Trim tab jack screw

[davidpa28140]

Long time listener first time caller.. Recently, I purchased a really nice 1946 85 hp BC12 that's been fun to fly and interesting to maintain. (particularly that time one of the tail wheel springs came off on landing, but that's another story lol)

My airplane had an extensive rebuild before my purchase and is generally in great shape but it came to me with an inop trim tab indicator. One rainy day I decided to take it upon myself to investigate why the trim indicator wasn't working and cranked the trip to it's stop not knowing this would jam the trim tab if the elevator wasn't in the neutral position. After reading several helpful post here by Robert Lees and many others I discovered my folly.
Due to the kind folks writing about their experiences I discovered that by loosening the tension on the trim cable that one could reset the trim tab to neutral by turning the pulley/jackscrew shaft at the rear.

I'm wondering if anyone could help a simpleton understand what is happening in that jack screw when this jam happens? I'm assuming the screw has reached its end? in this case its bottomed out and I'm assuming that if one where to have turned the trim wheel in the opposite direction the jack screw would have come out of the threaded portion of its carrier? Is that correct?

Lastly, why does this only happen when the horizontal stabilizer is out of the neutral position?? I understand that the travel is limited by the spring tensioner at the rear and the limiting bolt at the front of the trim wire but it makes me a little concerned for making large adjustments while in flight, like for slow flight/stall training.

Many thanks for your thoughts, this board is a great resource.

[Scott]

The bottom line is the trim should never jamb... in any position. If everything is installed and rigged correctly, including elevator travel, and parts are in good condition the trim will work in any position.

Here are some points that need to be concidered:

The effective cable length changes as the elevator is moved. This is because the jack screw is not on the centerline of the elevatore hinges.

Because if the above, having the correct spring in the correct position is critical.

The cable ends at the spring (behind the headliner) travel to the fairleads welded to the fuselage tubes. This is what creates the travel limits for the trim system and will prevent over extension or retraction of the jackscrew, and also provide adjustment of the tab travel. So, the more the spring is pulled open, the lower the total travel of the system from full up to full down.

To adjust neutral, the cable needs to be moved relative to the jackscrew pulley. In practice this means putting the crank and cable in close to neutral, then having someone hold the spring tension off the cable, while you rotate the jackscrew and pulley to the neutral position (with elevator neutral).

The pulleys are supposed to have 'V' grooves to pinch the cable. When these grooves get worn to a 'U' shape, the cable will slip and trim rigging will be lost.

Check the copper sleeves that the cable runs through, in particular at the stab rear attach location. The cable can wear through the copper and create a jamb point.

the lubrication used in the jackscrew can dry out causing viscous resistance. Use a synthetic lub of some kind to ensure this doesn't happen.

S

[Robert Lees]

Also some kind-thinking engineers may think the trim cable should be lubricated around the pulleys; this is incorrect.
They need to be taught the error of their ways with a small bat around the head behind the hangar where no-one can hear the whelps.

[Hank Jarrett]

Using Violin Rosin on the cable where it runs on the pulley will really help it NOT run but rotate the pulley. Also when the trim handle is turned to full up or down STOP TURNING IT!!!! If you need more trim you need to re-rig! Pushing the handle further will cause the screw rider at the front pulley to either come off the threads or jam and bent the arm at the bottom of the threads. DON'T FORCE IT!!! FIX it.

Hank