Prebennorholm From Denmark, joined Mar 2000, 6122 posts, RR: 55 Reply 4, posted (8 years 1 month 3 weeks 4 days 19 hours ago) and read 4045 times:
No, Stallionblue, the pointy thing cannot house a tailplane balance weight. The pointy shape would not allow enough movements up and down. But it is almost sure that the top of the leading edge of the fin - where the sweep-back ends - contains a tailplane anti-flutter balance weight.
I think that the pointy thing has two purposes:
1. It gives some drag reduction by "area ruling" the otherwise rather draggy tailplane / fin intersection.
2. It houses the VHF antenna.
Looking at the MD-80 we see that area ruling thing done in a somewhat different way. In front of of the tailplane the leading edge of the fin is excessively thick and seems to slim down - or at least not get thicker - at the tailplane leading edge.
I am pretty sure that this feature produces a slight drag reduction compared to the DC-9 which seems to have a more or less unchanged fin airfoil section from root to tip. But only at high cruising speed where the DC-9 has a widening fin profile combimed with widening tailplane profile and that way may produce high, local transsonic drag.