passages and nozzles.
Derivation
‘Con-di’ is a shop-floor shortening of convergent-divergent. ‘Convergent’ comes from the Latin convergere, meaning ‘to incline together’ — the duct walls come closer together. ‘Divergent’ comes from divergere, meaning ‘to incline apart’ — the walls move away from each other. Knowing the two halves of the name tells you exactly what the duct looks like.
Why Pilots Care
Technicians and pilots working with high-performance turbine engines need to recognize con-di ducts because their shape is critical to engine performance. A damaged or incorrectly assembled con-di section can dramatically reduce thrust or cause unstable airflow, especially at high speeds.
Analogy
Think of a garden hose nozzle that narrows down to make water shoot out faster — a con-di duct does something similar with air, but with a second widening section after the narrow point to handle very high-speed flow.
Intuition Check
Do not read “Con-di” as air-conditioning. In this term, it means “convergent-divergent”: narrow first, then widen.
Example Sentence 1
The supersonic fighter’s exhaust nozzle uses a con-di duct to accelerate the gases beyond the speed of sound.
Example Sentence 2
Con-di ducts allow the engine to produce additional thrust at supersonic exhaust velocities.