What is best method to combine the best parts of these two curves for S bend?


I am forming the S shaped twist bend on the inner bend surface.
I have curve E starting off correctly (tangent) at ‘C’ and F starting off correctly (tangent) at B.
These curves being on the surface to be used for ‘trim’. to remove unwanted part of tube surface that I started with.

How can I bring the middle parts together of curves E and F to get a single S curve on the surface giving a correct twist ?

Fillets were not doing it for me here so went with tube., especially up front.
Having to visualise bends and form them is a nightmare, but trying flow method failed dismally at front tip (other post). I am a lot closer to reality here., I have the actual item so can see (sort of ) whats happening.

The outer surface shape ‘A’ was dictated by the side view of the item there and its correct shape, but that cannot be used for the inner S twist., apart from the start of the curve at C, or can it ? It should dictate the other curve surely as they are a pair so to speak. I scaled it down and it fitted the inner surface ! …BUT its start tangent at C doesnt follow the curve ‘A’ long enough and heads off too quick not following the gentle shape of ‘A’ which it needs to follow else it twists far too early so that method failed ! Nice idea though !

file attached.

S bend how to combine two curves on surface.3dm (162.6 KB)



something like this maybe?

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thanks, that is a normal classic radiused corner for a bend, but on this occasion that loses curve A which is/was correct as the metal has that shape, I can see it as such on the item I am replicating. Curve A must remain untouched as per post.
I need to bring curves E and F together in the middle on the surface, keeping the correct vertical tangent at ‘C’ and the correct horizontal tangent at ‘B’.
How does one do that ?


well I had the idea of using blend, so I quadrated the quarter arc surface as lines, snipped the curves with these leaving 1/4 of E at C and 1/4 of F at B, ran blendCrv tangent for 1 and 2 and got a blend that looked pretty fair.
project to surface and trim, sweep2rail and the result.

that the curve must follow ‘A’ upwards for a while disallows a better twist there but it has to be that way.

Now was my BlendCrv the best way ?