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I understand the reason airfoils are cambered on the top: to create lift. But one would assume this would result in aircraft wings having a semicircle-shaped design. Why is the cross-section of an airfoil instead more reminiscent of a teardrop on its side and cut horizontally? Why is the front thicker than the back?

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    $\begingroup$ Having a longer path for the air over the top of the wing isn't actually all that important. The important part is having the wing at an angle to the airflow. The teardrop is to make it easy for the air to turn the corner around the leading edge of the wing, but difficult to turn the corner at the trailing edge. This page has a good explanation: av8n.com/how/htm/airfoils.html#sec-airfoils $\endgroup$ Sep 12, 2021 at 19:02
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    $\begingroup$ @RickyTensor, I think there's one more detail: A razor-thin leading edge would be better than a rounded leading edge if the air always hit the wing at just the perfect angle of attack, but it would stall at pretty much any other angle. The rounded leading edge offers more drag, but it performs without any abrupt changes over a significant range of angles of attack. $\endgroup$ Sep 12, 2021 at 21:50
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    $\begingroup$ ObXKCD: Airfoil. $\endgroup$ Sep 13, 2021 at 12:32
  • $\begingroup$ @SteveSummit Oh I completely understand that the equal time arguement is wrong $\endgroup$ Sep 13, 2021 at 19:19
  • $\begingroup$ Worth considering that a teardrop (or better, a raindrop) is defined as what happens to water when you put it in a stream of flowing air $\endgroup$ Sep 13, 2021 at 23:22

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The airfoil shape is optimized for two features

Maximizing the lift coefficient $c_L$ alone would, as you said, demand for a semi-circle-like shape, like this:
enter image description here

Minimizing the drag coefficient $c_D$ alone would demand for a tear-drop-like shape (with a round front end and a thin back end. See the streamlined body in the image below.

enter image description here
(image from Wikipedia - Drag coefficient)

Putting these two requirements together you arrive at the typical airfoil shape which is kind of a compromise between the semi-circle and the tear-drop shapes from above.
enter image description here
(image from Wikipedia - Airfoil)

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  • $\begingroup$ There was also a lot of practical research (e.g. measuring things in wind tunnels) that went in to determining optimal airfoil shapes for particular conditions. See for instance NACA (the predecessor to NASA) airfoils: en.wikipedia.org/wiki/NACA_airfoil $\endgroup$
    – jamesqf
    Sep 13, 2021 at 15:59
  • $\begingroup$ Why is a teardrop shape so streamline? $\endgroup$ Sep 13, 2021 at 19:18
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    $\begingroup$ @The_Sympathizer: If you're willing to assume "no turbulence," I imagine there's some ungodly differential equation you can try to solve, at which point you'll have a spherical-cow airfoil that doesn't actually work. If you admit that turbulence is a thing, then it's a really hard problem. $\endgroup$
    – Kevin
    Sep 14, 2021 at 5:18
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    $\begingroup$ @The_Sympathizer There is no "ideal" airfoil shape since every airfoil is a tradeoff. A glider has a different preferred airfoil family compared to an airliner which again has a different preferred airfoil family compared to a slow light aircraft. You choose airfoils based on what mission it needs to do. There is a an equation that can be applied as a field to figure out the behavior of any object in a fluid: the Navier-Stokes equation. $\endgroup$
    – slebetman
    Sep 14, 2021 at 7:21
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    $\begingroup$ @slebetman : Thanks. Your first comment answers my question (in the negative). Likely the answer would benefit from including some of this info and that in subsequent comments. $\endgroup$ Sep 14, 2021 at 17:52

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