Hi Pilou - in your case each piece of your "sine wave" like curve is a full 180 degree arc.

That's a lot more exaggerated of a bulge than a normal sine wave curve.

If you make the bulge of each of your pieces less extreme you will likely get a less bulgy result...

With that large amount of bulging, it would produce a very complex sweep as it tries to rotate the starting shape along each of those exaggerated bulges... That's why some kind of non-rotating method is needed.

You can kind of see what you are asking it to do if you do Transform/Array/Curve on it to make some copies along the curve that are rotated, you'll see a result like this:






You can kind of get an idea there of how your profile curve will swing around as it is rotated along the path, then it tries to produce a surface through that...

- Michael