TJ, et all,
even straight fletched, slight offset, my left wing have a natural helical. Not to the degree that putting a hard helical on the feather when fletching, but it's there-- and my straight fletched, single bevel, still spin all the way thru the target be it foam or meat.
Come to think of it while typing, when I shot STOS 2 blade double bevel + straight fletched, I had to rotate my arrows to pull back thru the block...just didn't get the S thru meat like with the single bevel...
Wouldn't that also suggest that the fletch starts the spin and continues it thru target mediums? The single bevel just enhances that perhaps? And the big reason I like them on game is the potential for a slight off mark hit and bone splitting ability... even with lighter draw weight but heavier arrows...