I think the key is that the ribs are actually the intended shape and the canvas deflects/sags down into the valleys of the span due to gravity and downward force from the laminar flow in flight stretching the canvas.
Hi Grendel
The Canvas is actually layed out over the wing and sewn together at trailing etch and wingtip. then
painted with something called "dope". That makes the canvas shrink and the ribs stick out.
Hope that helps.
Andy
Andy - That makes sense..you would want some sort of waterproofing to prevent the wings from retaining water and some way of pulling it taut over the frame.
Grendel - that dope or, taughtning dope, as they called it did all that. Some celluloses based compound.
Awfully smelling stuff. That why the name.
Andy