Rectangular helical sweep feature

Hello out there,

is there anybody out there who knows how to create a rectangular helical sweep Feature, as in the shown black and white Picture?

I tried my best to get a solution, but it is not to be used. The gradient is somehow not constant in the red area to the blue area. The reason is the great difference between the inner radius and the outer radius. If the shape of the required rectangle increases, or edge lengths are the same length (quadrangle). Then the slope becomes more linear.

Is there any help out there?

Thanks Heiko