7 ms·
Air moves faster on the upper side, creating a pressure differential.
by jobigoud 5y ago
Air moves faster on the upper side, creating a pressure differential.
- NickNameNick 5y agoWhy does the air move faster on the upper side of the wing? It's not because there's a magic force that requires air particles parted be the leading edge to rejoin thier partner at the trailing edge. The air particles on the upper surface reach the trailing edge much sooner than the ones under the wing.
- rcxdude 5y agoBecause the pressure on the top is lower :) (this is half-serious: the whole problem with these explanations is that cause and effect for all of these variables is not straightforward: you can see from the navier-stokes equations they are all dependent on each other).
- tomsto 5y agoKind of. Actually the real ‘cause’ in my understanding is 1) the curved geometry of the suction (upper) side of the aerofoil and 2) the fact that the flow remains attached to it. Everything else - you can actually approximate the curved surface to a circle and apply equations of circular motion to a parcel of air to satisfy yourself with why the flow is accelerating. And Newton’s 3rd law explains how lift is generated on the wing. In my view there’s no need to use Navier-Stokes to explain how an aerofoil works, if you simplify the geometry to make a special case. Most of the lift comes from the suction side. Actually, if you really want to test an explanation, try to apply the same reasoning to explain how a sailing boat can sail upwind (or at least up to about 45 degrees off).
- shadowgovt 5y agoThe devil is in that last detail. "Flow stays attached" is a description of the properties of the flow, not an explanation for what causes attached flow or why attached flow matters. It's semicircular reasoning to say that the plane gets lift because the flow stays attached... Attached flow and lift are correlated, but they may be two phenomena caused by the same underlying property.
- AstralStorm 5y agoDoes not move faster either. Otherwise, a flat wing would not work, and they do. Gravity or force creates the pressure differential. Wing pushes on air below it. (Why birds fly.) Additionally, for moving wing, edges create vortices that create local pressure differentials. (Why helicopters and planes and birds work better than floating pieces of paper.) Wings work very similarly to performance ship hulls in this regard.
- tomsto 5y agoFor anyone who’s ever tried building a robotic bird, there is a lot more intricacy to how birds fly than just “pushing air”. A better article might have been, ‘we still don’t understand how certain species of bird fly so efficiently’
- gpm 5y agoSurely it does move faster, because it's lower pressure/you're putting less resistance on it? If I have a wing shaped like ∖, air going in -> direction, which is what you need to generate lift with a flat wing, then the air on the bottom is running into the wing and slowing down, while the air on the top is being pulled into the region the wing swept clear of particles and speeding up.
- jjk166 5y agoIt does move faster. This can be readily observed in wind tunnel tests, and is a source of many issues once you get into transonic flight when the airflow can reach supersonic speeds while the plane in subsonic. Flat wings must be inclined to cause the air on the top side to move faster. The vortices cause air to move at different rates.
- mbrameld 5y agoMy understanding is that the air moving over the top of the wing is compressed against the air above it in the atmosphere, like a venturi. This may be extremely simplified but it's what we were taught in flight school.
- Ericson2314 5y agoMy Newton's-laws-only bullshit: bottom air: "bounces" down, simple enough. Force on wing up and back. top air: bounces up off front of wing (because it's not infinitely thin), but then is unimpeded by wing. It get's slightly more compressed at the very front, but then as the wing goes down this big gap is left. The air isn't going to bounce on the air above significantly because air compressed and this is laminar flow to boot: Viscosity > internia-ness. The about-to-be-vacuum means the bottom air pushes the wing up more easily, usually to the point where there is no more vacuum, just low pressure. But if you go really fast (or are a hydrofoil?) then there might be an actual vacuum. The vacuum "initially" just accelerates the air vertically, but once things get going since the airfoil "carves out a triangle", the air might speed up horizontally too. There is air behind it (front re aircraft heading) pushing on it but not air in front which is getting "untraffic jammed" away. There we go, I think this accounts for everything in the article without any Bernoulli. Screw Bernoulli.
