A system that redirects the air exiting a jet engine forwards or outwards, helping to slow the plane down (usually during landing).
A thrust reverser (often shortened to simply reverser) is a device or mechanism that allows the thrust from an aircraft’s engine (s) to be redirected forwards so it slows the aircraft down rather than speeding it up. Thrust reversers are mostly used during landing, but are even more useful in the rare event of a rejected-takeoff; in both cases, there benefits are most pronounced when there is considerable runway-contamination, where stopping an aircraft without the use of thrust reversers can take as much as three times as much distance as stopping it with reversers does. Thrust reversers are one of three main ways of slowing an aircraft down; the others are wheel braking, on the one hand, and drag-producing devices (spoilers and airbrakes), on the other hand.
Aircraft equipped with variable-pitch propellers can, with most types of variable-pitch propeller, move their propeller blades to a negative pitch angle, so that, when the propeller spins, it pushes air forwards, rather than backwards, and, thereby, slows the aircraft down rather than speeding it up.
Jet engines, in contrast, are not generally equipped with variable-pitch fan or compressor blades; as a result, thrust reversers for jets have to redirect the engine’s exhaust forwards after the rest of the engine has already accelerated it backwards.
There are four main ways to do this:
Target-type reversers are what most everyone probably thinks of when they hear the phrase “thrust reverser”; they have a pair of large, moveable doors mounted just behind the engine, which, when reverse thrust is commanded, swing backwards to form a “bucket” behind the engine which catches its exhaust and redirects it forwards. This type of reverser is typically found on turbojet and low-bypass turbofan engines, as using target-type reversers on a turbofan with a high bypass-ratio (and, thus, a large diameter) would require impractically-large bucket doors.
Clamshell-type reversers, instead of using external bucket doors that swing shut behind the engine, use swivelling doors inside the engine, which pivot to block off the air’s path out the tailpipe and open paths for it to exit the engine in the forward direction through openings in the engine cowling. There are two distinct variants of clamshell-type reversers:
- One variant, used mainly on low-bypass turbofans (although it was also used on a few early high-bypass turbofans), has clamshell doors that redirect the engine’s entire output - both the hot core air and the (relatively) cold bypass air. This variant produces more deceleration, for a given engine and power output, than the other variant (see below), but also has a shorter lifetime and higher maintenance costs.
- The other variant, used on high-bypass turbofans, redirects only the engine’s bypass air - the core air passes straight through, unaffected. This works because the vast majority of a high-bypass turbofan’s thrust is generated by the bypass air, with the engine core directly contributing only a small amount of thrust; therefore, redirecting a high-bypass turbofan’s bypass air, but not its core air, will still generate net reverse thrust. As the engine’s core airflow continues to produce forward thrust even with the reverser deployed, this type of reverser (all else being equal) produces less deceleration than one that also redirects the core airflow; however, as the clamshell doors are not exposed to the hot, corrosive combustion gasses and particulates in the core exhaust, they last much longer, are more reliable, and have lower maintenance costs.
The final type of reverser is the cascade-type reverser, used on high-bypass turbofans - especially on the very large high-bypass turbofans powering newer widebody airliners, although it also sees use on some smaller high-bypass turbofans as well. Cascade reversers redirect only the engine’s bypass air (as a result, they are also sometimes known as cold-stream reversers, although, as noted above, many clamshell-type reversers also redirect only bypass air); they do this by sliding the entire rear portion of the engine cowling backwards, which opens up a gap for air to blow forwards through and operates a mechanism that blocks off the rear portion of the bypass duct so that the air can’t continue to go backwards. As the redirected air can exit throughout the entire circumference of the engine rather than just through discrete doorways, cascade-type reversers work better than clamshell-type reversers for redirecting the huge amounts of bypass air from the largest turbofans; they also, unlike target-type or clamshell-type reversers, do not protrude sideways when deployed, which makes them very useful on engines with low ground clearance.
Alternatively, here’s a diagram explaining the differences between the different types of jet-engine thrust reversers:
For more information regarding thrust reversers, see the Wikipedia articles on: