Discussion of Design Rational

To what extent are we supposed to report on our design rational? I understand that we need to say why we chose the shape we did, but how much detail are they asking for? I could see this section being a short paragraph to a couple paragraphs depending on how much discussion they want.

Actually, there isn't much to the report devoted to this topic. You are writing the report as if you already had the final design (as if the other two models never existed) and all you did was create the scale model so you could test the performance of the full-scale model using the wind tunnel in the lab and scaling analysis.