Everyone:
34DD is spot on when he says that we don't have to understand every aspect of an aircraft inorder to use it.
In science you basically have a model of how a system behaves. By necessity, that model is usually a gross oversimplification of what is going on. But as long as the model holds good, for the operating regime that we are interested in, who cares?
Well, inadequacies in your model only become a problem when you try and extrapolate the behaviour of the system outside of the regime the model was designed to cover. At which point, one of two things happens:
1) Either the system still behaves as predicted by the model (you slap yourself on the back!).
2) The system doesn't behave as expected, the model has been shown to be imperfect.
When 2 occurs, you try a more complex model. Your simpler model either gets binned completely , or, since it is simple, you remember it as a way of introducing the system to students that are trying to get to grips with it.
The problem most pilots have with principles of flight is that they do not have a solid scientific education. Therefore most PofF books, by necessity, provide very simple explanations for what is going on.
view of a starting point for studying PofF.
This doesn't really represent a problem. The models are good enough to give everyone a rough idea of whats going on so that they can go and start poling the aeroplanes.
But don't be to hopeful in expecting the models to acurately predict more complex situations. You may even find that the pure behaviour of the system is the exact opposite of what you have been told, because secondary effects may be predominant.
The case in point:
You can say with absolute certainty (I'm not going to prove it here, because its lengthy) that when the line of action of thrust is above the line of action of drag a nose down tendancy will result (and vice versa), but you can only say this for sure when in the cruise with T = D).
It does not hold true when T <> D.
Lets extend my glider example. Having opened the airbrake on the top of the pole, you push forward on the control column to maintain the attitude. I.E. you have provided a pitching moment to counteract the pitching moment caused by the airbrake.
Now attach a JATO pod halfway up the pole. Its below the drag line (because the airbrake is providing the majority of the drag). Its still above the C of G. Fire the JATO pod. Aircraft will pitch nose down because the thrust is applied above the C of G. The fact that the drag line is above it is irrelevant.
CPB