Originally Posted by
OldnGrounded
Well . . . but what, exactly, is the aerodynamic problem? We've been told that it's a pitch-up tendency in some corners of the envelope and that MCAS was required to create linear stick forces to help keep the airplane out of those corners. OK, but using the H-stab for that job is pretty radical. Also, as Grebe and others have pointed out, stab trimming is inherently slower than the elevator movement controlled by the yoke, so it seems like an odd choice for that reason.
I think most of us are still trying to figure out how any of this makes good engineering sense -- even if the system design hadn't been so truly awful.
The stab only has to outpace the pitch rate of the entire aircraft and only slightly to linearize the control response to offset the contribution of the engine nacelles. It doesn't pitch up; it just doesn't fight elevator inputs quite as hard as the linear model requires. It's normal function would never keep anyone out of any corners. If the pilot wants to pull all the way into a stall MCAS does not prevent that; it certainly wasn't originally designed to even detect a stall.
It looks to me that the aerodynamic problem is that the engine nacelles produce non-linear lift at high AoA. The only ways to eliminate that would either to make the nacelles produce lift that is linear with AoA, a big trick for a round can or to put the Cp of the engine nacelle coincident with the Cp of the remainder of the plane - the latter is quite the trick as the Cp of the plane moves with AoA.
MCAS makes a lot of sense. It's inexpensive (should have been anyway) and does not generate drag and fuel costs under normal flight the way that adding a larger stabilizer and larger elevators and adding structure to the fuselage for their higher loads and heavier gear to handle the heavier load and bigger fuel tanks to make the range which add more weight for fuel and more weight for the tanks and more weight in the wings to carry that weight and more weight in the landing gear - and so on.
The AoA system that feeds it lies and also lies to the stall warning and stick shaker does not make sense.