This might be a silly question, and it might be answered elsewhere (link?), but why are diagonal ribs so uncommon in fabric-covered control surfaces? I know the Ercoupe uses them so they're obviously possible. I know that in a metal-skinned control surface the skin is a perfectly adequate shear member so there's no real win from diagonals. But it seems to me that in a fabric covered control surface a significant amount of structure is needed to provide lateral bracing both against aero forces and against the tension of the fabric itself; and it's not intuitive to me that straight ribs + additional structure should end up lighter than diagonal ribs. Is there concern about the diagonal ribs interfering with airflow since they "print through" the fabric?
