ORIGINAL: FILE IFR
My first thought by reading what is going on here is the servo arms are not exactly even with each other at rest.
As they travel during flap deployment, one arm is approaching the apex of the arc movement earlier than the other... it's a geometry thing.
It could also be the flap horns are not exactly in line at the hinge line and/or at different heights with each other, or the hole heights don't match (servo or horn).
You are exactly right Mike. Due the geometry issues you mention and the differences between various servos it is
extremely difficult to get flaps to move identically. The phenonomen is present with ailerons and dual elevators as well but not as noticed because they are normlly centered and only move 50% as far, minimizing the imperfections in geometry.
My experience has been that flap servos need to be individually adjustable through either separate channels, a secondary mixer (Matchbox, Powerbox, etc..) or programable servos, if you want to get them exactly even.
Craig