A thread just popped up with flat vs. compound curved surfaces. For the purposes of focusing the discussion the specific configuration being high wing, tractor, conventional everything. The flight regime to inspect being only high speed cruise (not WOT). What is the penalty for having a completely rectangular cross section vs elliptical or highly rounded and specifically where does the increase in drag come from and how would you design around it. My opinion is if you have trim flaps, a well faired 2 and 3D loft that aligns with the direction of travel that you wouldn't have much if any penalty other than a slight increase in wetted area between two similar designs. Having a highly rounded structure and having to then transition to the wing junction area would create more trouble than it seems it is worth for some configurations. I realize that there are a lot of other compromises to go completely boxy like crosswind handling and off axis handling and maneuvering, but I am looking to discuss the on-axis condition. I have a reason for asking that has to do with prototyping methods. I have a feeling that the answer changes with V, but maybe not as it is a streamline thing in general, so maybe it varies with Rn or FPE...?