It does exist: The Hovering Cobra
Yes Knife edge forever with just a tad bit of coupling. If I could get more throw out of my rudder I would say it would do knife edge loops with ease. (I'm using standard servo control horn and just can't get that much throw on the pull pull set up that I want.) Water Falls, Hhmm If I'm understanding what a water fall is I would say they look more like really tight loops than a flip. I can get mine to flip once but after that it just does a loop before I can get another flip out of it.