Definition
An air inlet duct on a supersonic jet aircraft, shaped so it first narrows (the convergent section) and then widens (the divergent section). At supersonic flight speeds, the convergent section slows the incoming air to the speed of sound at its narrowest point (the throat), and the divergent section then slows the air further to a subsonic speed before it reaches the engine compressor. This conversion of high-speed air into slower, higher-pressure air is necessary because jet engine compressors can only accept airflow at subsonic speeds.
Plain English
A specially shaped air intake on a supersonic jet that first squeezes the incoming air down through a narrow point and then lets it spread back out. This shape slows the air from supersonic speed down to a speed the engine can actually use, while raising its pressure at the same time.
Context Anchor
Seen in discussions of turbine engine inlets, especially on aircraft designed to fly faster than the speed of sound.
Derivation
Convergent comes from the Latin convergere, meaning to incline together — the duct walls angle inward. Divergent comes from divergere, meaning to incline apart — the walls then angle back outward. The name simply describes the duct's shape in the order the air passes through it.
Why Pilots Care
Enables safe and efficient engine operation at supersonic speeds by managing compression without causing damage or surge.
Analogy
Picture a passage shaped a little like an hourglass: it narrows, then opens back up. The shape is not just for fit; it changes how the moving air behaves.
Grounding Statement
On a very fast aircraft, the inlet duct has to turn extremely fast outside air into smoother, higher-pressure air the engine can use.
Intuition Check
Do not read “convergent-divergent” as two separate ducts. It describes one continuous duct shape: narrowing first, widening after the throat.
Example Sentence 1
The Concorde used a convergent-divergent inlet duct to slow the incoming air to subsonic speed before it reached the engine compressor.
Example Sentence 2
During the high-speed test flight the pilot monitored inlet performance to confirm the convergent-divergent inlet duct was managing airflow correctly.