The trouble with aligning to one normal below you is that its potentially very unstable. Where is the raycast coming from? Hovering along and coming up to a hill for instance unless the raycast is in the front then it'll probably ram any incline before the raycast hits it and reacts. What if you flew over small rocks? You'd be rolling around like crazy.
I remember seeing these videos a long time ago, and I think he has more that explains what he did to make this. iirc he put 4 raycasts, one at each corner, and either added force to points based on the distance they were returning or he averaged them out somehow to come up with an estimated safe normal direction for the ship to be set at.
Most of this sort of stuff is just smoke and mirrors, its just a matter of experimenting until you get something that is acceptable.
