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10-27-2009 | 12:07 AM

#28

Roy Dale

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From: , FL

RE: Golf ball dimples

Dimples cause the relative airflow that influences the ball be more turbulent, stickier and generate more surface drag. This causes the relative airflow around the non-spinning ball to stay attached longer before leaving the ball. This causes the ball to leave a smaller wake and generate less dynamic drag than a smooth ball with a more laminar flow. Because dynamic drag is the most prevalent reducing it reduces over all drag.



When the ball is hit with a spin things get more complicated. The relative airflow that influenced the ball while moving through the air is affected dramatically by the balls movement while in the air (rotation). Dynamic drag mostly opposes the balls linear motion while surface drag opposes the balls rotational motion.



Aircraft have reached speeds of 250mph while flying around the earth using drag exclusively as a means of horizontal acceleration. A lot of people may not know that drag in some circumstances can actually oppose motion but even while opposing one motion drag can cause another motion. When you pull a canoe paddle through the water the drag on the paddle is in the forward direction generating the thrust the canoe needs to propel itself. The drag from a powered prop causes the plane to tend to rotate around the propeller axes. The non-aerodynamic drag between a tire and the ground cause the rotating tire to move linear and this is very similar what is happening to the spinning golf ball.



If a spinning golf ball were to be pushed into the ground it would produce a dramatic linear force, if a spinning golf ball were pushed into the air by its linear motion in would produce a much less dramatic linear force that makes it curve called the Magnus effect inaccurately referred to as lift or Magnus lift. The Magus effect is caused by surface drag, dimples increase surface drag and therefore the Magnus effect, and this is not a coincidence





If the Magnus effect is caused by surface drag why is it called lift or Magnus lift? Well that’s a very good question and hear is the answer. Although texts state that the Magnus effect is caused by the ball spinning and the more it spins the more effect is caused, when determining the aerodynamic force that causes it the very large and obvious fact that the ball is spinning is intentionally and totally ignored. For this force to be lift you would have to determine it from the relative airflow caused by the balls linear motion through the air when in fact the relative airflow that is influencing the ball is made up of the balls linear and rotational motion. Calling this force lift is based on the false premise that the ball is not spinning. Not very scientific but it shows the amount of skew of actual occurrence that is acceptable when writing formula. Drag is in the direction of the relative airflow that caused it, and the drag from a spinning ball going through the air opposes its rotation as well as it linear motion, how many directions is that? When the even surface drag around the spinning ball starts to become uneven it also starts to become more linear and this is the cause of the Magnus effect.

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