3 ms·
Grumble — both this site and the newer one someone else posted [1] link to an old and somewhat misleading diagram of how an airplane wing works [2]. The proble
by interroboink 3y ago
Grumble — both this site and the newer one someone else posted [1] link to an old and somewhat misleading diagram of how an airplane wing works [2].
The problem with the diagram is that the "lines" both ahead of the wing and behind the wing are pretty much flat — as if the air ultimately returns to where it was as the wing moves through it (to be fair, the lines are a little wobbly in the diagram).
That's misleading because if the wing has a net force upwards (ie lift), then the air must have a net force downward. You can't invent forces out of thin air (literally, in this case :). Yes, there's Bernoulli’s principle and all that, but fundamentally: if the wing is pushed up, then the air must be pushed down. The mechanism for how that happens is a separate discussion.
This is an issue that has been debated back and forth for a long time, so I don't want to re-hash it in this comment, but I think it's fair to say at a minimum that those diagrams are misleading, and fly (ha) in the face of Newtonian laws of physics.
I expect better from NASA.
Here's good short Veritasium video on the subject: https://youtu.be/aFO4PBolwFg?t=13 https://youtu.be/aFO4PBolwFg?t=13
Also good: https://www.youtube.com/watch?v=HmDYbnGnhpA https://www.youtube.com/watch?v=HmDYbnGnhpA (part 1), https://www.youtube.com/watch?v=21YB4qCvIK4 https://www.youtube.com/watch?v=21YB4qCvIK4 (part 2), https://www.youtube.com/watch?v=iE27CXLXbp0 https://www.youtube.com/watch?v=iE27CXLXbp0 (part 3)
[1] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/ https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/
[2] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/aerodynamic-forces/ https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/aer...
- elteto 3y agoHonestly, I don’t think you are reading the diagram well. The lines are streamlines, i.e. tangential to the flow velocity at each point. They are not straight flat lines so they do show that there _is_ a change of momentum (that is, a force) being applied to the air. And the air in turn is pushing back on the wing. These are the normals also depicted on the diagram.
- interroboink 3y agoYou could be right, though the diagram is not well-labeled so it's hard to say for sure. What's the big black arrow at the left-hand side? Direction of flow? It would be nice if the accompanying text made any mention of deflection of air downwards, but it doesn't. I take some solace that the spotty explanations in the text (eg: a single mention of Euler's Equations without any other reference) will send a curious reader to better sources.
- elteto 3y agoI’m very certain that the curved lines are streamlines. The black arrow is labeled “Free stream velocity” meaning that, in absence of any perturbation, that is the direction that the air would freely follow. I.e. it would go in a straight line. There is a mention of flow turning as the source of lift here [0] but the page you mention is taking the more mathematical approach of defining lift as the integration of pressure over the wing area. I agree that the diagram could be better, most aerodynamics textbook diagrams could be :) [0] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/what-is-lift/ https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/wha... EDIT: I just reread the page with a bit more time and I have to disagree, it’s a very good explanation of lift, although it is a bit more technical than a layman’s. I can’t see anything obviously wrong with it, except that it doesn’t mention more advanced topics (e.g. turbulence).
- thomastjeffery 3y agoSo what? They are taking about the forces on the wing, not the surrounding area!