A Cure for Hooks and Slices? Asymmetric Dimple Patterns and Golf Ball Flight

Objective

The goal of this project is to test whether an asymmetric dimple pattern on golf balls can produce straighter flight.

Introduction

The dimples on the surface of a golf ball are there for a reason. A golf ball with a smooth surface would only travel about half as far as the dimpled ball. Why is this so? The answer has to do with the flow of air over the ball when it is in flight. When a solid object moves through a gas (or a fluid), the gas pushes back on the solid. In aerodynamics (or fluid mechanics) this resistive force is called drag. The dimples on the surface of the golf ball are there because they reduce the drag force on the ball (Figure 1).

Figure 1. The dimpled surface of a golf ball decreases the drag force on the ball as it flies through the air (Scott, 2005).

How exactly does this work? In order to understand, we'll need to take a closer look at the pattern of airflow around a ball as it flies through the air. Figure 2 compares the airflow pattern for a smooth ball (top) vs. a dimpled ball (bottom), in horizontal flight (or in a wind tunnel). In the case of a ball with a smooth surface, the airflow in the thin layer right next to the ball (called the boundary layer) is very smooth. This type of flow is called laminar. For a ball with a smooth surface, the boundary layer separates from the ball's surface quite early, creating a wide, turbulent wake pattern behind the ball. The turbulent wake exerts a drag force on the ball. When dimples are added to the surface of the ball, they create turbulence within the boundary layer itself. The turbulent boundary layer has more energy than the laminar boundary layer, so it separates from the surface of the ball much later than the laminar boundary layer flowing over the smooth ball (Figure 2, bottom). Since flow separation occurs later, the turbulent wake behind the ball is narrower, resulting in less drag force.

Figure 2. Comparison of the airflow over a smooth ball vs. a ball with a dimpled surface. In the case of the smooth ball (top), the boundary layer has a laminar flow pattern which separates from the surface early, creating a wide turbulent wake behind the ball. In the case of the dimpled ball, there is a turbulent boundary layer which separates from the surface later, creating a narrower turbulent wake behind the ball. The narrower wake results in less drag. Thus, given the same initial launch force, the dimpled ball travels further than the smooth ball (Scott, 2005).

In the real world, the situation is more complex than shown in Figure 2. First of all, golf balls don't fly horizontally through the air. When the club hits the ball, it launches it at an angle, determined by the golfer's swing and the loft angle of the club. The ball's initial speed and angle will be determined by the speed and orientation of the club face at the moment it strikes the ball, and exactly where on the surface of the ball that contact is made.

To make things even more complicated, the club generally imparts a spin to the ball. How does spin affect the flight of the ball? Let's consider the simplest case first. If the club strikes the ball squarely, the spin that is induced is called backspin (because the ball is spinning backwards, from the golfer's viewpoint). To be more precise, backspin is a spin around the horizontal axis, in a clockwise direction if viewed from the left-hand side (as in Figure 2, above).

Let's consider the effect that backspin will have on airflow over the ball. Since the surface of the ball is now moving in a clockwise direction, the airflow over the top of the ball will be sped up, and the airflow over the bottom of the ball will be slowed down. This has the effect of decreasing the pressure above the ball, and increasing the pressure below the ball. In other words, a spinning ball acts like an airplane wing and creates lift. Figure 3 shows how backspin affects the airflow over a golf ball in a wind tunnel. The smoke lines in Figure 3 show the airflow pattern. Notice how the flow pattern behind the ball is warped downward. This is the same type of pattern you would see for an airfoil at an angle to the wind tunnel air flow (like an airplane wing at takeoff when the plane starts climbing). The spin rate used in Figure 3 was less than the average spin for a golf ball hit by a club. The lift effect with real-world spin rates would be even greater.

Figure 3. Spinning golf ball in a wind tunnel. The smoke lines show the pattern of airflow over a golf ball with backspin. The air moves faster over the top of the ball, and more slowly over the bottom of the ball. The flow field is curved downward, indicating that the spinning ball is generating lift (F.N.M. Brown, in Veilleux and Simonds, 2004).

What if you don't hit the ball squarely? For example, say the club face is angled outward (away from the golfer's body) as it strikes the ball. Then the induced spin will have a component about the vertical axis. In this case, the spin would be clockwise, as viewed from above. The spin would result in an aerodynamic force pushing the ball off to the right, away from a straight flight path. In addition, the initial launch angle would be off to the right instead of straight ahead. These two combine to create what golfers call a slice. Instead of sailing straight down the fairway, the ball curves off to the right, perhaps into the rough, or trees, or (in the worst case) off to an adjacent fairway.

If the club face is angled inward (toward the golfer's body) as it strikes the ball, then the ball tails off in the opposite direction. Golfers call this a hook.

