Thanks Michael,
I tested your file and the result with revolve rail is perfect.
One thing I'm not sure to understand is the last step. You did use flow with the blue curve to deform the resulting shape?
What is the purpose of the straight red curve?

But thanks for the tips, I will try to use it.

I'm still trying to build my shapes and that's funny how this method help me to understand better the shape, it will improve my polymodeling at the same time ;)

So here is a shape I'm trying to build.
I tried to apply the different techniques you shown me, but I have some difficutly with this case.
I build the main shape using sweep, and then I wanted to use 2D curves to cut it and produce my final result. But it didn't goes as I planned it.
When I followed my template to draw my curve on the side and cut the shape with it, the projected cut didn't follow my shape in the front view. So I tried to cut an other time in the front, but as you guessed it that didn't worked.
On the pictures I tried to make the curve in 3D and project it on the surface to trim it, but it doesn't resolve the issue.
As I understand it, it is because my volume doesn't intersect at the right place to conform my needs.

The yellow curve is what I want and the purple curve is the result.
How would you approach this?

Thanks

Edit: I found lyed helmet tutorial thread but the videos are not available, is there an other place to get them as it seems really usefull.