I like Vanessa too.

The principle is simple: for two bodies hanging from the same point (or axle), the CG of each body will be located over the vertical (or plumb) line that extends from that point down.
This is true, regardless the position of each body around its own CG (or attitude) or the part of the body from which it hangs.

For this rig, one body is the airplane, the second body is the plumb.

Vanessa is the name of the daugther of the man who invented this balance rig: Jim Archer; unfortunately, he passed away some years ago.