Do they do the dragon's full hit points at all times? Do they decrease with damage taken? Moldvay/Mentzer/RC are clear that it's the dragon's current hit points. OD&D, Holmes and AD&D don't necessarily say one way or the other, but they read to strongly imply max damage.
Anyway, I finally finished up my new verse translation of Beowulf today (it sat on the shelf for quite a while) and in the battle with the dragon, there were two passages that caught my attention:
To assist his kinsman, that man in mail
aimed not for the head but lunged at the belly
of their vile enemy (in so doing his hand
was badly burnt); his sword, gleaming and adorned,
sank in up to the hilt and at once the flames
began to abate.
The 'man in mail' referenced above being Wiglaf.
Each time I slashed at that deadly enemy,
he was a little weaker, the flames leaped
less fiercely from his jaws.
Wiglaf recounting the battle to the cowards.
Quotes from the verse translation by Kevin Crossley-Holland (c) 1999.
If the Beowulf author says I'm doing it right, I'm doing it right.