> There are three basic divisions when it comes to Mach number and
> airfoils. Mostly an airfoil selection is chosen
> based on stall characteristics and that is a distraction to the thing
> that I'm talking about. A blimp, or dirigible, etc. has a Mach limit
> on the low end and it is generally believed to be Mach = 0.5, but
> don't quote me on that, it is just what I've heard through the
> grapevine. The Mach limit of an airfoil is based on what the fastest
> air is doing across the airfoil so at M=0.5, a blimp's fastest air is
> traveling, say, at around M=0.98. A supersonic airfoil is chosen based
> on other criteria such that the center of lift is at the halfway point
> from the edge of the airfoil.