Technologie: Aerodynamik: Page 5

3. March 2003

Common Myths

Golf ball aerodynamics is one of the most pervasively misunderstood (or just un-understood) technical subjects in the game. Not that there's a shortage of ''experts'', there's just a shortage of actual knowledge. Misinformation rules in print and broadcast media, advertising, and sometimes even in technical documents like patents. Here are some of the more common misconceptions, along with the truth.

Myth: Dimples give the ball ''traction'' on the air.

Sort of like a snow tire? This is one which appeals to our common sense notions about airflow. Unfortunately, common sense is often wrong about aerodynamics, and that's the case here. Truth #1 taken from Intro to Aerodynamics I: When a golf ball or any other solid object moves through the atmosphere, a thin layer of air (the boundary layer) sticks to it and is dragged along with it. There is no slipping between the object and the air. Therefore, the issue of ''traction on the air'' is completely irrelevant and without a shred of meaning. Slipping never occurs, no matter how smooth and slick the surface of the ball may be.

Myth: Dimples create lift

Amazing but true: A smooth golf ball will only fly about half as far as one with dimples. But why does this happen? Many a golf ball guru would explain that a ball without dimples creates no lift. But more than 250 years ago, B. Robins was able to demonstrate the lift force on a spinning dimpleless musket ball. And any serious table tennis match will provide example after example of wildly curving, floating, or diving shots produced by lift forces acting on spinning smooth balls. The common factor here is spin, not dimples. As we saw above, it's the spinning action of the ball which warps the airflow and makes the ball act like a wing.

This is not to say that dimples have no effect on lift. To the contrary, they can affect both the amount and the direction of the lift, especially at low speeds.

The graph shows the lift forces measured in a wind tunnel on both a smooth golf ball and a dimpled one at identical spin rates of 3,000 rpm (a typical value for the first part of a drive). While the smooth ball doesn't generate as much lift as the dimpled one, it does create a substantial amount - equivalent to about 1/3 to 1/2 of its own weight for much of the speed range. So in reality, it's the spin that creates the lift. The dimples just tailor the lift to be more useful for a golf ball.

While the improved lift on a dimpled ball is part of the reason for the extra distance, that's not the whole story. It's not even most of the story. As we've seen above and will verify below, the dimples cut the drag, which pays off even bigger.

Myth: Dimples increase the drag

It is often said that dimples increase the drag on the ball, but since they are necessary to create the lift, the tradeoff is worth it. Your average Joe Golfer would accept this at face value because it agrees with (guess what?) our common sense. A smooth ball would slide through the air with less friction, right? So a dimpled ball would have more air friction, and thus more drag. Makes sense. Unfortunately, it's completely wrong.

First, as we have discovered, the dimples don't create the lift, they only improve it. And second, as shown in History of the Golf Ball, Aerodynamics, the dimples substantially reduce the drag by creating a turbulent boundary layer which reduces the wake. Wind tunnel tests verify this, as shown in the graph above. Measured drag forces for a smooth and a dimpled golf ball are compared throughout the full speed range. Clearly, the dimpled ball generates much less drag, only about half as much as the smooth one. This drag reduction is the most important contribution of the dimples. Strike two for common sense.