henra:
With a little "back of the napkin" sketching, I arrive at this from
your post number 120 of this thread.
With the THS not stalled, though at an angle of attack significantly displaced from the usual "nearer to horizontal" (horizontal reference being THS airfoil chord line), the effective lift force, and thus corrective pitching moment, (roughly perpendicular to longitudinal axis of the aircraft, and then acting along the fulcrum to tip the aircraft's nose back down) is reduced roughly as the sine of the angle (AoA). The larger the AoA, from a nominal airflow parallel to chord line, or AoA absolute value of zero, the greater the lost force (vector subtraction). I din't factor in camber, as I both don't know it, and expect that it would wash out. This presumes no stall. Once a THS is stalled, it's a different problem.
I sketched it out this way based on expected control forces, controllability, and control response from an input to an airfoil movement. Each knot of reduced airspeed robs the lifting force of magnitude proportional to the 1/2*k*v^2 relationships in the lift equation.
What does this do to the aircraft's response ...
even with the THS not being stalled at that unusual angle of attack, it would take longer for both the elevator to influence THS, and for the lift acting on the THS to move the tail up (and thus the nose down) since the lift force isn't pushing as hard for a given flight control pitch command.
At some point at very low airspeed, depending upon CG, the airspeed ( airflow? ) may be so low that there isn't sufficient pitching moment (due to the combination of low speed influencing lift reduction, low air density, and sine of the angle vector subtraction) to move the nose at all.
Why sine of the angle?
As I sketched out the FBD on the THS, my vector component NOT adding lift increased with increased AoA, lift vector component decreasing.
Does that make sense to you?
EDITED since it was really confusing to me, and I wrote it.
Second EDIT: the more I think of this, the more it seems to me that an AoA indicator might be handy at high altitude. The AoA margins are reduced up there, a point which strikes home as I review HazelNuts39's graph on Mach Number and AoA and stall warning. If you lose speed at high altitude, that thin air really puts you at a disadvantage in terms of what your flight controls can do for you.
