Reynald's number
Dick,
Well, I guess my first comment would be that FXD could post a couple questions asking for clarification. In that exchange, I, or someone else, could ask him exactly what decision he wants to make, so that we could offer more helpful information, if possible.
Your example is an instructive one. If someone observes that a 12" chord wing with the same wing loading doesn't fly as well as a 15" chord wing, no simple, watered down explanation of Reynold's number is going to tell him exactly why. I am a little dubious that the observed difference in performance is a Reynold's number effect, myself. If such a big change is observed between those two wings, it probably means that we are operating in a critical range of Reynold's number ( or some other parameter ), and much more specific data would be required, likely more data that we could get our hands on. If someone wanted to gain some intuition about why the chord length can have such a big effect, that's pretty reasonable, and the posts in this thread so far may offer that. If he is a serious competitor, he will either use airfoils and dimensions that are supported by high quality data, or will simply avoid the dimension which experience showed to perform poorly. If he is not a serious competitor, he will pretty likely not notice, or just keep flying his plane and having fun.
As I said, I am a huge fan of accessible, clear explanations in everyday terms. I attempt to do that, as best I can.
banktoturn