3 ms·

This was one of the weirdest parts of the engineering course. There's so many wrong explanations of why planes fly, and we went through them. It's not equal-t

This was one of the weirdest parts of the engineering course. There's so many wrong explanations of why planes fly, and we went through them. It's not equal-time-to-the-end-of-the-wing, and it's not pushes-flow-down-so-goes-up. It's a good time to do some critical thinking, though: 1) We observe that planes can fly upside down. Does the theory under evaluation explain this? 2) It's hard to take off behind another plane that's just taken off. Why?

formerly_proven 5y ago
And some planes can go straight up without (?) being deflected to the side.
- lmilcin 5y ago
  Those planes work in a little bit different regime. For a plane flying straight up you have (usually) two things happening: 1. It is trading kinetic energy for potential energy. 2. They have so much power that their engines can literally lift the entire plane. These planes do not fly. They just use the air to keep the right direction but do not get meaningful lift when going straight up in a straight line.
  - meheleventyone 5y ago
    A wing generates lift in relation to the relative airflow over it, so a plane with some velocity "straight upwards" generates lift exactly equivalent to the lift generated when flying "horizontally" at the same velocity. You can have aircraft with engines powerful enough that they can overcome gravity with thrust alone so can fly upward indefinitely but their wings will still be generating lift, even if they are travelling vertically and it's not useful for overcoming gravity.
    - dTal 5y ago
      Through surfaces on the horizontal stabilizer, the pilot controls the angle of attack of the wing, and hence the "lift" force perpendicular to it. If the pilot doesn't want any lift, there's no lift. The "vomit comet" for instance achieves weightlessness by flying a totally ballistic trajectory; the wings produce no lift for up to 30 seconds at a time.
      - meheleventyone 5y ago
        Yes AoA and it's relation to the wing generating lift is just a technical term for "a wing generates lift in relation to the relative airflow over it".
    - lmilcin 5y ago
      You are wrong. "Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air." https://www.grc.nasa.gov/www/k-12/airplane/lift1.html#:~:text=Lift%20is%20the%20force%20that,the%20airplane%20through%20the%20air https://www.grc.nasa.gov/www/k-12/airplane/lift1.html#:~:tex.... Since gravitation always points down (BY DEFINITION), "the force that DIRECTLY OPPOSES IT" can only point up. I hope you don't plan to dispute NASA on what is lift.
      - meheleventyone 5y ago
        That's a simple, introductory explanation for kids and that sentence is contradicted twice on that page as well. For example: "Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction." And: "Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion. Drag acts in the direction opposed to the motion." So I'm not wrong, even according to the page you cite.
      - FabHK 5y ago
        Lift is defined to be perpendicular to relative wind (and drag parallel to it). What you cite is the simplification that works eg in unaccelerated straight and level flight.
lmilcin 5y ago
Actually, it is "pushes-flow-down-so-goes-up". You remember? Action requires opposite and equal reaction? Plane is (mostly) not a rocket and so to be able to fly it has to act on something and the only something up there is air. If the plane is being lifted it means a bunch of air needs to be pushed/dragged down. Helicopter is a better way to visualize it. Now, in a plane, once that air is pushed/dragged down it immediately starts doing funky things like vortices and so on, but at least initially the air is being pushed/dragged down. I write "pushed/dragged down" because the air is deflected on both sides of the wing. On the low pressure side of the wing (hopefully for most people this is the upper side) it is being deflected by creating low pressure in a laminar flow. If that laminar low pressure flow state is lost and instead we get turbulences, those turbulences are really bad at inducing the air to go down. We say that "it has stalled". EDIT: The only way for something to stay up is to push off something. When you stay on the ground you push off the ground but when you are up there you have to push something else even to just stay at an altitude. Wings, turbines, rocket engines, propellers, barn doors, these are just different ways to accomplish the same task -- push air (and very infrequently some other material) downwards. A plane is different from a rocket in that the material is mostly air (and very tiny amount of fuel) and main mechanism of pushing it are wings and air breathing engines/turbines. Some planes can have so much power that their main pushing mechanism is engine exhaust and additional air sped up by the turbine. But most planes have engines that do not produce anywhere near what is needed to stay up this way. These planes use the engines to push (mostly) forward (there is slight downwards vector due to engine exhaust being in line with the plane body but AoA positive). These planes rely on air stream being deflected downwards as seen by the plane to be economical.
- lordnacho 5y ago
  No, this is wrong. First of all, rockets are no exception. Just throwing things backwards will make the object fall to the ground, no matter how hard it pushes. It is indeed the air that pushes the wing up, but you have to ask yourself why the wing would need to be shaped in a certain way if it's merely deflection. If it was that, you could just have flat boards at an angle and be done with it. Vorticity has to come into the explanation, and explains both the points further up.
dTal 5y ago
>it's not pushes-flow-down-so-goes-up. Yes, it is. It cannot be any other way; momentum is always conserved. You need to get that 9.81m/s^2 acceleration from somewhere! To answer your two questions: 1) A simple wing is basically a teardrop shaped fairing around a flat plate - not unlike a rudder. Like a rudder, it can redirect the flow in either direction, depending on the angle it meets the air. (In fact, most aircraft struggle to fly upside down - if they're not aerobatic they tend to have wings that are like fairings around curved plates, to more efficiently redirect air in a single direction.) 2) The aircraft in front of you has just imparted a ton of downward momentum to the air. Compared to flight through clean still air, you're on an elevator that's going down.
- ertemplin 5y ago
  Regarding your 2nd answer, taking off quickly after another aircraft has departed is difficult because of wake turbulence. The most hazardous part of wake turbulence is the spiraling wingtip vortices which can be observed in this neat video: https://www.youtube.com/watch?v=dfY5ZQDzC5s https://www.youtube.com/watch?v=dfY5ZQDzC5s One notable wake turbulence incident was in 2006 at Boeing Field in Seattle where a Cessna 172 and Boeing 747 were both on final approach for parallel runways. The Cessna 172 got flipped almost upside down and recovered 150 feet over the terrain: https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20061122X01707&AKey=1&RType=HTML&IType=IA https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?E...

