Chapter 24: The Exhaust System and Thrust Reversal
24.1 The Exhaust System
- The exhaust system directs the hot gases from the turbine rearward to form the high-velocity
- jet that produces thrust.
- Components:
- Exhaust Cone: A cone behind the turbine that smooths the gas flow and prevents turbulence.
- Jet Pipe: The main duct carrying the gases to the nozzle.
- Propelling Nozzle: A convergent nozzle at the end of the jet pipe that accelerates the exhaust
- gases to their final exit velocity.
- Choked Nozzle: In a pure turbo-jet, the exhaust gas velocity reaches the local speed of sound
- (Mach 1.0) at the nozzle exit at most power settings above idle. At this point, the nozzle is
- said to be choked, and the only way to further increase the exit velocity (and thus thrust) is to
- increase the temperature of the gas (e.g., by using afterburner/reheat).
24.2 Thrust Reversal
- Thrust reversers are used after landing to redirect the engine's thrust forward, providing a
- powerful braking force that reduces landing distance and saves wear on the wheel brakes.
- Types:
- Clamshell or Bucket Doors: Used on pure jet or low by-pass engines, these doors swing into
- place to block the hot gas stream and deflect it forward through cascade vanes.
- Cold Stream (Blocker) Reverser: Used on high by-pass turbofan engines. Blocker doors
- are deployed inside the fan duct to block the cold by-pass air and redirect it forward through
- cascade vanes on the side of the nacelle. The hot core exhaust is not affected.
- Safety: Thrust reversers have numerous safety interlocks. They can typically only be
- deployed when the throttles are at idle and when the aircraft's weight-on-wheels switches
- indicate it is on the ground.