Originally Posted by
henra
What right yawing tendency? There is a left yawing tendency in the heading starting some time before the main upset. .
The heading reflects the combined result of a yawing tendency AND the rudder surface deflection together.
If you step back from the imminent main upset, and look at the early part of the takeoff roll instead, the pilot already had to input stronger left rudder force than he expected with the wind data. So there must have been an additional right yawing tendency during the early part of the takeoff roll, that he - successfully - compensated with stronger push on the left pedal.
(If there had been no such tendency, the aircraft would have ran off the left runway edge before becoming airborne. But it didn't. Something was causing a greater tendency to yaw to the right than expected, which disappeared around the moment when the aircraft rotated. The continued pressure of the left pedal remained, led to an even greater deflection of the rudder pedals and surface, and the over-compensation of the disappearing-reducing right yawing tendency resulted in a susprise turn to the left and yaw to the left instead, with practically no input force change from the pilot flying.)
The last appearence of that "hidden" right yawing tendency may be between 20:40:26 and 20:40:28. You can see a slight heading deviation to the right, followed by a significant 10 degree left rudder deflection, and a subsequent return to the normal heading. Would be good to understand, what "conditioned" the pilot to input larger-than-usual left rudder forces, because that pre-conditioning may be partially the answer why the deflection increased into the wrong direction at rotation.