Originally Posted by
bohpilot
In level inverted flight, the wing is still producing lift so why would the aircraft axis be above the velocity, would they not be the same ?
In level inverted flight, the wing has to produce negative lift, i.e., the lift vector is pointing out the belly instead of the roof. This requires negative AOA, which means the aircraft axis is above the velocity (seen by an upright observer)
Your last phrase "would they not be the same" belies a deeper confusion, which is enabled by the efficient cambered wings on our airplanes, that make lift at zero AOA at high speed. Imagine for a few minutes that all wings are symmetrical, and really inefficient at that [edit: earlier I typoed and wrote the opposite]. That means that to make any lift, it has to fly at a definite, high AOA. Zero AOA = zero lift. High AOA = high lift.
Even in normal cruise (level right side up flight), then, the aircraft axis has to be above the velocity. If they were aligned, then the AOA would be zero and no lift could be created. In level inverted flight, the exact same thing is happening. The aircraft axis has to be above the velocity, (or below, if this is being considered by someone sitting in the plane. Put it another way, in both cases the velocity is aligned exactly with the horizon, and the leading edge of the wing is pointed a bit at the sky.
When considering large scale effects of all-attitude flight, this simplification is more than close enough to use, and should hopefully make things clearer.