Originally Posted by
safetypee
Infrequent 789,
You appear to misunderstand the effects of 'g' and AoA; or is it memory
Oh it may well be memory, my aero engineering was a long time ago, my career ended up going in a different direction. I am pretty sure however that there was a (minimum) g threshold for MCAS to activate in the first version, which was later removed to allow MCAS to address the "low-speed high-AOA" problem area discovered in flight tests.
The need to change the magnitude of MCAS trim would more likely be an effect of Mach - difficult to replicate in a wind tunnel.
Although described as a trim increase, this could have been a larger value, or an earlier trigger point with an extended duration or change of trim speed.
Magnitude of trim needed is definitely going to be Mach-dependent, but again there was a change to this (probably to data in a lookup table) as they needed MCAS at lower speeds. My understanding is that it was a trim authority increase by increasing the trim run time, and trim speed didn't change (there are only two speeds for auto and I'm pretty sure it was already using the faster one - that is a hardware change and I don't think they made an MCAS
hardware change post-flight-test).
The aerodynamic intrigue with MCAS is why AoA was required at all. Existing technologies could manage low speed situations ( speed trim ), and Mach trim at higher speed. The Max modifications appear to be similar to the military 'Leading Edge Extension' which involves AoA and vortex management, but implemented with full FBY technology.
I think the lift from the engine nacelles is the problem, with pitching moment even worse because the engines are further forward. That lift is probably non-linear with AOA. Not sure why the effect wasn't found in wind tunnel or computer modelling given that they found it at high mach but not low - I doubt my aerodynamics was ever good enough to work that out. I'm also not sure why it isn't needed with flaps when it appears to be needed at same speed without flaps (since it seems it can kick in the minute flaps go up) - again, that may be my aerodynamics lacking.