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What airfoil (or family) has the highest $C_L/C_D$ ratio at speeds in the $50\:km/h$ range.

The use case would be an ultralight with a max speed of $50\:km/h$

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    $\begingroup$ That depends. How low is low? What about size? GA aircraft? Ultralight? Bird? Insect? As you get slower and slower and Reynolds number goes down, the optimal airfoil shape changes (at some point, at ultra low Reynolds numbers, the optimal airfoil is single surface; think bug wings, boat sails, hang gliders). So you need to set the size and speed range you're talking about. $\endgroup$ – John K May 26 '18 at 16:59
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    $\begingroup$ @David Teahay I've edited your title to match the question in the body. Since there is no single definition of best, I hope this will save the question from being too broad. $\endgroup$ – AEhere May 26 '18 at 17:32
  • $\begingroup$ A 5 ft chord ultralight wing at 50 km/h has a Reynolds # of about 1,400,000. So what you need are airfoil lift and drag polars for that Re value to compare. You might be able to find what you need here: airfoiltools.com $\endgroup$ – John K May 26 '18 at 21:12
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Please read the answer to this question first and try to answer the questions listed there. You need to provide more information!

Next, the best place to look is among glider airfoils. The Wortmann FX 63-167 is now 55 years old but still a good start. It was designed for the low speed of a human powered aircraft and has been used on gliders and motor gliders.

But the airfoil still can be improved, for example by adding flaps or by narrower tolerances for the surface contour, especially around the nose. But this will make construction more complicated, so the overall best compromise might still be the Wortmann airfoil.

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  • $\begingroup$ +1 for the Wortmann family, I used it in my end-of-studies project. Do you think the cusp on the trailing edge is worth mentioning at all? It complicates manufacturability, especially for a DIY project, which is the backdrop of OP's questions. $\endgroup$ – AEhere May 31 '18 at 13:14
  • $\begingroup$ @AEhere: You are right about manufacturability, and I linked to another answer which proposes the S3410 in the comments for exactly that reason. However, if you want the best performance at low speed, there is no way around more camber and a concave bottom. $\endgroup$ – Peter Kämpf May 31 '18 at 16:35
  • $\begingroup$ Thanks a billion times!.....Now I know what to do from here💃@Peter Kämpf $\endgroup$ – David Teahay Jun 23 '18 at 3:15
  • $\begingroup$ Pardon my ignorance @Peter Kämpf ,what do you mean by "narrower tolerances for the surface contour" $\endgroup$ – David Teahay Jun 23 '18 at 3:20
  • $\begingroup$ @DavidTeahay: Generally: Deviations from the ideal, like waviness, surface roughness, bumps in the surface. The early airfoils (like Clark Y) are more robust and tolerate more building errors. If you optimise an airfoil for one lift and one Reynolds number, the optimum will require a very high truthfulness of the actual surface contour to the calculated ideal contour. $\endgroup$ – Peter Kämpf Jun 23 '18 at 4:53

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