Trim tail mechanism

[RoN43220]

Hi. Couldn't find anything in the archives so ask for guidance. I tracked my trim tab play down primarily to the mechanism that's buried between the pulley in the tail and the rocker mechanism above the inspection cover. It must be a lead screw the way it converts rotary to push-pull. Before extracting that which is probably held by the castle nut on right side of the pulley and taking out the rocker mechanism so the mechanism in question can be removed I thought I'd check with the experienced.

Thanks, Ron

[RoN43220]

Re: Trim tail mechanism

And a related question - my trim cable sags for two reasons and I see no cure. One is that the spring isn't tensioned. I could either get a shorter spring or adjust the cable at one end of it or the other.

The other reason is that the weight of the spring and the distance it needs to travel causes a certain amount of uncorrectable sag. So it could oscillate but, probably better to be loose so it's low frequency and likely wouldn't cause flutter. Or is tight OK? Seems to be a designed in problem.

Thanks again, Ron

[Mark Julicher]

Re: Trim tail mechanism

Ron,
Here is a little tribal knowledge on trim tabs... specifically on the BC12 mode but applicable to others.

As you guessed, there is a jack screw with a buttress thread that is driven by the pulley and it is converting rotary to in-line motion. The screw is brass and many of them are somewhat worn. In some cases they have so much grease in them that they are difficult to turn.

The jack screw is actuating a bell-crank that makes the motion do the 90 degree turn. Sometimes the holes in the bell crank are worn. The holes can be welded and re-drilled. The amount of re-covering is not to great to do this task. You might get away with just bushing the bolt hole.

The trim tab hinges also wear. Sometimes you can increase the size of the piano wire in the trim tab hinge and eliminate a lot of play.

The cable should be reasonably tight... Meaning that it will hurt your fingers just a bit if you try to extract the cable from the front pulley. Too tight and the trim system will bind, too loose and it all slips. It should not sag very much.

If you have slippage and can't solve it, first make sure the pulley grooves are sharply V shaped. If not, you can gouge them with the tip of a knife while turning the pulley - be careful and wear eye protection. Some members use violin rosin on the cable to good effect in preventing slippage.

If the spring is worn out, Home Depot Aircraft Supply has a screen door spring I'm sure. the upper and lower stops in the trim are also adjusted with bugs in the cable and spring section.

I hope that helps some. Find a tribesman near you and look at their plane closely - you will hopefully learn a bunch.