Originally Posted by
david340r
I puzzled over this strange two way game of rock/paper/scissors some weeks ago, but I've not been able to find anything conclusive as to how it might actually work. However it's possible to imagine a fairly simple mechanical system that would allow it. For example there could be two discs, one attached to the jackscrew and one to the trim wheels, face to face and with several rollers between them and with a spring pressing them together. In one of the discs there would be grooves that were circular segments that varied in depth, so that they got deeper and then shallower and the rollers would normally sit at the deepest point, but any rotation between the discs would make the rollers run up the slope and push the discs apart, but they would soon hit the end of the groove so they couldn't roll any more and then the two discs would be forced to turn together. The moving apart of the discs would push apart a set of clutch plates/dogs disengaging the electric motor drive. I'm sure there are other ways it could be done.
If I follow that it seems possible, although a bit complicated at least to describe without pictures.
The implementation of the trim wheel overide clutch really does not matter, the point is that there is a mechanism that disconnects the stalled/runaway trim motor when the manual trim wheel is blocked from rotating or rotated against the electric trim. This allows the electric trim to have sufficient torque to move the stab under any conditions yet be overridden by the manual wheel.
Another 'proof of concept' design is detecting tension above a small amount in the manual trim cables, think spring loaded pivoting fan belt tensioner, mechanically 'unclutching' the motor with no electronics/solenoids/etc required.
Friction has very little to do with any this, to help visualize consider a 10 foot balance beam with a bearing at the middle with a 100 pound weight at one end, with a perfect bearing it will take 100 pounds of force to push the other end down, with a 10 pound weight it will take 10 etc.
Even a badly damaged bearing will only increase the force a small amount, the increase will be higher with higher loads but as a percentage will likely be the same or even less.
The 100 pound weight represents the aerloads on the stab.
To reduce the force required to move the 100lb weight shift the pivot closer to the weight, less force but more travel is now needed.