The notched spar scheme that is stock on the Brodak plans and Midwest kit is not the greatest IMO. With a strong .35 or .40, flown at a fast 2-cycle, that spar can crack. My 35 year old Midwest kit Magician has cracked its spar several times while being flown hard and fast.

A fix that seems to work well is to glue a 1/64" ply doubler about 1/4" wide and 12-18" long on the bottom of the spar. 1/4" balsa reinforcement would do about the same, but is a more cosmetically obvious mod with transparent finishes.