JollyPopper,
For the short answer, see the attached diagram.
Here is the 1000 words’ answer:
ORIGINAL: JollyPopper
If I hoist a plane in the loops of this machine and then manually level the horizontal stabilizer using a carpenter's level, the plumb bob actually does point diredtly to the center of gravity. However, if I rotate the dowel around which the loops are wrapped, raising or lowering the nose of the plane, the pointer points at another spot. This seems to be telling me that the CG changes with the attitude of the plane.
The CG
DOES remain constant regardless of the attitude of the plane.
The CG is just a concept:
Each cubic inch of the airplane feels a force pulling it directly to the center of Earth. We call that force the weight of that cubic inch of plane.
Since it is hard to compute the many weights of each cubic inch of any airplane, we try to find that unique point in the entire body of the airplane for which all the individual weights on the right side equal all the individual weights on the left side of that point.
We call that unique point CG, and consider that such a point is feeling a force equivalent to the addition of all the individual weights of the entire body of the airplane (on the right and left sides).
(That is only true for a bi-dimensional analysis (pitch balance), in actuality; CG is the balance point for all the individual weights surrounding that point spatially).
According to that,
the only way to relocate that theoretical point (CG) is by altering the balance of forces (individual weights) between right and left.
If we add a piece of lead to the tail, there will be more forces pulling down towards the tail than towards the nose.
Our old CG is not such “center” anymore; hence, we have to find the new CG, which will be a little closer towards the tail.
The weight of each cubic inch and its distribution in any airplane has nothing to do with the attitude or position of that airplane respect to the ground.
ORIGINAL: JollyPopper
I've read here on RCU a couple of times about a machine that you can hoist your airplane in and a plumb bob would point to the center of gravity. I could not visualize how the thing would work so I built one. And I still can't figure out what it is telling me.
In summary, the device is nothing more than two independent plumbs, pendulums, or hanging bodies pointing vertically down.
Naturally, the CG of each body will place itself at the lowest possible point.
If both bodies are suspended from a common point, then both CG’s will be located over the same vertical line that starts at the hanging point (although not necessarily at the same spatial point).
Once any swinging stops, the plumb points in the direction of that vertical line, and the airplane CG is located somewhere along than line.