# How to select the most efficient airfoil for low speed flight?

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 $50\:km/h$

• 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. May 26 '18 at 16:59
• @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. May 26 '18 at 17:32
• 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 May 26 '18 at 21:12