In each case, the servo was rotated 20 degrees from the top "Zero Degrees" setting. Notice the "No Differential" zero degree setup would form a perfect rectangle if you drew lines between the balls and the pivot points of the servo and the bellcrank. A 20 degree rotation results in the same rotation of the bellcrank in both directions.
In the lower illustration, the servo arm is not set square to the rod in the neutral position. A 20 degree rotation of the servo results in 16 degrees one way and 14.5 degrees the other way.
While the difference might seem small, differential plays havoc with the throttle and control inputs. Not only is the output smaller, it isn't the same (linear). In each case, the servo takes the same amount of time to make the input.
Bottom line...differential is usually bad. Work to eliminate it unless you are using it intentionally to improve symetrical geometry. I asked Elevated R/C to make this little square to help get linkages square. It doesn't work on all linkages, but it does work well in most throttle applications.
One point to remember. It's the angle between the ball and the center of rotation that's important, not the arm itself. Many arms are tapered or not square. If you are using levels to do the setup, always keep this in mind. Normally, when the designer uses bellcranks that aren't 90 degrees, they want you to have the rod square to the input arm. Always check the instructions.
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