Originally Posted by
abdunbar
Does anyone know? If you have a 737 Max , airborne, flaps up, really high speed, full nose down trim, electric stabilizer trimming motors shut off, how much back pressure does it take to raise the nose with the elevator? While applying this large force to the elevator, how much force is required to turn the stabilizer trim hand wheel? How many turns of the hand wheel per degree of stabilizer trim. In other words, is it possible to get into a position in which it is impossible to recover, not enough elevator authority due to force required and or inability to retrim the stabilizer manually due to it binding from a combination of speed and forces being applied through the elevator.
I cannot speak to the potential for binding of the horizontal stabilizer control (either via electric motor or via manual turn of mechanical trim wheel) but I can shed some light on the first question. Where the stabilizer needs to be for trim is a function of all of the contributors to pitching moments that must be balanced. CG is the biggest factor and will have the most influence on where the stabilizer sits for trim(i.e., no column push or pull required for steady flight). How "out of trim" the airplane is with the stabilizer moved all the way to its airplane nose down limit depends on where you start (i.e., what position represents "in trim"). If the CG is at its forward limit, trim will involve a fair amount of airplane nose up stabilizer so there will be a larger increment of stabilizer travel from there to the nose down limit. If the CG is at is aft limit, trim will place the stabilizer much closer to the airplane nose down limit so the increment from there to full nose down will be smaller.
Also with regard to your reference to "very high speed" specifics make a difference as Mach effects come into play along with dynamic pressure. At cruise speeds and faster elevator travel is limited by actuator force capability. The faster you go the less the elevator can be deflected and thus the more critical it is to have the horizontal stabilizer near its trim position.