Sometimes the geometry of paper has hidden pitfalls:
Hereâ€™s a common shape: a transition from an arc cross section to flat. Scoops and inlets have this shape, as do fuselages (particularly WWI fuselages that transition from the circular rotary engine cowling to the box girder fuselage aft, like the Sopwith Pup and Triplane).
The first thing to recognize (and it should be obvious) is that the paper shape that forms this piece is NOT a rectangle. The forward edge, being the perimeter of an arc, will be longer than the straight edge aft; it will also be a mild arc itself. The top and bottom edges will be straight, but not parallel as they angle out to meet the forward edgeâ€™s arc.
The problem, however, for the fastidious modeler who insists on authentic shape, is that a single piece CANNOT reproduce this shape accurately. Thatâ€™s because, if a single piece is used, there will of necessity be a triangular region of flat to allow the paper to transition from the straight edge aft to the forward arc, like this ---
The narrowing oval cross sections between the forward arc and the straight edge aft usually do NOT, on real aircraft, have any flatness across their arcs, but this piece DOES have flatness across those cross sections as the triangle of transition imposes its geometry.
To reproduce this shape accurately, therefore, requires pushing the tip of the transition triangle back as close to the straight edge aft as possible, the geometry of which requires two pieces, a forward curved piece that preserves the arc cross sections throughout its length, and a final, short, aft piece that has the transition triangle, but a short one.
With the exception of the straight edge aft, all the forward and aft edges of the two pieces will have slight arcs (and this is why it cannot be accomplished with only one piece); the arcs of the joining edges of the two pieces will be slightly different.
On small scale models the difference between using one piece with the large transition triangle or using two pieces isnâ€™t terribly noticeable, but as scale increases that flat triangle starts to show.