Mr Tullamarine and Genghis have provided a great deal of info. But I must be missing something here....
Academically speaking, if an aircraft is flying level at 250 kias and is trimmed, then there is no force required on the stick to keep it level. With slight changes to the flight path due to a variable air mass, the pilot can gently push and/or pull on the stick to maintain the altitude. The pilot can fly the aircraft without a problem.
Now the thrust is reduced slightly and the aircraft decelerates to 240 kias. Now there is a pull force required by the pilot to maintain straight and level flight. This pull force will likely vary slightly due to a variable air mass, but likely to remain a pull force (vice a push). Again, the pilot can fly the aircraft without a problem, although there is a trim system to help him which he would normally use.
So if I was flying this aircraft manually, and it had a zero stick force gradient, then there would be no trim required as the speed was reduced from 250 to 240. But it would surely be easily controllable.
So why is there a need for a (minimum?) positive stick force gradient of XXX lbs per knot?
VinRouge,
I'm assuming you mean A320/330 etc. Do you really think the aft CG condition allowable by these aircraft would be "unflyable" with conventional hydraulic/mechanical rods control? I can see that the the aircraft may not pass present certification criteria, but you said "unflyable".
F16 FBW not considered here, of course.