ORIGINAL: Indiomike
I was just trying to understand why an airplane needs to be inverted to check CG rather than upright......
The reason is the decalage, as explained in the attached schematic.
ORIGINAL: Indiomike
........and why the airplane needs to be at about 45 degrees on an up line as opposed to straight and level or down line or any other attitude.
The stability margin respect to the horizontal is reduced this way, making it less "pitch unstable" during inverted flight.
Same principle applies for a 45-degree down line, but it is less forgiving of mistakes.
Degree of decalage depends directly on stability margin.
During horizontal flight, the first compensates the second perfectly.
The lift force and the weight hanging from the wing gradually result reduced for any climb or dive, reaching both zero value for perfectly vertical flight (up or down).
Because of that, the down nose pitch decreases for any climb or dive; however, the up nose pitch produced by the decalage remains constant and even increases for increased speeds (diving especially).
The strength of that up nose pitch during climb or dive is directly related to the strength of the down nose pitch for level flight (which is determined by the separation distance between the points of application of the weight and lift=stability margin).
Check this related article:
http://www.charlesriverrc.org/articl...GMarkDrela.htm