# Why does reverse thrust work? **URL:** \ **Category:** Factual Questions\ **Created:** [January 10, 2013, 4:38pm UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758 "2013-01-10T16:38:48Z")\ **Posts on this page:** 7\ **Page:** 13
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [January 25, 2013, 2:25am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/241 "2013-01-25T02:25:58Z")
> [@virtually\_yours](#): > > On the vacuum issue let me know what is happening here: A glass vacuumed sealed tube on wheels. You break one end, if moves in the direction of the break. what and where is the action? thanks > vyVY Unbalanced pressure of the gas molecules on the exterior of the tube, on the opposite side from the break. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [January 25, 2013, 8:33am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/242 "2013-01-25T08:33:22Z")
> [@virtually\_yours](#): > > Actually my scenario was to try and show a no gravity situation.. and the plane would still fly.. imagine how newton did if the earth was whittled away and the plane was flying in a box of air, angle of attack now? In a no gravity situation you don’t need wings to “fly”. Since airplanes are designed to work in a situation with gravity you’ll get all sorts of weird effects, both from wing-shape and angle of attack. Try to imagine this: You’re flying along and your airspeed combined with the wings’ shape and angle of attack is giving you lift equal to the force of gravity, giving you level flight. All of a sudden gravity is turned off. Your lift force is now accelerating you “upwards”, but this changes the axis of your direction of travel and changes both the angle of attack and the direction of air flow towards the wings, and you no longer have “lift”. I expect calculating the behaviour of most aircraft design in a gravity-free airspace would be a quite complex affair. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [January 25, 2013, 11:24am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/243 "2013-01-25T11:24:39Z")
They can and do fly aircraft in a sort of zero gravity…freefall. The vomit comet does it all the time. It is not at all complex to work out, just remove the gravity vector from your free body diagrams. To maintain level flight you lower the nose to put the wing at zero lift AOA and thus have no induced drag, so you can go much faster on the same power. A blimp at neutral buoyancy exploits aerodynamic forces to maneuver. ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 25, 2013, 4:23pm UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/244 "2013-01-25T16:23:22Z")
> [@naita](#): > > In a no gravity situation you don’t need wings to “fly”. Since airplanes are designed to work in a situation with gravity you’ll get all sorts of weird effects, both from wing-shape and angle of attack. > > Try to imagine this: You’re flying along and your airspeed combined with the wings’ shape and angle of attack is giving you lift equal to the force of gravity, giving you level flight. All of a sudden gravity is turned off. Your lift force is now accelerating you “upwards”, but this changes the axis of your direction of travel and changes both the angle of attack and the direction of air flow towards the wings, and you no longer have “lift”. > > I expect calculating the behavior of most aircraft design in a gravity-free airspace would be a quite complex affair. I guess you didn’t understand.. my point was not the flying or floating aspect, but what the air does to the actual aircraft in respect to it.. in a gravity situation, nose up, with not enough speed the aircraft would stall. in a no gravity situation you would have to have movement through the air for the the plane to change direction .(not talking about using thrust vectors) so if the nose was straight up in the air it would be the same as if it was in level flight . and if it was upside down it wouldn’t make a difference because there is no ground reference. So the aircraft doesn’t need gravity to fly.. it is the air that it is flying in so to speak.. gravity is just a force vector acting on the aircraft so to speak.. vyVY ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 25, 2013, 4:29pm UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/245 "2013-01-25T16:29:33Z")
> [@Absolute](#): > > Unbalanced pressure of the gas molecules on the exterior of the tube, on the opposite side from the break. To be clear.. you are saying that on the actual other end of the tube, the closed end, is getting pushed on? I laid the sheet of paper flat on the table and it still sucked up to my mouth when I got close.. where is the air pressure underneath the paper? Or the little bit of air there is enough, and how did the little bit of air under the paper _know_ that there was a vacuum sucking on top? If you get what I am trying to picture. vyVY ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 25, 2013, 6:45pm UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/246 "2013-01-25T18:45:56Z")
> [@virtually\_yours](#): > > To be clear.. you are saying that on the actual other end of the tube, the closed end, is getting pushed on? Yes. Air on the outside is the only thing that could conceivably be pushing on it. > [@](#): > > I laid the sheet of paper flat on the table and it still sucked up to my mouth when I got close.. where is the air pressure underneath the paper? There’s lots of air under the paper. Oxygen and nitrogen atoms/molecules have dimensions measured in _pico_meters; the roughness of the paper (and whatever surface it’s sitting on) is millions of times greater, allowing room for lots of air underneath. You have to get surfaces really smooth and really close together to eliminate a signficant percentage of the air between them. This is actually one mechanism by which [gauge blocks](http://en.wikipedia.org/wiki/Gauge_block) (which are extremely flat and extremely smooth) are suspected to adhere to one another > [@](#): > > …and how did the little bit of air under the paper _know_ that there was a vacuum sucking on top? The air under the paper does not know or care what’s going on above the paper; it’s pushing upwards against the underside of the paper at all times. The paper moves because you have reduced the pressure above it that was pressing down and opposing the pressure on the underside. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [January 25, 2013, 6:50pm UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/247 "2013-01-25T18:50:28Z")
> [@virtually\_yours](#): > > . So the aircraft doesn’t need gravity to fly.. it is the air that it is flying in so to speak.. gravity is just a force vector acting on the aircraft so to speak.. > vyVY So long as it has an engine. Gliders are pretty much powered by gravity. Take it away and they would eventually stop due to drag. Disclosure: I am a glider pilot. [Previous page](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758.md?page=12)