Interesting. My copy is the Sixth Edition, published in February, 1996. In the introduction, Riblett describes his ID system, with no reference to a “U”. Wild guess, IFF the U designation is from an earlier edition: the U stood for what is now designated as an A, the cusp has been replaced with a straight line.Hello BJC, first time I found it in SD1 specs and in SD1 plans (without coords). Second time I found it on picture of H.Ribblet airfoils book - see my post 32.
Unfortunately picture of page with GA37U-A315, that I found, doesn't show year or edition. Marc, what version of airfoil is in Your 5th edition?I dug out my Fifth Edition of GA Airfoils. On page 5 it says the "U" means "upper surface optimized". In other words the airfoil is designed to maximize laminar flow on the upper surface. I think the airfoils are the same in the 5th and 6th editions. Riblett just dropped the "U".