@DanHulme there's a video now: youtube.com/watch?v=gkNoLSrzTQ8

@FreeMan see above

So I'm trying to think through why, in an airship, the gondola automatically stays at the bottom, and ailerons are not required in order to keep it there.

I'm used to thinking that the only effect that gravity has on an aircraft is on the aircraft's flight path (more precisely, its velocity).

gravity?

In particular, gravity doesn't cause an aircraft to roll, or otherwise have any effect on an aircraft whatsoever, besides constantly adding downward velocity to the aircraft (which is usually balanced out by an approximately equal force in the opposite direction).

That's why we need turn indicators and attitude indicators to fly through a cloud—it's because the instruments on an aircraft have no way of determining the direction of gravity.

Anyway, it seems like airships are some sort of weird exception to the rule or something.

But now that I think about it, I think I know the reason.

Airplanes are held up by dynamic lift, which is completely independent of gravity. Airships, on the other hand, are held up by buoyant lift, which always acts straight up; and the center of pressure for buoyant lift is in a different place than the center of mass.

airships tend to be a lot slower than fixed wing aircraft

and held up by buoyancy of the lifting bag up top

the centre of pressure for dynamic lift is also in a different place than the centre of mass

that's why we have a horizontal stabiliser - otherwise gravity would cause the airplane to pitch nose-down