Trim Tab Problems

[jdlowrey]

Hi all:

I am currently, and have for some time, been experiencing difficulties with my trim tab adjustment in my 1940 BC-65. When turning the handle it seems like it's slipping as if it were stripped and I do not get any movement from the trim. With the trim currently "stuck" in the nose up position I am hoping to find a way to get it fixed.

Wondering if this is a common problem and or what solutions maybe available.

Thanks.

Jeff

[Robert Lees]

Re: Trim Tab Problems

Jeff,
a quick "search" brings up a thread called "Trim Common Problems"

here

It is a common problem, usually caused by trimming without the yoke in neutral, or grease/oil on the cable.

Read through the above thread and see if that answers your problem.

Rob

[jgerard]

Re: Trim Tab Problems

1940 - Does that mean you have the little flippers under the tail?

Jason
N43643

[henryawehrli]

Re: Trim Tab Problems

There are a lot of problems in the basic design of the trim tab design. The indicator can be out of sync with the trim tab or may not work at all. However, this should not prevent you from adjusting the trim tab by turning the crank inside the cockpit. You may have grease or some other substance on the micarta pulley which drives the cable that turns trim tab. Or you may have something wrong in the tail that is causing excessive friction that forces the cable to slip. How is the tension in the cable? I don't know the tension specs but it is damn tight - as when I have had problems, I check what is happening by removing the cable from around the micarta drive wheel and it takes a lot of pull! But this is the only way I know of identifying the real problem (remove the cable from the drive pulley).

If after removing the cable everything else works well, then the problem may be in the micarta pulley. It may have been slipping for a long time and wore a grove in the micarta. I had this happen to me when the the micarta pulley was badly worn. I could turn the crank and the cable would just slip. Everything else was working properly.

I have a lot of experience with elevators (the kind you ride in buildings - made by Westinghouse) and most of them are cable driven. The cables may be 1" in diameter but the theory is the same. You NEVER want the shape of the groove to match the diameter of the cable! To guarantee the cable is not slipping on the pulley, you want the groove in the pulley to be shaped like a "V" so that the cable is pinched by the "V" and this provides much more traction between the pulley and the cable. The cable should NEVER be allowed to touch the bottom of the "V"!!!!!

On my BC12-D I took a VERY fine pitch hacksaw blade (the more narrow the better) and held it firmly against the center of the groove in the drive pulley and then turned the crank about 20 turns (until I could see a lot of micarta particals). The direction of rotation does not matter but try to hold the cutting edge very steady. Clean the pulley with alcohol or some other cleaner/degreaser when finished. When I reinstalled the trim cable, it worked perfectly!

EMAIL me if you have any questions

Hank

[email protected]

[Robert Lees]

Re: Trim Tab Problems

One of the other causes for slipping trim is inadequate tension, often caused by a weak tensioning spring. (the long one on the trim cable in the fuselage).

I had a batch of new ones made by a well-known Milwaukee spring manufacturer. They are 8" long, 20 coils per inch, stainless steel, shot-peened. Photo of the spring here

If anyone would like to assist me recoup some of my development costs, then send me a Private Message or email.

Rob

[Carl Ellis]

Re: Trim Tab Problems

Hey Rob,

The link is broken.

- Carl -

[Robert Lees]

Re: Trim Tab Problems

Seems to work for me...try www.taylorcraft.org.uk/trim-spring.jpg
Rob

[jdlowrey]

Re: Trim Tab Problems

As always I greatly appreciate the feedback. Thankyou Rob and Hank. I am able to unzip my overhead liner and manually move the cables thus moving the trim tab. I put it back to neutral until I can get if working properly.

Jason - I do not know about any flippers under the tail. I was told when I bought the plane last year that the tail section had been replaced with the "D" style tail sometime between 1940 and 1948. I have no logs prior to 1948 so do not know whether that is accurate.

I was wondering, mostly for my own edification, as to what the differences are between the BC-12, the BC-12D, and the BC-65.

Jeff

[jgerard]

Re: Trim Tab Problems

Early ships had small "flippers" under the horizontal for trim control. The BC aircraft have the trim tab on the left elevator. I can't say exactly what year the switch was made.

Jason
N43643

[Jim Herpst]

Re: Trim Tab Problems

switched in 41

[lktiller]

Re: Trim Tab Problems

cable tension can be too tight.
after a rebuild of my trim system, I had to adjust the cable tension to a minimum or the front pulley would slip.

[selje]

Re: Trim Tab Problems

A few years ago, I had trouble with the trim system and received a lot of good advise to fix the problem. One suggesting was to apply violin bow rosin to the cable near the bell crank. This made it work well.

The trim fairlead wears down if the trim is adjusted with the elevator up or down. The Taylorcraft is to be trimmed only during level flight. I use mine only to get level flight at cruise. During the pattern and on landing the mild back force needed helps guage speed and decreases the chance on a stall.

[Richard Boyer]

Re: Trim Tab Problems

I don't know........If I left my trim at level flight settings on landing I think my left arm would be bigger than Arnold Schwarzeneggers!! I need a little more than some "mild backforce". Maybe because it's heavier? BC12D/19

I wind that sucker all the way to the stop and it sets me at the perfect attitude for landing with my approach at 60 to 65 depending on wether the wind is gusting or not.

When I first started flying my T I would wind it really fast wich would cause it to "slip" at the end. This in turn messed up my trim indicator. After the punishment of fixing it, I wind much slower and pretty much know when it is going to hit the stop.

Richard Boyer
N95791
Georgetown, TX

[jrzangger]

Re: Trim Tab Problems

When rebuilding my plane I found the jackscrew mechanism to be dry and very stiff. Once lubricated properly the trim system works quite well. I would believe that the jackscrew and mechanism in the tail has not been lubricated periodically.

Jim