It probably also indicates that for unflapped, unslatted, fixed configuration airfoils at low subsonic flight conditions that there is just one good shape. The designer of the Clark Y had some good intutitive vibes going on. I am impressed with both airfoils and the work that went into them.

The trips we used in the McDonnell Douglas (now Boeing) wind tunnels (a Polysonic 4X4 ft and Low Speed, a 12 ft cross section) were made from a strip of fine grit like sparse sandpaper at the 5% aft of the leading edge location.

We would mask off a 1/8 inch wide stripe, spray on some clear Krylon and sprinkle the grit onto the wet paint. It was felt that this was more subtle/finesse way of triping the boundary layer. The grit drag was in the noise of our ability to measure the force.

I often wonder why the same thing is not used by modelers some times? Especially in the glider duration type of contest.