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.
MechEngr,
You have very cogently made the case -by reasoning- for the need for something like MCAS; of course if a new force is introduced it would be desirable to offset it if one wishes to use an extant control model.
However you haven't really explored the issues which lead to this need, in other words what piloting a MAX without MCAS would look like.
Also it is not at all obvious -in fact extremely improbable- that a given control model can be perfectly grafted on a different airframe by means of a simple mechanical accessory. Of course one could make the case that the mechanical aid creates a similar linearised behavior around equilibrium, bu it would still be required to demonstrate that the relinearised zones share the same volumes of state space, and that these volumes are the only regimes which are of practical importance during flight. Hence it would be very difficult to demonstrate by maths alone, without extensive testing that the MAX shares a control model with the NG and thus can share its certification. There is no evidence to date of the results of such testing.
My understanding that under current regulatory règimes for civilian aircraft, acceptance of the airframe design WITH MCAS is conditioned on the behavior WITHOUT MCAS. Namely if without MCAS the airframe cannot be flown easily ie. can tend to stall or dive or do other wierd things, then the design would be classed as necessitating active control (MCAS) and couldn't be certified. Which is why Boeing so prudently talks of "linearizing" stick feel.
I continue to believe that in a rush to acquire a grandfathered certification, Boeing was overly economical with the truth in its descriptions of the 737 dynamics, which explains Boeing's reluctance to reveal the exact design goals of MCAS.
Most of the pilot readers here would appear to have no issue with MCAS certification if it were "just" a stick-pusher, but if it is a more complex entity as now seems likely, then a more careful certification process seems necessary.
Edmund