Re:My Letter to the Editor of New York Times Magazine
by ncmncm on Wednesday October 16, 2019 @04:11AM
(#21268137)
Yes: the distinction between pitching force from engine thrust, and pitching force from engine nacelle aerodynamic lift, is essential and, here, frequently elided.
MAX has more powerful engines with center of thrust lower than in NG, and they are both physically bigger and mounted such as to exaggerate an already increased aerodynamic pitching force.
The distinction is frequently neglected in discussion here. Both effects contribute to MAX's different handling characteristics, but differently in different flight regimes.
The result is that it is tricky to make a MAX fly like an NG, particularly while trying to conceal the system used to do it.
Minor side note: More power is supposed to increase altitude. (An aircraft is trimmed for a given airspeed; adding power injects more energy into the system. Then, it doesn't go faster, it goes up. To go faster, lower the nose. To go faster without losing altitude, you lower the pitch and increase power.)
For aircraft with engines below the center of mass, thrust tries to rotate the aircraft upwards around the center of mass. In un-accelerated flight condition, this is counteracted by other forces. But adding thrust increases the force and causes the aircraft to pitch up to a new angle based on the new thrust. This is aside from the more power = more altitude effect.
A second-order effect is that the engine nacelles act aerodynamically. (They're wings, albeit poor ones.) When pitched up, they generate more lift. Because the 737 MAX's nacelles are in front of the wing, their increased lift causes another pitching up force.
None of these forces affect stability; the aircraft will arrive in another stable state shortly after you change the power. However, they can lead to other effects like the control force changes you clearly describe in other comments.
Edit: Source: I slept in a Hilton last night.