Curve which keeps constant length ?

 From:  amur (STEFAN)
8414.6 
Interesting topic! May i ask for what you need this? And how about this, in case you have a flatbed scanner and the curves are planar. Cut a parcel twine to the desired length, scan it, deform it, repeat etc. and then redraw in MoI...
No math required. Maybe rigging in an Animation Software should work also.

Regards
Stefan