Originally Posted by
tdracer
One of the linked Seattle Times articles stated that they initially tried to solve the force problem with aerodynamic tricks - vortex generators, etc. Nothing did much good. Then someone came up with the basic MCAS idea and it immediately solved the force problem.
If they couldn't figure out how to fix it aerodynamically before, I wouldn't be optimistic they can fix it aerodynamically now.
TD,
maybe... but the basic pitch issue is relatively quantifiable, at least a limit case can be made for it. We know the problem is a clean config issue, that strongly suggests that the Cm of the flaps is sufficient to remove the problem from the board. It is also an aft CG case issue...
Adding a tab or wedge outboard of the aileron will increase the section CLmax of that area by about 40% (its a size thing... ) It does bring down the AOA slightly where CLmax occurs, but at the same point where the normal stall would occur, there is a greater CL than would otherwise be the case. Adding VGs at the front of the wing, (and these are for high AOA so they would be between 3-5%c from the LE) increases the AOA slope, and much higher CLmax, as well as higher AOA before the break can occur. VG's for the issue if used need to be on the slat TE itself. The local Cm from a tab or wedge is considerable, but the overall effect due to the sweep effect and increased loading of the wing tips is much greater, about a 3%MAC shift, which is about what you get with the flaps... more or less. Boeings usual VG stands proud at about 20mm, and that is too large for a VG at the TE of the slat, for effectiveness and drag, they need to be around 6-8mm in height if located around the 3-5%C location, which makes them effective at high AOA and less so at any other time.
Adding a reflex to the B737 has been done by STC previously. Was also done to other aircraft. That can be accomplished with a piece of foam rubber along the upper edge of the main flap aft element, which constitutes the wing TE of the yehudi section. That would give a modest local CL and Cm reduction in that area, but shifts Cp outboard and aft, and give an overall increase in Cm. Its handy to have swept wings. It is possible that can result in some buffet at very low AOAs at high Mach, but it would otherwise reduce buffet at high AOA.
Removal of the strakes is viable, but comes with a TO performance penalty, the other options aerodynamically all alter the span wise lift distribution, but that can be tailored to occur at only high AOA which equates to low speeds, and is otherwise straightforward to apply.
MCAS was a simple and relatively easy fix with no expected downside, whereas all aero fixes alter loading, including torsion and bending, and some are adverse to parts of the performance envelope, but some are beneficial at all points within the envelope. Altering span wise loading is not for the faint hearted structurally, but having looked at that area previously, there is not a big impact from doing that for this problem.
The manual trim, get a bigger wheel, or add another stby trim motor, there should have been one from day one, the design has always been less than stellar. The real problem for this was the unknown unknowns that came to light with MCAS. Pitch up issues can be dealt with, but absolutely need training to be safe, pitch down is just nasty. Get rid of the MCAS by aero mods, and train for the pitch up, and apologise for the missing link in the training of the crews from the 60's to 20's... what is old is new again.
The pitch issue is not that big a problem the fact that MCAS didn't function with flaps deployed quantifies that... and any of the aero mods achieves that level of Cp shift that will offset the Cm sufficient to normalise the control force.
A point of clarification, the aero issue is not that big a deal, there are a few phone numbers that TBC could call to get that sorted, but the MCAS was and is a major deal, and it introduced unintended consequences that are severe. The manual trim issue was a lousy design that was adequate in the past but thrust into prominence with MCAS. It is fixable or trainable, MCAS is fixable or replaceable. After the shock of the events, the confidence of the design system is in question, and that doesn't make it easier to get the work done that is necessary.
I hope TBC does get their act together soon. Their products are competent usually, but the corporation itself needs some self reflection.