The return value is in [-1 - 1] according to the
angle between the object Y axis and the world Z axis
If our object is a plane then this function will return the
cos() of the angle between the plane wing and the world Z axis
0 means the object is strait.
1 means one wing is pointing up to the sky and the other wing into the ground.
-1 means that the other wing is pointing up to the sky and the first wing into the ground.
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