Helical fletch will stabilize your broadheads better than straight. Even with helical fletch your arrow has no appreciable spin. The benefit of helical fletch is by exposing more surface area of the feather or vane you get more drag and that makes it harder for the broadhead to plane or steer off on its own. Also the bigger the cutting area of the broadhead generally the more fletch length and helical degree you may have to use to keep it under control.