Can you install an airconditioner window unit vertically?

No part of the air conditioner should require gravity-down, but it's likely that the drain won't work quite right if you turn it sideways.

I'd find a nice local a/c shop or small hardware store and ask them about it.

I will admit, however, I've never seen one installed that way.

that's what I figured... thanks!

I don't know what kind of space you're trying to fit, but I did a web search for:
vertical "air conditioner"
and there are models designed for sliding and casement style openings. Give it a try!

thing is that I already have an AC unit that would fit perfectly in one of my windows if i installed it vertically, which it wasn't specifically designed for

I understand; but I wouldn't recommend it. Could run up some costly repair bills from water damage if it's not draining properly.

well folks, I went ahead and did it. so far it's draining fine. the a/c is cheap enough that if it breaks, no big woop. two days running and it still works, though it's a bit noisier than I think it would be if installed properly

It won't last long! The reason it's noisy is due to improper oil return to the compressor!