formerly_proven 5y ago

And some planes can go straight up without (?) being deflected to the side.

lmilcin 5y ago
Those planes work in a little bit different regime. For a plane flying straight up you have (usually) two things happening: 1. It is trading kinetic energy for potential energy. 2. They have so much power that their engines can literally lift the entire plane. These planes do not fly. They just use the air to keep the right direction but do not get meaningful lift when going straight up in a straight line.
- meheleventyone 5y ago
  A wing generates lift in relation to the relative airflow over it, so a plane with some velocity "straight upwards" generates lift exactly equivalent to the lift generated when flying "horizontally" at the same velocity. You can have aircraft with engines powerful enough that they can overcome gravity with thrust alone so can fly upward indefinitely but their wings will still be generating lift, even if they are travelling vertically and it's not useful for overcoming gravity.
  - dTal 5y ago
    Through surfaces on the horizontal stabilizer, the pilot controls the angle of attack of the wing, and hence the "lift" force perpendicular to it. If the pilot doesn't want any lift, there's no lift. The "vomit comet" for instance achieves weightlessness by flying a totally ballistic trajectory; the wings produce no lift for up to 30 seconds at a time.
    - meheleventyone 5y ago
      Yes AoA and it's relation to the wing generating lift is just a technical term for "a wing generates lift in relation to the relative airflow over it".
  - lmilcin 5y ago
    You are wrong. "Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air." https://www.grc.nasa.gov/www/k-12/airplane/lift1.html#:~:text=Lift%20is%20the%20force%20that,the%20airplane%20through%20the%20air https://www.grc.nasa.gov/www/k-12/airplane/lift1.html#:~:tex.... Since gravitation always points down (BY DEFINITION), "the force that DIRECTLY OPPOSES IT" can only point up. I hope you don't plan to dispute NASA on what is lift.
    - meheleventyone 5y ago
      That's a simple, introductory explanation for kids and that sentence is contradicted twice on that page as well. For example: "Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction." And: "Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion. Drag acts in the direction opposed to the motion." So I'm not wrong, even according to the page you cite.
    - FabHK 5y ago
      Lift is defined to be perpendicular to relative wind (and drag parallel to it). What you cite is the simplification that works eg in unaccelerated straight and level flight.

