Originally Posted by
yanrair
Well spotted. You trim nose down during acceleration because centre of lift moves forward. From memory.
nope, Re-read the quote, and get a white board and draw out the forces to see the couples.
read my follow up comment, any further questions feel free to PM.
CsubP moves forward on a section as AoA increases, however it is dependent on section camber, and also to an extent the section LE radius. A section that has a laminar separation bubble will have a slight wobble in moments for lift and pitching but not a big deal. Mach tuck occurs due to reduction in lift inboard, on a swept section this loads the tips giving a pitching moment, as well as a movement rearwards of Cp and the resultant pitching moment from that change, Cp follows normal shock movement on the section. As the wing section and T/C result in shock formation being more pronounced inboard than outboard, the total result is an increase in pitching at higher Mach. At speeds below Mcrit, lift and section a-slope, and inflow to the tail result in pitch moments leading to nose down trim being needed as speed rises above trim speed. Shock formation is not relevant to JT or ET’s events, they had normal pitching moments going on other than the MCAS establishing a major trim error for desired speed.
The annoyance of MCAS is that it has authority to schedule trim for a period of more than 3 times the miss-trim case considered for certification, per 25.255, which seems like a lousy concept.