....and now you need to calculate the bending loads with the forces distributed as they are in real life. They are not evenly-distributed along the span. You have more force at the root than at the tip, so the support structure must become stronger as you progress from tip to root. I don't know the exact distribution, but I think a plot would show something close to an elliptical curve.

....and to paraphrase Dick Hanson: "If it's light enough, any structure will do. If it's too heavy, structure is irrelevant". (heh, heh)