What's spiral spin? What's the difference between hitting a twist serve and a topspin-slice, when apparently both just involve brushing 8 to 2? All my kick serves spin to the left in the air then bounce further to the left (I'm right handed), although I don't really get very much RHS
Spiral spin is a
Z-axis spin = the spin axis is in the same direction as the flight of the ball. The spin that we observe for badminton shuttles or for a forward pass in American football is spiral spin (or
spiralspin). TW physicist has also referred to it as
gyrospin on this website; however, in his book
Technical Tennis, her refers to it as spiralspin. Ping pong players often refer to this type of spin as
corkscrew spin (or corkspin).
The spins that are more familiar to tennis players are X-axis spins (topspin and underspin) and sidespins (Y-axis spins). The spin axis for the types of spins are perpendicular to the flight of the ball and have an associated
Magnus force (or Magnus effect). It is this Magnus force that is responsible for the left/right or up/down deviations in the trajectory of the ball -- they create the curves in the flight of the ball (aside from that up/down curves due to gravity).
Spiralspin, whose axis is in line (parallel) to the trajectory, does not have any associated Magnus force and does not cause any left/right (or up/down) deviations in the trajectory. Think again of badminton shuttles, football passes, bullets and other projectiles with spiralspin (but no other spin types). None of these object will curve left or right in flight unless acted on by a cross wind. The same is true for the spiral spin on a tennis ball (or a ping pong ball).
While spiral spin has no effect on the flight of the ball, it has a very distinct on the bounce of the ball. (Y-axis side spin has absolutely no effect on the bounce direction). A clockwise spiral spin will cause the ball to bounce to the right whereas a CCW spiralspin causes the ball to kick off to the left.
The difference between a regular
topspin-slice serve and a
twist serve is where the ball is contacted. That 8 to 2 brush for the topspin-slice is applied directly to the back of the ball. A right-hander will brush the ball a bit to the left side of the ball for a twist serve in order to increase the amount of spiral spin generated.
From
Technical Tennis (Rod Cross), here is an image that shows the relative contact locations for ball spins with and w/o spiralspin:
http://img.photobucket.com/albums/v623/soulcutterx13/Spiralspin.jpg
.