How to define quarter-chord sweep angle for a cranked wing?
I think you have to define the sweep angles for every section on the cranked wing. You can use the same definition for each segment as for a "normal" wing. So practically speaking: Root Segment quarter chord --> create line between quarter chord of root and quarter chord of kink. Then you can measure the sweep angle of the root segment. Continue with next segment. The quarter-chord sweep angle is the angle between the y axis and the quarter chord line.