Originally Posted by
Meikleour
IIRC the 707-436 had a parallel yaw damper which had to be disengaged for take-off and landing and even with the extra fin area (main + ventral) their was a continuous mild tendency to dutch roll. This was not the case with the -321C which had a series yaw damper and thus no ventral fin. It would have been interesting to hear DPD's opinion of the stab trim motor stalling under large mistrimmed conditions.
Further to that, and the interesting response by Alan Baker, it may be worth mentioning that some of the early -320Cs may have retained the parallel yaw damper, while having the taller fin and no ventral fin. British Caledonian obtained a -373C from Britannia Airways (registered G-AYSI) that had been manufactured in 1963 and first operated, I believe, by World Airways.
AFAIK it was the only BCAL -320C that was not designated as "Advanced". All our "Advanced" a/c had the serial yaw-damper. In my experience, G-AYSI had a tendency to Dutch-roll slightly in the cruise with the parallel yaw-damper engaged.
It may be worth mentioning that full-span (except at the pylons) L/E flaps on B707s are of the Krueger type - unlike the VC10s, all of which have full-span L/E slats.