# commercial aircraft reverse thrust questions **URL:** \ **Category:** Factual Questions\ **Created:** [April 29, 2016, 2:44pm UTC](https://boards.straightdope.com/t/commercial-aircraft-reverse-thrust-questions/753427 "2016-04-29T14:44:53Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [April 30, 2016, 5:15pm UTC](https://boards.straightdope.com/t/commercial-aircraft-reverse-thrust-questions/753427/21 "2016-04-30T17:15:22Z")
> [@Machine\_Elf](#): > > The problem is that air isn’t sucked into the engine from just straight in front of the intake; it’s sucked in from pretty much every direction near the intake. You can see this when the engines are running while there’s water on the pavement, [as in this video.](https://www.youtube.com/watch?v=VUDu9n2bsjQ) (another good one [here](https://www.youtube.com/watch?v=0JuvarRAbfM)). The presence of the ground makes that vortex point toward it; without any ground near the engine (imagine it’s 100+ feet above the ramp), that downward-pointing vortex would likely disappear, and the inlet flow would exhibit a lot more radial symmetry. So if air is coming into the intake from pretty much all directions, and it’s being deliberately exhausted in all directions, you won’t develop much forward thrust. > > On a related note, the fact that an inlet draws in fluid from all directions around it (instead of just parallel to the orientation of the inlet) is the reason [pop-pop boats](https://en.wikipedia.org/wiki/Pop_pop_boat) can generate a net forward thrust: very little thrust when sucking in water, and a good pulse of forward thrust when farting it out. > > Things look a little different when the engine is moving forward through the air, because now (from the engine’s perspective) it looks like air really is coming straight into the intake. Except the engine isn’t pulling that air in, the air is being crammed down the engine’s throat by the slipstream. If you scatter the exhaust stream in all directions at equal velocity (relative to the engine itself, which again is moving forward through the air), then you end up generating reverse thrust (really just aero drag). Thanks, **Machine Elf** for a lovely explanation. I loves me aerodynamics and that’s a good example of an intuitive look at mass flow and momentum change. Well done. ---
**Author:** ![joema](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@joema](https://boards.straightdope.com/u/joema)\ **Post date:** [April 30, 2016, 11:27pm UTC](https://boards.straightdope.com/t/commercial-aircraft-reverse-thrust-questions/753427/22 "2016-04-30T23:27:44Z")
> [@Machine\_Elf](#): > > The problem is that air isn’t sucked into the engine from just straight in front of the intake; it’s sucked in from pretty much every direction near the intake… Here is a good example of this: [![](https://photos.smugmug.com/photos/i-g27vpFz/0/XL/i-g27vpFz-XL.jpg) ](https://photos.smugmug.com/photos/i-g27vpFz/0/XL/i-g27vpFz-XL.jpg) ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [April 30, 2016, 11:54pm UTC](https://boards.straightdope.com/t/commercial-aircraft-reverse-thrust-questions/753427/23 "2016-04-30T23:54:51Z")
Thrust reversers always amazed me, and remind me of the “astern elements” of a naval steam engine (i.e. in a nuclear powered vessel), in their effectiveness, if not their actual implementation. They typically have two small turbine sections at the [ends of the low-pressure turbine](http://www.massengineers.com/Image788.gif) that are designed to work in reverse, and those provide about 10% of the power as the main engines, and are highly inefficient. If your ship has a total of 200,000 shaft horsepower going forward, you probably have 20,000 going in reverse. So when you want your ship to stop, you shut the forward throttles and pour loads of steam in the very poor astern turbines, but efficiency just isn’t what you are looking for at that point, you just are trying to slow down. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [May 2, 2016, 11:04pm UTC](https://boards.straightdope.com/t/commercial-aircraft-reverse-thrust-questions/753427/24 "2016-05-02T23:04:04Z")
> [@JKellyMap](#): > > That is, the mere \*\*sucking in of air \*\*would pull it forward – enough, even, to get some speed going, and maybe even get airborne? No. There is no such thing as “suck air”. Its pushed. Its pulled by gravity though, but then that creates air pressure, which means it can push itself… So anyway the push from the engine requires the inbalance of pressure from front to back… So if the system reduced pressure at the front and produced pressure at the back, that represents a force forward… Thats why everyone talks about thrust, its the increase in pressure , which creates higher pressure behind , that pushes… When at idle the engine’s intake isn’t acting as a dumb static air brake. But when they put it into reverse, it starts to act as an air brake, braking about the same amount as if a flaperon of that size was there. After considering all the ineffeciencies, it seems “reverse” is just to get that extra braking surface for free. (no extra weight,etc) ---
**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:** [May 2, 2016, 11:36pm UTC](https://boards.straightdope.com/t/commercial-aircraft-reverse-thrust-questions/753427/25 "2016-05-02T23:36:52Z")
> [@Isilder](#): > > After considering all the ineffeciencies, it seems “reverse” is just to get that extra braking surface for free. (no extra weight,etc) It’s not \*just \* a brake. As mentioned earlier aircraft can (but typically don’t anymore) taxi backwards using reverse thrust. [Previous page](https://boards.straightdope.com/t/commercial-aircraft-reverse-thrust-questions/753427.md?page=1)