But we're only concerned with real world practical applications. Who cares about the fantasy and the absurd? Of course, if a plane could be so light that air would be like molasses, it virtually wouldn't fall due to gravity, not mention it could hardly move in any direction, thus c.g. doesn't matter (the fantasy) and why would we want to build a plane so heavy that it couldn't fly? (the absurd) I understand what you're saying. It just seems pointless.