David, there's not that much background. You may know the Wikipedia articles about the [link=http://en.wikipedia.org/wiki/Mudry_CAP_10]CAP 10[/link] and [link=http://en.wikipedia.org/wiki/Mudry_CAP-20]CAP 20[/link]. The 21 just has a new wing with slightly different planform and a modern airfoil which is flat on top and bottom from about the middle to the trailing edge (but not concave like some funflyer airfoils).

Just for comparison, you may look at the [link=https://mraerodesign.securicom.ca/en/products/info.cfm?ProduitID=15]CAP 10b plans[/link] (bootom of page) for Martin Rousseau's model, which will be too big, though. As well look at the [link=http://www.toni-clark.com/english/modelle/cap21/cap_info.htm]CAP 21 fuse plan[/link] of [link=http://www.toni-clark.com/index_en.htm]Toni Clark[/link]'s model, which seems to be too big and heavy, too. (1285 sqin, 18 lb, 33 oz/sqft)