what about vertical cg?

[wmsvirsky]

I see lots of talk about CG, but it's always about the horizontal CG. What about the vertical CG? The reason I ask is that I've seen messages on various forums where flyers mention that a plane has a tendency to flip over on it's back suddenly if you bank it too sharply during a turn. The same thing happened to me when I was trying to find the horizontal CG of a new lightweight foamie airplane - the Albatross 3D, a profile foamie with a flat wing. I was using masking tape to hold the battery pack - 2xQualcomm 830s (about 2oz) flat against the fuselage while resting it on the top of the wing (fig 1). I did have problems with the plane wanting to flip over when I was turning it. At the time it didn't occur to me that the high battery pack might be the problem. Next time I flew it I put the pack under the wing (fig 2). The plane tracked much better and turned much better. Unfortunately, it was already pretty dark and I had to bring it in almost immediately, so I didn't get a chance to really put it through its paces, but the little I did get to fly looked much better. When I thought about, it did make sense that too high of a vertical CG would make the plane want to flip over. Taken to an extreme, as in fig 3, it would be almost impossible to fly without flipping over, and taken to an extreme the other way, as in fig 4, it would be impossible to invert the plane at all. Eventually, once I find my ideal horizontal CG, I will mount the pack as in fig 5.

So, first of all; does this make sense? If so, why doesn't anyone talk about vertical CG? Is vertical CG usually not a critical factor in designing airplanes?

Thanks.

Bill

[Sparky Paul]

Any "sudden flip over" is due to stalling the airplane, not the c.g. location.
The air surrounds the plane on all sides.
Any change in orientation regardless of the c.g. isn't going to result in anything "sudden" unless it's aerodynamic, not mechanical in nature.
A low placement of the c.g. in a high wing airplane lends "penudulum stability", and moving the c.g. up makes it less stable.. or more manuverable depending on what you desire, but there's not a point at which the plane will "flip" unless the c.g. is really poorly located, in which case it probably wouldn't fly well anyway.

[Ollie]

I agree with Paul. The sudden flip is caused by the wing tip stalling before the rest of the wing. Possible causes of tip stall are flying too slowly (at too high an angle of attack) in conjunction with turning too tightly, a highly tapered wing planform, wash in or, sweep back.