I think the statement that "metal wings are stronger" is a generalization when we're talking about the Ercoupe. An airplane designed with metal wings CAN be stronger and we have many examples of airplanes that are, fron the DC-3 to the 747. Many engineering factors go into wing design, including g-stress and wing loading. To assume that metal wings on an Ercoupe are inherently "stronger" is probably a fallacy. The internal structure is responsible for load-bearing on our airplanes, and any additional strength from the covering medium is incidental. Reasons to favor one covering medium over another could be exposure to the environment, maintainability, cosmetics, and cost. That's a valid debate, and anyone choosing metal or fabric should consider those and make the best choice for their circumstances. But choosing metal over fabric based on strength is simply a false choice, IMO.
By the way, the B-52 I flew had metal wings and was a 2 G aircraft. My B-1 had metal wings and was a 3 G aircraft. My Ercoupe is rated at 3.5 G's so I could make the argument that "fabric Ercoupe wings are stronger". We all know that's not the case, and that many other factors go into max G loading and wing design. Dave
