I'M BUILDING A ROLLERCOASTER

Is this a vertical or horizontal (relative to the ground, ie gravity) loop?

It would be a vertical loop I think, as it would be on a rollercoaster.

Here are a couple of sites that might help you:
http colon //seanelvidge dot com/2013/02/velocity-required-for-loop-the-loop/
http colon //science.howstuffworks dot com/engineering/structural/roller-coaster7.htm

You'll need to reconstruct the links. I don't know what constitutes a valid (won't be deleted by a moderator) link at this place.

far as I figure, useful links by known posters (not spam, not selling something or other, not bots) are ok. I recognize your username and avatar and the mods probably do too.