The concept of the D-Plane introduced by T.P. Jorgensen in The Physics of Golf, second edition, 1999, describes what happens to the flight of the ball immediately after it impacts the clubface -- we reviewed it in our previous blog, D-Plane and Ball Flight. Here we continue our discussion on how knowledge of D-Plane helps you understand your ball flight.
Upon impact, the ball slides up the clubface and ball spin is generated by frictional resistance between ball and club. The rotational direction of the spin is under and back -- we think of it as underspin or back spin. The spin occurs around an axis that is perpendicular to the direction of the spin.
In a straight shot where the clubface orientation and the club path are in the same vertical plane, the axis of ball spin is horizontal. When the face angle is open relative to the club path, the ball spin axis is tilted to the right as illustrated below.
When the spin axis is tilted to the right, the ball flight trajectory will fade or slice for a right-handed golfer. If the club path is straight towards the target and the clubface is open the rightward tilt of the spin axis sends the ball slicing to the right.
If the club path is towards the left with an open clubface the spin axis is still tilted right. The ball trajectory will start out to the left then fade or slice to the right. The amount of curvature in the direction of the ball flight is a direct function of how much tilt there is in the spin axis. There is no separate side spin affecting direction.
When the ball contacts the center of the clubface and the face angle is not the same as the club path, the spin axis tilt is perpendicular to the orientation of the D-Plane. However, when ball contact is elsewhere on the clubface in the horizontal (heel to toe) direction, the force of the impact will cause the clubhead to twist slightly. Clubhead twist changes the face angle and causes the spin axis to tilt.
When the ball impacts the heel of the clubface, the clubhead twists in a counter clockwise direction causing the ball to rotate in the opposite direction -- clockwise to the right. The spin axis tilts right and the ball flight goes right.
The opposite rotations of the clubhead and ball at the moment of impact can be described in terms of a gear effect. The ball and clubhead always rotate in opposite directions. The side spin from the gear effect adds to the spin of the D-Plane and tilts the spin axis. Gear effect is helpful for understanding ball flight and we'll consider it in a future blog post.