Lift force on sports balls with minimal spin: an examination of the knuckleball

Mark Adam Fong

It is long held that the trajectory of a knuckleball is random and not controlled by the pitcher, with sudden directional changes not seen in other pitches. Wind tunnel studies indicate that seam orientation of the baseball initiate or delay flow separation and impart lift on the ball. When orientation is controlled these lift forces are repeatable. When held stable in moving air baseballs rotate to a predetermined seam orientation due to air friction imbalances on the surface of the baseball. This work aims to study a knuckleball in-flight to examine if propelling a ball through still air will create a repeatable trajectory as suggested by wind tunnel experiments. 3-D pitch trajectory was tracked by both radar and stereo video tracking, while video tracking was also used to quantify the orientation of a baseball with respect to the global frame. It was shown that a knuckleball trajectory over a 60 foot pitch distance is smooth and not as erratic as baseball pitchers and experts have described. The largest displacement from smooth sphere trajectory observed was 23 inches, also less than anecdotical descriptions and average curveball displacement. However, baseballs launched with the same initial conditions demonstrated coefficient of lift values as different as .45 during a 60 foot pitch. This can result in as much as an 18 inch difference in ending position. Over a 27 foot laboratory environment, it was shown that in-flight lift values were within .05 coefficient of lift to wind tunnel measured lift values for the same orientation. However, it was also shown that individual baseballs will have independent air friction rotation magnitudes and directions. With identical launch conditions, this difference in air friction is can cause up to a 50 degree difference in orientations between baseballs after 24 feet of flight. After 16 feet of flight coefficient of lift values can differ by as much as .2. The variation of air friction induced rotation on the baseball creates differences in lift force and results in the unpredictable trajectory of a knuckleball.

Lift force on sports balls with minimal spin: an examination of the knuckleball

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