There are many theories of how <a href="https://en.wikipedia.org/wiki/lift_(force)" class="extiw" title="w:lift (force)">lift</a> is generated. Unfortunately, many of the theories found in encyclopedias, on web sites, and even in some textbooks are incorrect, causing unnecessary confusion for students.  The theory described on this slide is one of the most widely circulated, incorrect explanations. The theory can be labeled the "Longer Path" theory, or the "<a href="https://en.wikipedia.org/wiki/Equal_transit-time_fallacy" class="extiw" title="w:Equal transit-time fallacy">Equal Transit Time</a>" theory. The theory states that <a href="https://en.wikipedia.org/wiki/airfoils" class="extiw" title="w:airfoils">airfoils</a> are shaped with the upper surface longer than the bottom. The air molecules (the little colored balls on the figure) have farther to travel over the top of the airfoil than along the bottom. In order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing. Because the upper flow is faster, then, from <a href="https://en.wikipedia.org/wiki/Bernoulli%27s_equation" class="extiw" title="w:Bernoulli's equation">Bernoulli's equation</a>, the pressure is lower. The difference in pressure across the airfoil produces the lift.  Source: <a rel="nofollow" class="external autonumber" href="http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html">[1]</a>  Image was cropped from original slide.
<a href="//commons.wikimedia.org/wiki/User:%D7%A8%D7%95%D7%A0%D7%90%D7%9C%D7%93%D7%99%D7%A0%D7%99%D7%95_%D7%94%D7%9E%D7%9C%D7%9A" title="User:רונאלדיניו המלך">רונאלדיניו המלך</a>, translated by <a href="//commons.wikimedia.org/wiki/User:%D0%A0%D0%B0%D0%BD%D0%BA%D0%BE_%D0%9D%D0%B8%D0%BA%D0%BE%D0%BB%D0%B8%D1%9B" title="User:Ранко Николић">Ранко Николић</a>
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<ul><li><a href="//commons.wikimedia.org/wiki/File:Equal_transit-time_NASA_wrong1_es.svg" title="File:Equal transit-time NASA wrong1 es.svg">Equal transit-time NASA wrong1 es.svg</a></li></ul>