Originally Posted by
MemberBerry
it seems what's preventing the use of the trim wheels when severely out of trim is somehow friction related.
Clamp a hex nut tightly in a vice.
Thread a six inch long bolt through the nut and keep turning. Now apply a side load to the bolt and repeat the exercise. The more side pressure you apply the more torque is needed to turn the bolt. This is because the side load is relieving pressure on one side of the threads at the top and opposite bottom side but is increasing it on the other two sides. If you keep applying the side load eventually you won’t be able to turn the bolt at all. The bolt is the stabiliser screw jack. The side load is the aerodynamic forces transferred from surface of the stabiliser to the jack screw. The point where it no longer turns is the point where the clutch disengages the electric trim motor or the trim wheel no longer turns without using the elevator to momentarily unload the aerodynamic pressure on the screw jack.
As I said in an earlier post, a redundant electric servo to assist/replace the manual trim wheel will not only need to be able to apply more torque than the existing electric trim motor but the clutch and screw jack themselves may need to be beefed up to cope with the increased stresses on those parts. Adding an electric motor and motor control to each of the 7,500 NGs is not something that I believe Boeing will relish. Redesigning and retrofitting an HS screw jack is something that they will relish even less.
