Originally Posted by
Lord Farringdon
They were not just forward but also up. That's significant becaue the line of thrust against drag becomes way more active in pitch up, hence the need for MCAS to maintain the stability in longitudinal pitch in conditions of flight that required it.
No, actually, raising the thrustline of underslung engines would
decrease the tendency of a power application to cause a pitch-up.
The reason for the pitch instability is the aerodynamic effects of the engine nacelles. At low (cruise) AoA the nacelles produce little or no lift, by design. At high AoA, they begin to produce lift. Because the MAX nacelles are mounted further forward, the lift produced by the nacelles at high AoA is further ahead of the wing, thus providing a pitch up tenancy. Effectively this shifts the center of lift of the wing-engine assembly further forward, whcih reduces pitch stability at high AoA, and reduce control force gradient for pitch-up control inputs.