Originally Posted by
MechEngr
An airplane in flight is essentially a teeter-totter balanced on the Center of Pressure (CP) for the entire airplane and is considered to be the point where the lift acts. One element of this is that lift from positive AoA produces a nose-down pitch torque. To counter that, the horizontal stabilizer produces a nose up pitch torque by pushing down on the aft end of the fuselage. When these are balanced everyone is happy, or at least the AoA isn't changing. Of note is that the stabilizer functions as a wing that produces lift opposite to the lift of the wing and has its own local AoA.
If one looks at all possible stable AoAs one would like to see a linear relation between the AoA of the wing and the stabilizer trim position required to balance it. But the Max seems to have another player on the teeter-totter and that is the nacelle of the engine which starts producing noticeable nose up pitch torque at high AoA. So the stab trim position is no longer quite as linear as it was. Since the elevator also affects the pitch torque, that's where the effect could be noticed by the pilot as they move the controls without adjusting the trim. The function of MCAS is to make it so the pilot doesn't experience this new player.
Because it's just to offset the new player and depends on AoA and airspeed (because the amount of lift the stabilizer produces depends on those things to generate nose-up torque) it really isn't moving much or fast; it just has to be fast enough to keep up with AoA changes to the aircraft and to speed changes, neither of which ought to be particularly high. It gets more pronounced at low speeds because not only does the requisite high AoA needed to provide lift at low speeds increase the effect from the engine nacelles, the lower speed also means the stabilizer has less dynamic pressure to work with. This is no different than, say, rudder authority at low speed; the rudder has to move a lot farther to get the same effect at low speed than high speed.
AFAIK that's the intended MCAS function. To meet a linearity requirement for pilot controls by rebalancing a larger input from the engine nacelles than was existent, but could be ignored, on earlier models. It's not fast enough for a negative stability situation, so that's not it.
Nicely explained, MechEngr, but that's the trim stability of a glider... Now add two large bonks with thrust/drag not vectored through cg and in line of flight, and you get another big factor influencing pitch moments. As we do not know in what flight regime(s) the unacceptable pitch up tendency occurs, the power (or idle drag) of the engines add to influencing factors you mentionned. What I cannot figure out - maybe some of you can - is the fact that the unwanted non-linearity does not occur with flaps/slats slightly extended.