Originally Posted by
BEagle
Deployment of mainplane flaps and drooped ailerons alone would cause a significant n-d pitching moment. To counter this and avoid the need for significant retrimming of the varialble incidence tailplane by the pilot, simultaneous upward extension of a tailplane flap provides an equal and opposite n-u pitching moment. The pilot does not need to make significant control column movements during flap extension and only needs to retrim with IAS change as normal.
Blown flaps meant the tailplane flap was blown as well if hazy Q course memories from 38 years ago spring to mind.