Editing Thrust reversal (section)

==Principle and uses==
[[File:Rolls Royce RB.199 2.jpg|thumb|Half-deployed [[Target-type thrust reversal|target-type]] reverser of a [[Turbo-Union RB199|RB.199]] engine for the [[Panavia Tornado]], one of very few fighter aircraft with thrust reversal]]

A landing roll consists of touchdown, bringing the aircraft to taxi speed, and eventually to a complete stop.  However, most commercial jet engines continue to produce thrust in the forward direction, even when idle, acting against the deceleration of the aircraft.<ref name="Airplane handbook2011">{{cite book|author=Federal Aviation Administration|title=Airplane Flying Handbook:Faa-h-8083-3a|url=https://books.google.com/books?id=Zd5pt-nu6HUC&pg=PT635|access-date=9 July 2013|date=1 September 2011|publisher=Skyhorse Publishing Inc.|isbn=978-1-61608-338-0|pages=635–638}}</ref> The brakes of the [[landing gear]] of most modern aircraft are sufficient in normal circumstances to stop the aircraft by themselves, but for safety purposes, and to reduce the stress on the brakes,<ref name="Croucher2004 JAR" /> another deceleration method can be beneficial. In scenarios involving bad weather, where factors like snow or rain on the runway reduce the effectiveness of the brakes, and in emergencies like [[rejected takeoff]]s,<ref>{{Cite web|url=https://www.boldmethod.com/blog/expressjet/how-to-handle-an-engine-failure-on-takeoff-v1-in-a-jet/|title=How Jet Crews Make Their Go/No-Go Decision During Takeoff|access-date=2020-06-16|archive-date=2020-06-17|archive-url=https://web.archive.org/web/20200617004621/https://www.boldmethod.com/blog/expressjet/how-to-handle-an-engine-failure-on-takeoff-v1-in-a-jet/|url-status=live}}</ref> this need is more pronounced.<ref name="Soares2011 Gas turbines">{{cite book|author=Claire Soares|title=Gas Turbines: A Handbook of Air, Land and Sea Applications|url=https://books.google.com/books?id=rTPZp1YCQBkC&pg=PA315|access-date=11 July 2013|date=1 April 2011|publisher=Butterworth-Heinemann|isbn=978-0-08-055584-3|pages=315–319, 359|archive-date=8 September 2024|archive-url=https://web.archive.org/web/20240908042140/https://books.google.com/books?id=rTPZp1YCQBkC&pg=PA315#v=onepage&q&f=false|url-status=live}}</ref>

A simple and effective method is to reverse the direction of the exhaust stream of the jet engine and use the power of the engine itself to decelerate. Ideally, the reversed exhaust stream would be directed straight forward.<ref name="MacIsaacLangton2011 Propulsion" /> However, for [[aerodynamic]] reasons, this is not possible, and a 135° angle is taken, resulting in less effectiveness than would otherwise be possible. Thrust reversal can also be used in flight to reduce airspeed, though this is not common with modern aircraft.<ref name=Purdue>{{cite web|title=Thrust Reversing|url=https://engineering.purdue.edu/~propulsi/propulsion/jets/basics/reverse.html|publisher=Purdue AAE Propulsion|access-date=10 July 2013|archive-date=13 March 2019|archive-url=https://web.archive.org/web/20190313163405/https://engineering.purdue.edu/~propulsi/propulsion/jets/basics/reverse.html|url-status=live}}</ref> There are three common types of thrust reversing systems used on jet engines: the target, clam-shell, and cold stream systems. Some propeller-driven aircraft equipped with [[Variable-pitch propeller (aeronautics)|variable-pitch propeller]]s can reverse thrust by changing the [[Pitch plane|pitch]] of their propeller blades. Most commercial jetliners have such devices, and it also has applications in military aviation.<ref name="MacIsaacLangton2011 Propulsion">{{cite book|author1=Bernie MacIsaac|author2=Roy Langton|title=Gas Turbine Propulsion Systems|url=https://books.google.com/books?id=cCd_-bNmtmcC&pg=PA152|access-date=11 July 2013|date=6 September 2011|publisher=John Wiley & Sons|isbn=978-0-470-06563-1|pages=152–155|archive-date=8 September 2024|archive-url=https://web.archive.org/web/20240908042237/https://books.google.com/books?id=cCd_-bNmtmcC&pg=PA152#v=onepage&q&f=false|url-status=live}}</ref>