# Why do thrust reversers on jets work? **URL:** \ **Category:** Factual Questions\ **Created:** [October 11, 2007, 11:49am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339 "2007-10-11T11:49:27Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Dorjan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dorjan/32/12440_2.png) [@Dorjan](https://boards.straightdope.com/u/Dorjan)\ **Post date:** [October 11, 2007, 2:08pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/21 "2007-10-11T14:08:02Z")
[QUOTE=Vetch] Depends on what aircraft it is. I’ve seen intakes there for a third engine, an APU, cooling for a component etc. Passenger air is usually taken from the engine(s) and then conditioned. [/QUOTE] If the intake is at the base of the tail, it’s almost certainly for the APU. ---
**Author:** ![Ponder\_Stibbons](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponder_stibbons/32/343_2.png) [@Ponder\_Stibbons](https://boards.straightdope.com/u/Ponder_Stibbons)\ **Post date:** [October 11, 2007, 2:13pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/22 "2007-10-11T14:13:46Z")
[QUOTE=Mangetout] Just to add - if the exhaust gas could be considered to have left the system before hitting the thrust reverser, then those cartoon devices consisting of a raft with a sail, driven by a fan _mounted on the raft_ would work. They don’t. [/QUOTE] They _don’t_? Next thing you’ll tell is I’m wrong in my long-cherished belief that all I need to survive a fall from a cliff in the Grand Canyon is a small sign that has “Ouch!” printed on it. ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [October 11, 2007, 2:20pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/23 "2007-10-11T14:20:07Z")
[QUOTE=Ponder Stibbons] They _don’t_? Next thing you’ll tell is I’m wrong in my long-cherished belief that all I need to survive a fall from a cliff in the Grand Canyon is a small sign that has “Ouch!” printed on it. [/QUOTE] No, silly. The key is jump up just before you hit the ground. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 11, 2007, 2:20pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/24 "2007-10-11T14:20:11Z")
[QUOTE=Ponder Stibbons] They _don’t_? Next thing you’ll tell is I’m wrong in my long-cherished belief that all I need to survive a fall from a cliff in the Grand Canyon is a small sign that has “Ouch!” printed on it. [/QUOTE] You don’t even need that - just stand on some object for most of the fall, then step or jump off it just before it hits the ground. ---
**Author:** ![Scuba\_Ben](https://avatars.discourse-cdn.com/v4/letter/s/278dde/32.png) [@Scuba\_Ben](https://boards.straightdope.com/u/Scuba_Ben)\ **Post date:** [October 11, 2007, 2:26pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/25 "2007-10-11T14:26:04Z")
[QUOTE=Mangetout] You don’t even need that - just stand on some object for most of the fall, then step or jump off it just before it hits the ground. [/QUOTE] Bad idea, **Mangetout** – if the object is big enough to stand on, then when you step off it, it’ll slow down and land on top of you. You get boggled both from hitting the ground and from the object hitting you. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 11, 2007, 2:35pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/26 "2007-10-11T14:35:19Z")
[QUOTE=Scuba\_Ben] Bad idea, **Mangetout** – if the object is big enough to stand on, then when you step off it, it’ll slow down and land on top of you. You get boggled both from hitting the ground and from the object hitting you. [/QUOTE] Well, obviously not a piano or a safe - that never ends well. ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [October 11, 2007, 2:37pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/27 "2007-10-11T14:37:33Z")
I find it easiest to think of it this way: The engine has to provide a push on the air to stop it from moving backward and get it to move forward (all relative to the plane). Because of Newton’s third law, that forward push on the air results in the air pushing backwards on the engine. You can invoke bypass streams and compression and combustion all you want…but this is what it boils down to. It’s the end result, no matter what the means are. Same thing happens if you shoot a fire hose at your friend. In the process of his body pushing against the water, the stream is brought to a stop or even reversed a little bit. Because momentum must be conserved, and actions result in equal and opposite reactions, he experiences a backwards push from the water he just pushed against. If he were to hold a U-shaped pipe to catch the water and turn it around at you, he’d feel even more of a backwards push by more efficiently changing the water’s momentum. It would also be really funny. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [October 11, 2007, 2:45pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/28 "2007-10-11T14:45:25Z")
The exhaust gas is greater in volume and temperature due to the fuel added and converted to gas. Just temperature by itself would be enough if not then rocket engines would not need to burn the fuel, just spray it out. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [October 11, 2007, 2:49pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/29 "2007-10-11T14:49:35Z")
Thought experiment: Suppose you are sitting on a zero friction platform. You are on a surface which has rocks scatterd about, some always within your reach. If you reach out, grab a rock, and set it on your lap, you will move slightly toward where the rock was, such that the combined center of gravity of you, your hovercraft, and the rock remains unchanged. Note that this operation produces a change in position, but NOT a residual velocity after it is complete. Now take that rock off your lap, and heave it some direction. In this case, you WILL have a residual velocity. Conservation of linear momentum demands that m \* v = M \* V. Since you now have a velocity, you will coast in the direction opposite you threw the rock. Note that your movement is determined solely by the direction you throw the rock, NOT the direction you grab it from. It is a bit more efficient to reach forward and grab the rocks from the direction you wish to go, but it is not a requirement. You can repeat this operation and gain velocity…thus you are producing thrust. You are doing this by taking rocks that were stationary, and giving them velocity opposite your direction of travel. Now substitute “blocks of air” for the rocks. It matters not from what direction you grab the air…only that it ends up with a velocity opposite the direction in which you wish to produce thrust. It is not a zero sum game, because the air ends up moving forward. The force that accelerated that formerly stationary air forward is balanced by a rearward force on the airframe. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 11, 2007, 3:10pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/30 "2007-10-11T15:10:37Z")
