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.
The ventral fin was to prevent ground stalling through over rotation. The -320C had three section leading edge flaps which altered the lift distribution and allowed the elimination of the ventral fin. Early -320Bs had the ventral fin, but the -320C wing mods were applied to this variant and it was eliminated. Some at least of the early -320Bs were modified to the later spec (including the VC-137C 62-6000). The 707-436 was mechanically and aerodynamically the same as all -320 and -420 variants, the only difference being that the -420 had Conways instead of JT4As.