- MaxBarraclough 5y agoYou might get something out of this 2013 talk by former Boeing engineer Doug McLean on misconceptions about lift. [0] [0] https://youtu.be/QKCK4lJLQHU?t=834 https://youtu.be/QKCK4lJLQHU?t=834 (watch for 5 minutes to get some idea of his main points, or 35 minutes to watch in full. The link will skip the introduction.)
- cryptica 5y agoI thought it was mostly because of the slight upward angle of the wing which creates air compression under the wing and suction above the wing. If you try to move a flat object through water, it creates pressure at the front and suction at the back. If you tilt it diagonally (and move it right to left), you get pressure in the bottom right and suction in the top right.
- tomsto 5y agoThis is true of a symmetrical aerofoil (e.g. most helicopters) but not for an asymmetrical aerofoil (most fixed wing aircraft). It is true that a slightly positive angle of attack generates more lift than none (because the pressure/lower side starts making a contribution)
- AstralStorm 5y agoCorrect. Still incomplete. Angle of attack causes a vortex at the trailing edge which has nothing to do with raw air speed and everything to do with fluid dynamics (which involves speed but is much more complex) Short version is that you created a hole (lower pressure area) in air which it now tries to fill. Air and gasses have finite limited velocity known as speed of sound, which is why you get these pressure differentials while the wing is moving. With a flat wing, they're rather small and low pressure vortex is located behind the wing. In an angled wing, some of it is located below the wing and the air trying to fill the low pressure area exerts a lift force on the wing. (It's unlike a balloon. Bernoulli has very limited impact, unlike essentially wind.)
- tomsto 5y agoI’m not sure I fully agree. Do you not get this trailing edge vortex with an asymmetrical aerofoil at 0 angle of attack? (Just less strongly because less pressure difference between suction and pressure sides)
- AstralStorm 5y agoYou do. And you get small vortices on wing surfaces too.
- IshKebab 5y agoA common and wrong explanation. The pressure differential is created by the fact that the air is being pushed into the lower side of the wing. It's much simpler than that anyway. The wing forces the air downward, so the plane must be forced up.
- beckingz 5y agoOverall you are correct that the plane receives an upward force due to the air it interacts with, and that the air receives an equal amount of force downward. In level flight the vertical force components must equal zero (or the plane falls/rises). But equally, if the plane is forced up, the air must be forced down. Cause and effect are not obvious from a force diagram.
- jjk166 5y agoAnother common and wrong explanation. The air is pushed down by induced rotation. An inclined wing is one way to do it, but is not necessary.
- belter 5y agoIts complex...The example normally given is, the wing is shaped a little flat in the under side and curved on the top. So that would explain the flow as you mentioned. However when an airplane flies upside down, its not sucked into the ground ;-) It seems nobody really knows: "No One Can Explain Why Planes Stay in the Air" https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/article/no-one-can-explai... Edit: Added brief from article above: ------------------------------------------------------------ - On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs. - There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations. - Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists. ------------------------------------------------------------
- Someone1234 5y agoIf an aircraft flies level upside down it will lose altitude towards the ground (as opposed to right side up wherein given adequate thrust it should keep its current altitude). In order to stay at a fixed altitude upside down you have to bring the nose of the aircraft up several degrees (increasing based on air speed).
- sigmoid10 5y agoIf wings only generated lift in one direction (i.e. towards the curved side), then even flying with your nose up would pull you down if you are inverted. What people here are missing is that curved wings in level flight generate lift, but any shape of wing can generate lift with a positive angle of attack. Just stick your hand out the window while driving on the highway and tilt it slightly, you'll see.
- clairity 5y ago> "Just stick your hand out the window while driving on the highway and tilt it slightly, you'll see." this is really all the intuition most people need to understand flight, even if it leads to an incomplete understanding. it's easy to feel the air pushing on the bottom of your hand when you tilt it up (or top, when tilted down). what's not obvious is that there is also lift created on the top side at the same time, but that can subsequently be learned in high school physics (or fluid dynamics in college, which is where it really stuck for me).
- marvin 5y agoI'm confused. I was in a plane that flew upside-down, and it didn't fall down. Am also a pilot.