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Exhaust
Propelling nozzles, the exhaust cone, convergent-divergent nozzles and thrust reversers.
Exhaust — summary notes
FreeMain ideas
- The exhaust system collects the turbine-exit gas and accelerates it through a propelling nozzle to produce (or complete) thrust. In a convergent nozzle the gas speeds up as the area reduces; most subsonic engines use a fixed convergent nozzle.
- The exhaust cone (bullet) and struts straighten the swirling gas leaving the turbine and prevent the flow from expanding back into a low-pressure, turbulent wake behind the turbine disc.
- A convergent-divergent nozzle is used where the gas is supersonic (afterburning/military engines) to expand it efficiently; nozzle area may be variable on such engines.
- Thrust reversers redirect the exhaust (and/or bypass air) forward to help decelerate the aircraft on landing — cascade/blocker-door types for fan air, clamshell/bucket types for the core; they must stow positively and are interlocked to ground/idle logic.
- Exhaust-gas temperature is measured here or upstream of the turbine as a key engine limit; noise-suppressing mixers and chevrons reduce jet noise.
- ⚠ Exam trap: a convergent nozzle ACCELERATES the subsonic gas (area down, velocity up) — the opposite of the divergent intake; thrust reversers act mainly on the high-mass bypass/fan air on a high-bypass engine.
Solved examples
On a high-bypass turbofan, which airstream does the thrust reverser mainly act on during landing, and why?
Mostly the bypass (fan) air, because that is where most of the engine's mass flow and thrust comes from. Cascade-type reversers with blocker doors redirect the fan air forward to produce reverse thrust; acting on the small, hot core flow alone would give little deceleration.
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Exhaust quiz
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