[Trivial Psychic]

When it comes to FS ships, one could also assume that a form of lift could be generated by some sort of anti-gravity repulsion system, at the cost of the ship's power.  Perhaps if the engines are damaged, the ship loses repulsion power and begins to sink... unless the pilot pulls the nose up to divert more primary thrust down.

[Nuke]

the whole antigravity theory does mesh well with some of the cutscenes. the one where the shivans are bombing vasuda, you see a toth flying in the atmo, the ship is just going too slow to stay up by means of aerotdynamic lift (unless vasuda had really thick atmo). on the other had i dont think every feature in the engine needs to revolve around fs cannon. in fact if you stuck with fs cannon, missiles would launch from bays and this whole topic would be pointless.