There are fads in airfoils. For a decade the SD7037 was by far the most popular choice for thermal duration contest models. It is still a good and popular choice but, it is being replaced by thinner, lower mean camber airfoils that have become practical through superior spar materials, design and construction. The popularity of the SD7037 is still being used to market planes with built up wings and open bay construction whose actual shape is not close enough to the true SD7037 to do any good. The selection of an airfoil carries with it the need to keep the contour of the airfoil very, very close to the true contour in order to achieve the potential of the choice. This means substantially no covering sag between the ribs, thin trailing edges and leading edges contoured to an accuracy of a few thousandth of an inch. If you don't come very close to these standards, you are spinning your wheels with an excercise in airfoil selection.

A case in point is the Spirit 100. It comes with two sets of wing ribs The builder can choose the "beginners" S3021 airfoil or the "advanced" SD7037 airfoil. The S3021 is a bit thicker so the wing will be stronger and can be launched a bit higher. It also will have less covering sag and the contour will be more accurate because the trailing edge stock will come closer to the true airfoil contour. Not only that but, the S3021 will out penetrate the SD7037.

So, the person who falls for the advertising hype and chooses to build the SD7037 will end up with a weaker wing that doesn't penetrate as well and is harder to build for no advantage. If the kit manufacturer had just provided the S3021 rib set they would have saved money, served their customers better but not sold near as many kits.