Originally Posted by
safetypee
fdr #44,
Interesting views, no disagreement.
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This particular deficiency is going to be more than a minor matter …’
It might be premature to discuss specific solutions for a deficiency which as yet is not clearly understood (at least publicly).
Elevator effectiveness, yes; but also in normal operation consider the combined horizontal tail surfaces - trim drag. What do you imply with TE tabs; corrective effect must overcome the failed trim condition, but not detract from normal operation.
Alternative thoughts could question why it was necessary to increase the tail area, yet retain the same size elevator. More trim range required whilst the pitch control appeared to be adequate.
The obvious longer, heavier, cg, arguments apply, but pitching moment with varying thrust levels could add another dimension.
Another question is why the trim range was chosen (horiz stab angles), is this relatively large, what are the limiting aerodynamic conditions; high, low speed, configuration, cg, thrust.
Have these changed with the evolving variants.
Background ref;
https://www.satcom.guru/2019/04/stab...and-range.html
Increasing the span of the stab increases the tail volume for the stab system. It also increases the slope slightly of the CL/AOA curve for the stab and the elevator as a system. It does however as you have intimated, reduce the effective balance of stabiliser to elevator authority, which could be a problem. The increased slope may be a factor that is being observed in the creep of the trim in ET302, and if so, that will need some sober thought to resolve. I would think that a solution for a possible authority mis match between the stab and the elevator is resolved in the first instance with a T tab on the TE of the elevator, which markedly increases the control authority. That can also be achieved elegantly by a thicker TE section, much as is seen on some Airbus aircraft and others.The addition of wedges would achieve the same outcome. Any addition at the TE of a flight control comes with it's very own aeroelastic issues. Altering the elevator authority has considerable knock on effects in certification, but is doable. Another method of increasing authority is low profile VGs in the cove of the stab-elevator, that become exposed on deflection of the elevator. That sort of device only makes a difference when the control deflection has exceeded around 5-8 degrees of deflection, below that there is not much separation effect on the surface. Blowing is a viable solution as well and has very large authority change possible, but it would be a reliability headache for certification.
I think any potential problem in this area is able to be viewed from the other end, the stab manual movement needs to be reworked to permit manual control in all cases, or the industry has to dust off the old training information, and make sure that the crews are fully trained to deal with stabilisers that are defeated by air loads. Defeated, not immobile, as ET appears to show that the trim was creeping in the wrong direction all by itself, and that is not a happy place to be.
Alternatively, the industry will shrug it's shoulders, and look at the probability of the event and just suggest that the passengers be careful in buying tickets that are unlucky.
The article on the development of MCAS is depressing reading. Incremental change is usually reasonably safe, but only until trigger points are crossed. The problem is that the risk resulting from further change from known processes is not linear.