The Aerodynamic Forces
on a Golf Ball


 

Simple Wing
Aligned with Airflow

Given that golf is the most technical sport, and scientists are the explorers of technology, it was only a matter of time before the two got together. From an aerodynamics standpoint, that didn't happen until near the turn of the century, by which time golf was already hundreds of years old. The science of aerodynamics, however, was still young at that point. It should come as no surprise that the momentous event occurred in Scotland, when physicist Peter Guthrie Tait began publishing a series of scientific papers in 1890, which were pioneering in their recognition that air had a lot to do with a golf ball's amazing trajectory.

No doubt it is counterintuitive that the overall effect of air on the flight of the golf ball is, in fact, very positive. After all, wouldn't wind resistance slow the ball down and make it drop, rocklike, to mother earth? Believe it or not, a shot that flies 230 yards in the normal atmosphere would only fly about 160 yards in a vacuum. How can this be? Strangely enough, golf balls are brethren to wings, and wings don't work if there's no air. By the magic of aerodynamics, the spinning ball makes lift, suspending itself against gravity. So it flies farther even though wind resistance (or drag, as aerodynamicists call it) is slowing down. If the air were to disappear, then the drag would disappear, but so would the lift. The net result? You'd be pulling out the Big Stick for a middling par 3 (assuming you could swing it while wearing a space suit).

Simple Wing at
Angle of Attack
 

Every time we stick a hand out the window at 65 mph (don't try this at home if you live in a 55 state!), we're reminded that air exerts a force on any object moving through it. Scientists like to break this force down into two basic components: drag, which acts directly opposite the motion to slow the object down; and lift, which acts at right angles to the drag and generally upward.

Ball with No Spin

Ball With Backspin

To the uninitiated, watching a golf ball fly is an amazing experience. It hangs in the air for an astonishing length of time, as if supported by a force field. And it flies twice as far as a towering second-deck home run. All of this goodness is possible because of the aerodynamic lift force. But where does it come from?

While a person wouldn't confuse a golf ball with a 747 wing, a wind tunnel might. To the air, they look very much the same. When a simple wing is placed in an air stream and aligned with the flow direction, it simply slices through the air with minimal hoopla, and generates no lift. However, if it is inclined to create an angle of attack, then interesting things start to happen. It deflects the airflow downward, creating an upward reaction force (Newton's Third Law: "To every action there is always opposed an equal reaction" - which we know as the lift.)

A golf ball may look portly next to a streamlined wing, but it manages to do similar things to the airflow. When a golf ball is placed in an air stream, it pushes through the air creating a considerable disturbance (that's the portly part), but generates no lift. Here's the good part: given some backspin, it warps the airflow very much like the angled wing, deflecting it downward and creating lift.

Ball Punching
Through the Air


 

Dimpled Ball Punching
Through the Air


 

Golf Ball with
Spin and Dimples

Move any object through the air, and you'll get some drag. Most flying bodies (not including the occasional golf club) have a streamlined profile by design or by nature, so that they cut through the air cleanly with minimal drag. But a golf ball has to be (guess what . . . ) a ball, so it is destined to be an air punch rather than an air knife. This makes for a large drag force.

The air hits the front of the ball, creating a high pressure area, and splits around to the sides. But like a teenager in a Camaro, it's going too fast to make the turn to the back of the ball. It separates from the surface, leaving a low pressure wake like the one a boat leaves in the water. This combination of high pressure on the front of the ball with low pressure on the back is the main source of a ball's drag.

This may seem hopeless, but it's not. Maybe you can't control the teenager, but you can put better tires on his or her car. The solution? Dimples. When the surface of the ball is covered with dimples, a thin layer of air next to the ball (aerodynamicists call it the boundary layer) becomes turbulent. Rather than flowing in smooth, continuous layers (a laminar boundary layer), it has a microscopic pattern of fluctuations and randomized flow. Here's the good part: a turbulent boundary layer has better tires. It can better follow the curvature of the ball's profile. It travels farther around the ball before separating, which creates a much smaller wake, and much less drag. In fact, a dimpled golf ball has only about half the drag of a smooth one.

Wind Tunnel Smoke Flow
(Picture F.N.M. Brown)

If we combine the spinning motion, which warps the airflow and creates the lift, with the dimples, which reduce the wake and cut the drag, we get the flow pattern around a golf ball.

Proof, you say? Professor F. N. M. Brown of the University of Notre Dame was famous for his work in visualizing flow patterns by injecting streams of smoke into a wind tunnel.

To the left is one of his pictures of an actual spinning dimpled ball. While the spin rate he used appears to have been quite low by golf ball standards, you can still see the warpage of the flow field, especially in the angle at which the wake trails away from the back of the ball.
 

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