The flow of air around a moving baseball knuckler is always fairly irregular. This is because a baseball is a blunt object, as opposed to a streamlined object such as an airplane wing, and the air through which is passes must do quite a bit of hurrying to get out of the way.
Generally, however, the flow is relatively smooth on the top and the sides of the ball. But once the air stream reaches the rear of the sphere it becomes confused. It no longer adheres smoothly to the surface of the ball but breaks away, some of it whirling on back into space, some of it sucked in close behind the ball to form a turbulent wake. This is much like the wake behind a boat, a whirling vortex of eddies and currents and agitated air.
Aerodynamicist Dr. Stanley Corrsin of Johns Hopkins is fascinated fan of Hoyt Wilhelm’s jitterbugging phenomenon
The point at which the smooth air breaks away from the ball is known as the separation point, and the line formed all around the back part of the ball by these countless separation points is known as the separation line. It is never a smooth line but a zigzag, erratic one, for the air breaks away sooner on some points on the sphere than at others. This is caused by a number of factors, including the raised seams of the knuckler, the imperfection of any sphere, gusts of wind, etc.
In the case of the curve ball, which is thrown with tremendous spin, or a fast ball, which is thrown very hard and also with a relatively high rate of spin, the very fact that the ball is spinning tends to have a stabilizing effect on its flight. It smooths out the streamlined air flow even more, causes the separation point to occur further back on the sphere and reduces drag. The spin, since it is a somewhat overpowering force, also enables the curve or fast ball along a relatively smooth path, unlike the knuckler.
Because the curve ball has both sideways and forward rotation, the streamlines which flow along the top side are vastly accelerated as opposed to those which travel along the bottom. This increased velocity on top sets up a pressure difference and forces the ball in a downward arc. (continued below)
Upper Left Caption: Curve ball spins forward. Unequal pressures force it into downward path. Upper Right Caption: Fast ball spins backward, will “hop” if pitcher can throw it hard enough. Lower Caption: Knuckle ball does not spin, so dominant force is setup by erratic turbulence created when smooth air stream breaks away at back of ball. This is called separation line. In rear view of ball (right) separation line is irregular, constantly shifting.
The fast ball reacts in an opposite manner. Here the ball spins with a bottom-to-top or backward rotation, those streamlines are faster which pass beneath the ball and the pressure difference established in this case tends to force the ball upwards. In a normal fast ball, this only offsets the always-present force of gravity, and the ball proceeds on a straight path. In the case of an exceptionally hard-thrown fast ball, such as those thrown by Herb Score or Don Drysdale, the ball will actually rise toward the end of its flight. This is the “hop” on a good fast ball.
The knuckle ball, however, spins little or not at all; any slight rotation it might produce is so small as to have little effect on the ball’s course. The dominant factor, therefore, is the interaction between the separation line and the turbulent wake. And these confused, swirling eddies not only slow the ball down, they cause unbalanced sideways pressure forces. These forces will eventually cause the ball to go off course. This is why the knuckler darts and jumps.
If the separation line was perfectly straight, for the pressure forces would be even. But since the separation line is highly irregular, so is the course of the ball. And since the separation line is constantly shifting and changing in its irregularity, the course of the knuckle ball may shift or change. The knuckle ball can change direction several times in flight. It is also well to remember that regardless of other forces acting upon the ball, gravity is always exerting its influence, too. Gravity does not make the ball break, but it does accentuate any downward movement. And that is why Wilhelm’s down-breaking knuckler is much more abrupt than the one which rises.
Although the batter may be hard to convince, no knuckle ball — or any other baseball — breaks as sharply as it seems to. No blunt object obeying established physical laws can execute a sharp angle during flight.
“That is why,” Dr. Corrsin says, “flying saucers can’t make sharp turns.”