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Where did the false "equal transit-time" explanation of lift originate from?

68 点作者 IdealeZahlen30 天前

14 条评论

smallbugfound30 天前
No idea why you would provide the equal transit time theorem to students, it makes such a low amount of sense that you&#x27;re inevitably going to get your students extremely confused if they are paying any attention at all.<p>&quot;Why does the air have to transit in the same time period?&quot;<p>&quot;But _why_ is the air moving over the top faster? Weren&#x27;t you going to tell me how a wing works?&quot; Etc etc etc<p>It is the worst kind of lie-to-children (and adults) in my opinion, it&#x27;s not a simplified true answer it&#x27;s a whole cloth fabrication that vaguely gestures in the right direction, partially, if you are being generous.<p>The idea that people get tested on regurgitating it for a pilots license is crazy.<p>It&#x27;s up there with those ridiculous tounge maps with taste regions on them.
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jgord30 天前
Handy video showing relative speed above and below the curved wing surface, and non-equal times :<p><a href="https:&#x2F;&#x2F;www.cam.ac.uk&#x2F;research&#x2F;news&#x2F;how-wings-really-work" rel="nofollow">https:&#x2F;&#x2F;www.cam.ac.uk&#x2F;research&#x2F;news&#x2F;how-wings-really-work</a>
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dguest30 天前
Glad to see this posted more!<p>I was asking about this 9 months ago and it started quite a thread [1]. I remember learning about this in grade school, finding it pretty confusing, and wondering why a simple &quot;newton&#x27;s third law&quot; wouldn&#x27;t suffice. That&#x27;s incomplete but at least not wrong.<p>[1]: <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=40835223">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=40835223</a>
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dr_dshiv30 天前
“Just like the &quot;explanation&quot; of seasons by the Earth&#x27;s changing distance to the Sun, or textbook pseudo-explanations of history, like the &quot;crisis&quot; over the discovery of irrationals, or Maxwell&#x27;s &quot;mathematical&quot; reason for adding an extra term to Ampere&#x27;s equation.”<p>Love it! Anyone have any others?
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bowsamic30 天前
I remember being taught this in A level physics case study of lift alongside a better third law explanation, with absolutely no acknowledgement or justification for one over the other. It caused a bit of a scandal among the class. Gave me a very sceptical view of physics education in general which I took to my physics undergrad and PhD
bobbyraduloff29 天前
If you google “how does a plane generate lift”, the first result you get is a link from nasa.gov which claims that lift is generated this way. Kind of funny considering the SE question includes a screenshot of another NASA resource claiming this is false.
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magicalhippo29 天前
William Fraser over on YouTube has a video[1] on aerodynamic lift which I found interesting.<p>In it he briefly touches on the equal transit time explanation, and how the steady-state snapsot presented doesn&#x27;t really have enough information to tell how the flow field developed.<p>He&#x27;s been writing a particle-based simulator which he wants to use to show how lift develops from that perspective[2], still a work in progress.<p>Just sharing as I found them interesting and cleared up some confusions I had.<p>[1]: <a href="https:&#x2F;&#x2F;youtu.be&#x2F;ZUBwc67c5_Y" rel="nofollow">https:&#x2F;&#x2F;youtu.be&#x2F;ZUBwc67c5_Y</a><p>[2]: <a href="https:&#x2F;&#x2F;youtu.be&#x2F;IVLpbOQUdqU" rel="nofollow">https:&#x2F;&#x2F;youtu.be&#x2F;IVLpbOQUdqU</a>
marcus_holmes30 天前
I recently encountered this in a sailing class when the teacher was explaining how &quot;pull-mode&quot; works in sailing, where the wind is coming from ahead of the vessel and pulls the sails rather than pushing them. I knew this theory to be debunked and yet couldn&#x27;t work out the answer from non-debunked physics (and certainly didn&#x27;t want to disrupt the class by arguing physics with someone who&#x27;s been sailing for 50+ years - if it worked for him, it&#x27;ll probably work for me, even if debunked).<p>Modern sailing vessels <i>always</i> sail into the wind, because they&#x27;re always going faster than the wind blows. I do find the physics of this fascinating.
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nonrandomstring30 天前
Fluid mechanics has always seemed so complex that scientists joke about it. Supposedly [0] Werner Heisenberg and&#x2F;or Horace Lamb quipped that when they died they&#x27;d ask God about relativity, quantum electrodynamics and turbulence... and they didn&#x27;t expect he would have an answer for the last.<p>[0] <a href="https:&#x2F;&#x2F;boards.straightdope.com&#x2F;t&#x2F;did-heisenberg-really-say-this-about-what-he-would-ask-god&#x2F;631918" rel="nofollow">https:&#x2F;&#x2F;boards.straightdope.com&#x2F;t&#x2F;did-heisenberg-really-say-...</a>
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samuelfekete30 天前
It’s still the explanation on NASA.gov<p><a href="https:&#x2F;&#x2F;www.grc.nasa.gov&#x2F;www&#x2F;k-12&#x2F;UEET&#x2F;StudentSite&#x2F;dynamicsofflight.html" rel="nofollow">https:&#x2F;&#x2F;www.grc.nasa.gov&#x2F;www&#x2F;k-12&#x2F;UEET&#x2F;StudentSite&#x2F;dynamicso...</a>
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bandrami29 天前
What if we put the airplane on a treadmill though?
financetechbro29 天前
Truth is we don’t fully understand how lift happens
artemonster30 天前
So what is correct explanation? Is there a TLDR?
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lordnacho30 天前
Ok so let&#x27;s see if HN can put something together:<p>- The wing deflects the air down, so that&#x27;s one way of creating lift, but most wings are not just flat<p>- An airplane can fly upside down<p>- It&#x27;s a bad idea to take off behind another plane<p>- Modern wingtips have special shapes that makes them more efficient<p>- Answer has something to do with vorticity, but what exactly?<p>Hopefully we can get something better than whatever AI uses to explain these. I haven&#x27;t asked it yet, but I get the feeling it would produce something plausible sounding that I won&#x27;t be able to easily refute, ie it would trick me into thinking I understood it.