Originally Posted by
STS-134
I never said it had anything to do with engine thrust, but about the differences in how engine thrust affects pitch compared to how it affected it in the 737NG and previous models.
The pitching moment from engine thrust depends on how far the engines are below, or above, the center-of-gravity (CG).
A tail-mounted engine's thrust-line is very close to the CG. This is not because the engines are on the tail, but because the height of the engines is near the height of the CG. Such an airplane (Entire DC9 line, CRJs, E145s, most corporate jets, etc.) have little, if any, pitching-moment from thrust changes.
The larger diameter engines on the MAX were too low to the ground. In order to raise them higher they had to be moved forward, away from the wing. Raising the engines higher reduced the distance between the engine's thrust line and the aircraft's CG so thrust-induced pitching moments are REDUCED, not increased. This improves the handling of the MAX aircraft through most of its flight envelope.
The issue that MCAS was designed to address is that the larger nacelles, mounted farther forward, created more lift, and were further forward from the CG, so this lifting-moment was greater and created increasing amounts of nose-up tendency as the angle-of-attack (AoA) increased.
The MAX 10 will not need MCAS because of the longer fuselage making the nose-up aerodynamic pitching moments from the nacelles relatively less.
But the entire 737 MAX would be airworthy without MCAS so long as the pilots were trained properly, no?
No.
Certification rules require that, as the AoA increases, the control forces necessary to hold the increasing AoA increase proportionally. In other words, as the AoA increases, the nose gets "heavier". The greater nose-up pitching moment from the larger, farther-forward, nacelles negated the natural increase in pitch forces. MCAS increases those pitch forces by introducing a nose-down bias through the application of stabilizer trim. Without this, the aircraft would not meet certification requirements.