Forward trim pulley for BC12D

[flexflyer]

Good afternoon all. I recently bought my Taylorcraft BC12D back after 6 or 7 years and am trying to get a few items sorted out. The forward trim mechanism is stuck and the trim is not working at all. After closer inspection the forward pulley appears to be shot (pulley won't turn cable but cable can be pulled) and I believe it is the 3 1/8" pulley. I've searched the internet heavily an I can't seem to find one. Anyone here have source for these?

Thanks,

Mike

[3Dreaming]

You need to tackle the whole system starting in the rear of the airplane. Anything that puts drag on the system will keep the front pulley from being able to drive the cable.

[flexflyer]

Thanks for the advice. The trim tab has an excess amount of slack also. I have not had time to look inside yet.

[Hank Jarrett]

Tom is right, even though it looks like a front pulley problem, START with the tail. First eliminate ALL play in the tab hinge. Be sure the wire or piano hinge moves freely and is properly lubricated but most of all ELIMINATE ALL of the play. Next is to make sure the push rod bolt holes and the hole in the tab are snug and not wallowed out. You can peen the edges of the holes to tighten things up a bit. Be sure the bolt isn't worn.
We will be getting to the front pulley, but you need to be sure the rest of the system is right or you are wasting your time up front.
Next is the bell crank. ALL holes snug and NO PLAY. You should be able to hold the screw jack and NOT be able to wiggle the tab! Next is the biggest problem I have seen in almost every trim system that has given problems.....the screw jack. All the holes need to be slop free, but the BIG problems here are worn out threads in the jack itself and missing spacer washers at the fuselage end. Worn threads are a KILLER because the threaded parts are hard to get (I think one of the Tribe recently made some but just hope yours are good). The easy one, and the one most often messed up is people assemble the parts at the fuselage and leave out the spacer washer! When everything is in place and the inboard pulley is on YOU SHOULD NOT BE ABLE TO MOVE THE SCREW JACK LENGTHWISE! If it moves, ADD SHIM WASHERS! Even a tiny bit of end play will show up as play in the tab.
Check the cables and the fairleads where the cables are supported in the fuselage. Don’t laugh here, but THE CABLE SHOULD NOT CROSS ITSELF! If it does it is routed wrong. This happens way more often that it should and in a couple of cases it was like that a LONG time. Make sure the cable is rigged so that the cable bugs hit the stops with full up and down trim and make sure the cable spring makes the cable snug but not so tight you can play the banjo solo from Deliverance.
Now the part you asked about. First inspect the pulley groove for wear. You CAN dress it up with a fine jewelers file. There was a picture in the Forum so if someone has it please post it. Hopefully your pulley isn’t totally trashed. It “can” be repaired with some E-poxy if the pin slot is damaged or the hole is wallowed out. Best would be a new pulley if it is really bad. Now the secret sauce to get the pulley to grip again. Violin Bow Rosin. You rub it into the groove and it REALLY makes the pulley grip the wire. Can’t believe how many things from my past life in music I have found useful as an engineer. Again, NO SLOP ANYWHERE!!
You didn’t say anything about the trim indicator but if it is run past the end points (remember getting the cable bugs rigged?) it can really mess up the trim indicator and it CAN jam the trim system. If you need it I have drawings of all the parts to build a new trim indicator.

And they say I talk too much! ;-) Let us know where you find problems. There is NOTHING you could find wrong one of us hasn’t had to fix before.

Hank

[astjp2]

I may have one or two trim screws left, I need to get the tooling made to make the pulleys for the front and back, also you need to make sure the shaft up front turns freely and the pin is still there. Also, I used a spring from a Volkswagen Baja bug exhaust to provide the correct tension on the cable, too much causes wear, too little and it will slip.
another thing, if a 1/16” drill goes to the bottom of groove, it means that the pulley is worn out, that does not mean it won’t work, it’s just not ideal. the original pulleys had almost a sharp groove with a specific taper so it maintains good traction on the cable.

[skip]

One other thing that got me a few years ago: Never adjust the trim with the controls full forward. It looses adjustment and screws things up. I imagine that lots of annuals are done with the checklist item "ensure full trim travel" and the trim tab gets cranked full up and down with the elevator down and the wheel forward. Then the trims slips out of adjustment and things bind.
I had to release the tension spring and start from the back and work forward like Hank describes to get everything back in good shape. Blocking the elevator into mid position or pulling the wheel back during the trim check is the correct procedure. The good news is that it gives an opportunity to look at everything in the trim system and ensure it is OK.
So there is a possible and maybe common way that your trim could have gotten messed up without the pulley being worn out.

Skip
N34237

[astjp2]

I have found a way to not have to not release the spring, just put a wrench on the jackscrew nut and grab the cable and pull it to center by the tensioner with the trim tab centered, pull it so the cable will go slack. The cable will just slip to where you pull it to. You also can turn the crank handle to adjust it. I have done it on the 3 different airplanes to get the trim back into adjustment. Tim

[flexflyer]

Thank you all for the advice. I'm heading to the hangar now to do some investigating.

Mike

[Ngroark391]

I just had a learning experience on the trim system myself. As it turns out all trim adjustments are to be made with the elevator in a neutral position otherwise the trim cables will bind on phenolic blocks just ahead of the trim tab jack screw. Hank Jarrett has some great tips above on where to look for issues in the trim; fwiw I am missing a few spacers and the hole itself in the trim tab is elongated allowing more play in the trim tab than a school yard at recess, still my trim system will do the job if I only adjust with a neutral elevator. The issue I ended up causing (and then needing to learn to remedy) was a locked up trim tab with full nose up.... not a fun flight but very educational!

[astjp2]

You have other problems, it should work in any position, make a new phenolic block, which I believe is causing your cable to bind

[Scott]

Yes, the purpose of the phenolic block imediatly forward of the elevator is to hlod the cable near the centreline of the elevator hinges such that as the trim pulley moves up and down through its arc with the elevator, cable length/tension remains relitively constant.

So... if this block is setup properly, trim will operate irespective of elevator position.

The cable makes a rather sharp turn through the block however so there's potential for wear and jamming. The block has a soft copper insert that improves function but also wears more quickly.
s

[Hank Jarrett]

I don't trust the block or copper/brass insert to not jam. I inspect mine carefully every annual and mine look brand new, but it will still jam if the elevator is full down. The furthest aft guide is "supposed" to keep this from happening, but doesn't always work. The good news is the plane is fully flyable with full trim, it just isn't very fun and your arms will be tired when you land.

Hank

[Ngroark391]

I don't trust the block or copper/brass insert to not jam. I inspect mine carefully every annual and mine look brand new, but it will still jam if the elevator is full down. The furthest aft guide is "supposed" to keep this from happening, but doesn't always work. The good news is the plane is fully flyable with full trim, it just isn't very fun and your arms will be tired when you land.

Hank

* arms tired and maybe a bit weak in the legs haha, but yes fully flyable (with nose up from experience). Subsequent flights have been entirely enjoyable and allow me to trim for hands off airspeed (and altitude provided the 36’ wings don’t find a thermal).

At this point I do not trust mine not to bind either and find that mine works great with the elevator between stops.