To extend **Kevbo** ’s illustration, now mount a small trampoline on the back of your hovercraft throw a rock at it so that it bounces back at you and disappears over your shoulder - the net effect is that the system is ejecting rocks in a direction behind you - so you move, by reaction, in the direction you are facing. ---
**Author:** ![Themenin](https://avatars.discourse-cdn.com/v4/letter/t/ee59a6/32.png) [@Themenin](https://boards.straightdope.com/u/Themenin)\ **Post date:** [October 11, 2007, 5:01pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/31 "2007-10-11T17:01:16Z")
[QUOTE=Mangetout] Just to add - if the exhaust gas could be considered to have left the system before hitting the thrust reverser, then those cartoon devices consisting of a raft with a sail, driven by a fan _mounted on the raft_ would work. They don’t. [/QUOTE] Wouldn’t the sail act as a (inefficient) thrust reverser ? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 11, 2007, 10:48pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/32 "2007-10-11T22:48:12Z")
It would, but I think so much air from the fan would spill past the sides of the sail that it would cancel out or possibly net out to a thrust in the opposite of the intended direction. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [October 12, 2007, 7:49am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/33 "2007-10-12T07:49:12Z")
Oh God it would be SOOOOOO much fun to add a certain piece of exercise equipment to all these cartoon physics thoughts!!! \ ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [October 12, 2007, 8:49am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/34 "2007-10-12T08:49:21Z")
[QUOTE=Dorjän] If the intake is at the base of the tail, it’s almost certainly for the APU. [/QUOTE] It’s hard to tell if it is an intake at all. I doubt it is an APU intake because I don’t think the aeroplane it’s on is big enough to have an APU, it looks more like a ram air intake that may be used as part of the cabin air system. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [October 12, 2007, 8:52am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/35 "2007-10-12T08:52:09Z")
[![](http://www.quartershares.com/jet/Lear25/aLear%2025%20N800GG%20002.jpg) ](http://www.quartershares.com/jet/Lear25/aLear%2025%20N800GG%20002.jpg) It’s a Lear 25 as shown in the above link. Definitely not an APU. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 12, 2007, 12:33pm UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/36 "2007-10-12T12:33:41Z")
Pilot type … The intake at the base of the tail is _probably_ cooling air for the air conditioning system. The overall process is that hot compressed air is pulled off the engine ahead of the combustion section, then cooled via one or more expansion turbines, trading high pressure for lower temperature. All that machinery needs cooling. In cruise the blast of cold air coming in a duct like that is plenty. There’s probably also a fan plumbed in there somplace to provide additional machinery cooling air when needed at low altitude or on the ground. ---
**Author:** ![terrapin52](https://avatars.discourse-cdn.com/v4/letter/t/53a042/32.png) [@terrapin52](https://boards.straightdope.com/u/terrapin52)\ **Post date:** [March 11, 2012, 2:00am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/37 "2012-03-11T02:00:19Z")
[/INDENT] > [@kanicbird](#): > > The exhaust gas is greater in volume and temperature due to the fuel added and converted to gas. > > Just temperature by itself would be enough if not then rocket engines would not need to burn the fuel, just spray it out. FINALLY, after twenty or so asinine and irrelevant answers, someone got it right. Thank god jet engines and rockets aren’t built by committee. It would be a ship of fools which I would not want to board. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [March 11, 2012, 2:08am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/38 "2012-03-11T02:08:56Z")
> [@terrapin52](#): > > [/INDENT] > FINALLY, after twenty or so asinine and irrelevant answers, and _four and a half YEARS!_ ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [March 11, 2012, 2:17am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/39 "2012-03-11T02:17:06Z")
The thrust reverser works because it’s attached to the plane. If it was just blowing against something that was attached the the ground, even if it kept up with the plane, it would make no difference. It works for the same reason that [this](http://tosh.comedycentral.com/video-clips/leaf-blower) works. (Skip to the second half of the clip.) If the sail wasn’t there, he would have gone in the other direction. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 11, 2012, 5:07am UTC](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339/40 "2012-03-11T05:07:52Z")
> [@terrapin52](#): > > FINALLY, after twenty or so asinine and irrelevant answers, someone got it right. Thank god jet engines and rockets aren’t built by committee. It would be a ship of fools which I would not want to board. [Moderator Note] **terrapin52** , please note that insulting other posters, even indirectly, is against the rules of this forum. Also note that this thread is more than four years old, so many of the original posters are no longer participating. Colibri General Questions Moderator [Previous page](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339.md?page=1) [Next page](https://boards.straightdope.com/t/why-do-thrust-reversers-on-jets-work/422339.md?page=3)