CG vs MAC

[Crossunder]

For a jet transport, like the B737; where is the CG usually located in relation to MAC and Aerodynamic Centre?
Does the tail produce negative- or positive lift? Or both, depending on AC movement?

[18-Wheeler]

The CoG will be in front of the centre of pressure, just like we were taught in basic aerodynamics.
All Boeing airliners (the fly by fencing wires ones at least) generate nose-up forces as they go faster, so they must have a conventional arrangement of forces.

[Mad (Flt) Scientist]

As usual, it's all a bit more complex than that....

Firstly, the Aerodynmic Centre (AC) is usually used to refer to the tail-off aerodynamics (for a conventional layout). The critical concern for stability is that the c.g. should remain forward of both the Neutral Point and the Manoeuvre Point, both of which are determined tail-on. Since the tail is (usually) pretty stabilising and it is not desirable to have excessive stability, it is common for the progression, moving aft, to be AC - CG - MP - NP, certainly at aft cg conditions.

The centre of pressure is not directly related to stability characteristics - although the location of the CP does bear some input to the trim requirements, the AC is of far more significance.

(The CP is the point through which all lift may be considered to act; the AC is the point about which the pitching moment does not vary with change of angle of attack. The two are not the same, except for trivial and impractical cases)

18-Ws statement refers to static stability (speed stability) and is a consequence of the c.g. being forward of the Neutral Point. Even then, making such a deduction from control column forces depends on a knowledge of the control system, since it's a regulatory requirement there may be artificial stability added. Any aircraft with any form of Mach trim, for example, is using that to restore stability lost as the NP moves forward with Mach.

[Crossunder]

I'm using the Oxford/Jepp PoF learning material. They discuss stability using AC of course, and the tail is also considered, showing a positive resultant force. I'm confused about this, as I've always thought the tail to be producing a downforce?