http://www.tap46home.plus.com/mechanics/fspin.htm
"It is a common practice to fit fletchings at an angle to the shaft or use fletchings which have a built in angle to cause the arrow to spin in flight around the shaft axis. Why do this and how does it work?"
SPINNING ARROWS
It is a common practice to fit fletchings at an angle to the shaft or use fletchings which have a built in angle to cause the arrow to spin in flight around the shaft axis. Why do this and how does it work?
An individual arrow can have a bias to curve in flight. This may result from the arrow not being perfectly symmetrical e.g. slightly bent, or from having a non symmetrical pile e.g. a broadhead. When the arrow flies, any asymmetry will result in a torque on the arrow increasing the offset angle and resulting in a curved flight path. By making the arrow spin around its axis the curved flight path is converted into a helical path as the 'bias' is rotated in all directions. This approach works equally well with all arrows, all with different biases and tightens the arrow groups. The diagram illustrates the effect on arrow flight path of arrow spinning.
