Originally Posted by
Smythe
When in doubt , add vortex
tabs
it often appears to be the solution, but not so much in this case.
VG's generally control separation, or introduce shock forming structures into the boundary layer.
The general problem with the Max is change in the Cp on the wing around the engines at varying AOA, that is the result of the lift being generated by the nacelle. There may be some interference effects from the nacelle at low AOA that add to the relative shift and that would be able to be altered by VGs, but otherwise not going to do much there. A possible solution would be to reduce the strake effectiveness on the nacelle, but that comes with total lift penalties over that gained at present for modest AOA values.
The Stab itself is not going to benefit from VG's.... The elevators may, for low profile VGs that are well forward on the elevator surface, which would improve elevator authority at high deflections, but then that also results in a load change that may affect the existing stick force per G which is the symptom that is being dealt with. Increasing elevator authority can improve the controllability against a jammed stabiliser, but that does not benefit directly freeing up the stabiliser itself when it has become immovable due to aero loads when using manual trim. Trailing edge tabs, as T, L's or ramps increase the elevator authority (thickening the TE), but that is the same outcome, improved control margin over the jam, but no change to the immovable stab case directly. Presumably sufficient elevator authority would permit getting the plane to a condition where the crew can then unload the control system sufficiently to permit manual trim in the period that the aircraft is following the incorrect trim. Maybe.
VGs' are not certainly the solution in this case, there is a fair bit of CFD needed to sort out the underlying issue, and then a whole bunch of playing with the subsequent control forces to ascertain the real extent and solution of the problem, if, and only if it is considered that having a stab that cannot be controlled due to excessive airloads in manual mode is a bad thing. I still think it is, in the absence of training and analysis of the operational envelope safety that comes along for the ride of this phenomena.
The kludge is that the operators should be aware of the issue, and should ensure that they never get that far out of trim, but sods law is that crews won't stay in that area at all times. As it is an underlying characteristic of the design of the B737 particularly, where a manual backup exists, then it is a product related issue, but, I hazard to guess that the fundamental issue of a locked stab from airloads floats nearby all trim systems, and the margin in actuator force required would bear looking at for other types as well to ensure that the emperor has clothes.