The Polara golf ball has an asymmetric pattern of dimples. There are six rows of deeper dimples on either side of the equator. At each pole, the dimples are shallower. This creates an airflow that tends to correct sidespin, and reorient the ball toward straighter flight. Does it have a significant effect on where the ball ends up? That's what this project is designed to find out.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Newton's Third Law
• Golf ball aerodynamics:
  • Drag
  • Lift
  • Boundary layer
  • Flow separation
  • Wake
• Loft angle
• Spin
• Magnus effect

Questions

• How does backspin on the golf ball create lift?
• How does side spin cause the ball to hook or slice?

Bibliography

• For information on golf ball aerodynamics, try these resources:
  • Portz, S., date unknown. "Why Does a Golf Ball Slice or Draw?" Physlink.com [accessed April 11, 2007] http://www.physlink.com/education/AskExperts/ae423.cfm.
  • Scott, J., 2005. "Golf Ball Dimples and Drag," Aerospaceweb.org [accessed April 11, 2007] http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml.
  • Cislunar Aerospace, 1996. "Aerodynamics in Sports Equipment: Why Does a Golf Ball Have Dimples?" Cislunar Aerospace, Inc. [accessed April 11, 2007] http://www.fi.edu/wright/again/wings.avkids.com/wings.avkids.com/Book/Sports

  /instructor/golf-01.html.

  • Titleist, 2006. "Titleist: Technology: Principles of Aerodynamics," Titleist.com [accessed April 11, 2007] http://www.titleist.com/technology/aerodynamics.asp?bhcp=1.
  • Veilleux, T. and V. Simonds, 2004. "How Do Dimples in Golf Balls Affect Their Flight?" Scientific American, Ask the Experts [accessed April 11, 2007 http://www.sciam.com/askexpert_question.cfm?articleID=000BB2B1-AD6A-1FF0-AD6A83414B7F0000.
• This webpage describes a method for visually estimating wind speed:
  NWS, 2007. "Beaufort Wind Scale," National Weather Service Forecast Office, Miami-South Florida [accessed April 11, 2007] http://www.srh.noaa.gov/mfl/hazards/info/beaufort.php.
• This website has descriptions and calculators for several statistical tests, including Student's t-test that you can use in this project:
  Kirkman, T., date unknown. "Student's t-Tests," Department of Physics, College of St. Benedict & St. John's University [accessed April 11, 2007] http://www.physics.csbsju.edu/stats/t-test.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Golf club (nine iron or pitching wedge)
• Polara golf ball with asymmetric dimple pattern (available from Polara Golf; see the Variations section for a less expensive alternative)
• Regular golf ball (any brand)
• Empty football field
• Tape measure

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Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions above.
2. Set up at one end of the football field to hit a ball from the center of the goal line.
3. Use the same club for each shot, and do your best to use a consistent swing for all of the shots.
   1. Note that the Polara ball is designed to correct the aerodynamics of balls that are mis-hit. It should have little effect on balls that are hit squarely.
   2. It would be a good idea to collect data for two different types of swing for each ball:
      • hitting the ball squarely so that it goes straight,
      • hitting the ball with the club face angled to deliberately hook or slice the ball.
   3. The trick is to do each type of swing as consistently as you can.
   4. Because your swing is not likely to be the same each time, you will need to do a large number of trials for each type of ball and each type of swing (at least 20, more is better).
4. Use the yard lines on the field to measure the distance of each shot, and use your tape measure to distance away from the center of the field (amount of hook or slice). Keep track of these measures for each type of ball.
5. You can alternate which end of the field you hit from to save walking.
6. Since the wind can have an effect on the flight of the ball, you should note the wind speed and direction in your lab notebook (see NWS, 2007).
7. Calculate the average flight distance for each type of ball, and the average amount of hook or slice for each type of ball.
8. Calculate the standard deviation for the flight distance and the amount of hook or slice for each type of ball.
9. Illustrate your results by making graphs that show the distribution of the two types of balls with each type of swing.
10. More advanced students should also do a t–test (Kirkman, date unknown) to see if any differences in the flight characteristics of the two types of balls are statistically significant.

Variations

• Rather than purchasing a Polara golf ball, you could make your own asymmetric dimple pattern by increasing the depth of some of the dimples on a regular golf ball. (You could also try filling, or partially filling, some of the dimples to create a smoother surface. You'll need to use a material that will stay put even when the ball is whacked repeatedly with a golf club.) You shouldn't need to remove (or add) a lot of material to get an effect—just a few thousandths of an inch should be enough (Veilleux and Simonds, 2004). You could do it by hand with a drill bit of appropriate size, or you could use a drill press with a mechanism for setting the maximum travel (wear safety goggles, adult supervision required). This experimental approach will save you some money (the Polara golf balls are expensive), and give you the flexibility to explore different patterns. For example, do you think you could make a dimple pattern that would increase the amount of hook or slice?
• This procedure assumes that you will make your shots on an empty football field. The clubs were selected to be appropriate for this distance. If you have access to a larger open space, you may want to modify the experiment accordingly.
• For a more basic experiment on the flight of golf balls, see the Science Buddies project Golf Clubs, Loft Angle, and Distance.

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