[caldrail]

Ok, I guess I might have come across as a bit harsh. But I must say that you have been making some obvious annotations simply for the sake of arguing.

No, I've been making it clear that your explanation is poorly defined, and provided a better one. Positive and negative lift are only present if both components are present, otherwise all you have is a measure of positive and negative air air pressure, which is the actual case.

Most light aircraft have one main spar (there are obviously smaller spars in the wing as well). When an aircraft has more than one "main spar" (if you still want to call it that), then it will obviously not have multiple centers of gravity; the center of gravity will be between them.

No, that isn't obvious. It depends on the design. Furthermore, as you claim to be a practising pilot of some experience, you will know that the position of the centre of gravity is not fixed. It varies according to the load carried on any particular flight, and one of the essential responsibilities of an aircraftas captain (as defined by CAA regulations) is to ensure the CoG is within safe limits for that flight. It isn't physically possible to keep the main spar and the CoG in the same place.

I think it is pretty obvious and truly didn't think it even needed mentioning, but apparently you didn’t consider it to be obvious. When discussing these basic aerodynamic principles it is also absolutely absurd to hammer on about every little variation in design just in order to be argumentative. The center of gravity also shifts slightly as cargo and passengers are loaded on the aircraft, but let’s not fret about it shifting a few cm to this side or that side just for the sake of arguing. And let’s not get into every slight center of gravity variation for each aircraft on the planet, please.

You're damning yourself as a careless pilot and for that reason please stay the heck out of my airspce. You're a danger to yourself and everyone else.

The reason a tailplane is not curved at the top and flat underneath, is because the function of the tailplane and elevator combination is to create either positive or negative lift at the tail in order to control pitch.

No, the function of the tailplane is to maintain stability. The elevator is there to control pitch. It's no good trying to teach me to suck eggs.

However, you still don't seem to understand that when the elevator is deflected it causes both the angle of attack and the camber of the combined tailplane and elevator surface to produce either a lift or down force,

I understand it perfectly well. There is increased air pressure on the deflected side plus the vector effect. The idea that increased camber causes this positive and negative lift ignores the fact that such aerofoil sections are inefficient and derive most of their lifting effect from air impacting one side, not from the camber, which does not in itself functionally change the speed of airflow across it the convex surface and thus does not create reduced air pressure on that side, thus does not produce 'lift'.

How do you think flaps work?

Irrelevant to the point. Especially since the trim changes of lowering flaps are usually in opposition to the deflection of the surface.

Ps. Before you PM me, please read up a bit on how the elevator changes lift at the tailplane by altering its camber.

I have absolutely no intention of PM'ing you whatsoever. Further, I don't consider further reading is necessary - I was after all responsible for an aircraft design thirty years ago. Also I suggest you take a course of further education in physics, in which you'll discover that definitions of properties are very important and that the sloppy explanations you insist on are inaccurate.