Would I be correct in saying that whatever force is required by the tailplane for level trimmed flight is just exaggerated depending on whether airspeed increases or decreases?
The following bit I find a bit confusing though:
Now, suppose that the aircraft is loaded so that its CG is further rearward, coinciding with the point from which the lift force acts, as in Figure 121. In this case, the original tail down-force would now pull the tail down, thus pitching the nose up.
To prevent this, the pilot would have to use his controls to reduce the tail down-force to zero, thus restoring the equilibrium. But now, if a gust upsets the aircraft, the tailplane will have no stabilising effect, because, regardless of the aircraft's speed, the tail down-force would remain zero.
If an aircraft with a tailplane applying zero TDF is displaced by a gust into, say, a pitch up attitude, wouldn't the AOA increase on the tailplane causing the tail to rise? Or is the airflow the tail experiences purely from the wings downwash?