Understanding the Determining Factors of Ball Flight
Earlier this year I purchased a Trackman®, and working with it has allowed me to understand impact and ball flight at a new level. One of the great things about Trackman® is that it tracks the full trajectory of every shot from 6 foot pitches to 400 yard drives. It then displays the shots 3D trajectory as well as 26 impact and ball flight parameters in real time. A few examples of these parameters include: club path, face angle, attack angle, launch angle, ball speed, smash factor, spin, among many more. Basically it is an MRI for your golf swing. It tells us what your club is doing at the all important moment of impact, as well as tracking the full flight of the ball, which then allows us to figure understand the golf balls flight. More information on Trackman® can be found at www.trackmangolf.com.
Whenever I start working with a new student one of the first things that we do is discuss, in depth, the ball flight laws and the “ D-Plane”. Basically, golf in its simplest form, is the physics of the collision between the club face and the golf ball. The ball does not know, or care, if someone who has never broken 100 or a multiple major champion is holding the club. All it knows is what the club head is doing at the moment of impact.
I believe there are two crucial things that every serous golfer needs understand…Why does the golf ball curve? What are the determining factors of the golf balls flight?
Lets take a closer look at these questions.
1. Why does the golf ball curve?
Well thats an easy answer, we know that a ball curves because of side spin! Incorrect! The golf ball curves due to a tilted spin axis. There is no such thing as side spin. With the emergence of launch monitors such as Trackman®, we have come to realize that what we thought was happening isn't necessarily what is actually happening.
Trackman® defines spin axis as a representation of the amount of curvature of a golf shot. A negative spin axis represents a ball curving to the left, a positive spin axis represents a ball curving to the right, and a zero spin axis represents a shot that has no curvature.
Below (Figure 1) we can see spin axis can be associated to the wings of an airplane. If the wings of an airplane are parallel to the ground, this would represent a zero spin axis and the plane would fly straight. If the wings were banked/tilted to the left (right wing higher than left wing), this would represent a negative spin axis and the plane would bank/curve to the left. And the opposite holds true if the wings are banked/tilted to the right. In general, a spin axis between -2 and 2 can be considered a straight shot. Under normal conditions, it would be difficult to see curvature on a shot with a spin axis between -2 and 2. The higher the number of the spin axis, the more curvature that should be visible.
So, what determines the spin axis of the golf ball? That brings us to question number 2.
2. What are the determining factors of ball flight?
The quick answer to this question is… the D-Plane. Now your wondering what is the D-Plane. In 1993, Dr. Theodore Jorgensen, a University of Nebraska physics professor who had worked on the Manhattan Project during World War II, published The Physics of Golf. In it, he coined the term "D-Plane," which he called "the plane located between the intersecting lines created by the club face angle and club head path”.
Lets take a look at the two determining factors of the the D-Plane, club face angle, and club head path. These are the two determining factors of ball flight. Actually there is another determining factor in ball flight, and that a centred strike on the club face. When we do not strike the shot in the centre of the club face there is gear effect applied to the spin axis of the ball. But that is a discussion for another time. From this point on we are going to assume that the shot was a centred strike on the club face, and there was no gear affect present.
Club Face Angle
For short we will call club face angle as face angle. Trackman® defines face angle as; the direction the club face is pointed (right or left) at impact and is measured relative to the target line. Most golfers refer to this as having an “open” or “closed” club face. A positive value means the club face is pointed to the right of the target at impact (“open” for a right-handed golfer) and a negative value means the club face is pointed to the left of the target (“closed” for a right-handed golfer).Face angle is the most important number when determining the starting direction of the golf ball. Lets take a look at that last sentence again. Face angle is the most important number when determining the starting direction of the golf ball. What does this mean? The face angle, at the moment of impact, will determine the direction the golf ball will start on, It is not the only factor of where the ball will start as the launch angle will be pulled a bit in the direction of the club path, but for this discussion we are going to assume it is.
Club Head Path
For short we will refer to club head path as club path. Trackman® defines club path as the direction the club head is moving (right or left) at impact and is measured relative to the target line. Most golfers relate this number to hitting the ball “in-to-out” or “out-to-in”. A positive value means the club is moving to the right of the target at impact (“in-to-out” for a right-handed golfer) and a negative value means it is moving to the left of the target (“out-to-in” for a right-handed golfer). The difference between these 2 parameters is known as face to path, and is the primary determinant of spins axis of the ball. So lets take a look at the face to path parameter.
Face To Path
Trackman® defines face to path as the difference between the face angle and the club path. For a right-handed golfer, a negative face to path would represent a face angle that is “closed” to the path and a positive face to path would represent a face angle that is “open” to the path. A zero face to path represents a face angle and club path that have the same value.
Face to path is a key factor in determining the expected curvature (spin axis) of a golf shot. Assuming centred contact, the ball should curve towards the face angle and away from the club path (if face to path is not equal to zero).
Ok, now that we understand what each of the terms means, lets start looking at some practical examples of golf shots and what if anything may need to be worked