Definition
A duct used in supersonic jet engine inlets and exhaust nozzles whose internal cross-section first narrows (the convergent section) and then widens (the divergent section). The shape is used to slow supersonic airflow to subsonic speeds at the engine inlet, or to accelerate subsonic exhaust gas to supersonic speeds at the nozzle. The narrowest point between the two sections is called the throat.
Plain English
A pipe-like passage that gets narrower, reaches a small middle point, then gets wider again. This shape is used to control the speed of air or exhaust gas in fast jet engines.
Context Anchor
Seen in turbine engine, jet exhaust nozzle, and high-speed airflow discussions.
Derivation
Con-Di' is a shortening of 'convergent-divergent.' Convergent comes from Latin 'convergere,' meaning to come together — the duct narrows. Divergent comes from Latin 'divergere,' meaning to move apart — the duct widens. Knowing this makes the shape easy to picture: walls that come together, then spread apart.
Why Pilots Care
Enables efficient thrust and stable engine operation at high and supersonic speeds.
Analogy
Think of a wasp-waisted hourglass laid on its side. Air enters the wide end, squeezes through the narrow waist, and expands out the other side.
Intuition Check
Con-Di does not mean conditioning, as in an air-conditioning duct. Here it means convergent-divergent: narrowing first, then widening.
Example Sentence 1
The supersonic fighter's engine uses a con-di duct at the exhaust nozzle to accelerate the hot gases past the speed of sound.
Example Sentence 2
During supersonic cruise the inlet uses a con-di duct to keep airflow stable to the compressor.