lmilcin 5y ago

Actually, it is "pushes-flow-down-so-goes-up". You remember? Action requires opposite and equal reaction? Plane is (mostly) not a rocket and so to be able to fly it has to act on something and the only something up there is air. If the plane is being lifted it means a bunch of air needs to be pushed/dragged down. Helicopter is a better way to visualize it. Now, in a plane, once that air is pushed/dragged down it immediately starts doing funky things like vortices and so on, but at least initially the air is being pushed/dragged down. I write "pushed/dragged down" because the air is deflected on both sides of the wing. On the low pressure side of the wing (hopefully for most people this is the upper side) it is being deflected by creating low pressure in a laminar flow. If that laminar low pressure flow state is lost and instead we get turbulences, those turbulences are really bad at inducing the air to go down. We say that "it has stalled". EDIT: The only way for something to stay up is to push off something. When you stay on the ground you push off the ground but when you are up there you have to push something else even to just stay at an altitude. Wings, turbines, rocket engines, propellers, barn doors, these are just different ways to accomplish the same task -- push air (and very infrequently some other material) downwards. A plane is different from a rocket in that the material is mostly air (and very tiny amount of fuel) and main mechanism of pushing it are wings and air breathing engines/turbines. Some planes can have so much power that their main pushing mechanism is engine exhaust and additional air sped up by the turbine. But most planes have engines that do not produce anywhere near what is needed to stay up this way. These planes use the engines to push (mostly) forward (there is slight downwards vector due to engine exhaust being in line with the plane body but AoA positive). These planes rely on air stream being deflected downwards as seen by the plane to be economical.

lordnacho 5y ago
No, this is wrong. First of all, rockets are no exception. Just throwing things backwards will make the object fall to the ground, no matter how hard it pushes. It is indeed the air that pushes the wing up, but you have to ask yourself why the wing would need to be shaped in a certain way if it's merely deflection. If it was that, you could just have flat boards at an angle and be done with it. Vorticity has to come into the explanation, and explains both the points further up.

dTal 5y ago

>it's not pushes-flow-down-so-goes-up. Yes, it is. It cannot be any other way; momentum is always conserved. You need to get that 9.81m/s^2 acceleration from somewhere! To answer your two questions: 1) A simple wing is basically a teardrop shaped fairing around a flat plate - not unlike a rudder. Like a rudder, it can redirect the flow in either direction, depending on the angle it meets the air. (In fact, most aircraft struggle to fly upside down - if they're not aerobatic they tend to have wings that are like fairings around curved plates, to more efficiently redirect air in a single direction.) 2) The aircraft in front of you has just imparted a ton of downward momentum to the air. Compared to flight through clean still air, you're on an elevator that's going down.

ertemplin 5y ago
Regarding your 2nd answer, taking off quickly after another aircraft has departed is difficult because of wake turbulence. The most hazardous part of wake turbulence is the spiraling wingtip vortices which can be observed in this neat video: https://www.youtube.com/watch?v=dfY5ZQDzC5s https://www.youtube.com/watch?v=dfY5ZQDzC5s One notable wake turbulence incident was in 2006 at Boeing Field in Seattle where a Cessna 172 and Boeing 747 were both on final approach for parallel runways. The Cessna 172 got flipped almost upside down and recovered 150 feet over the terrain: https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20061122X01707&AKey=1&RType=HTML&IType=IA https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?E...