How does the trim cable work?

[Helicopter D]

Ok can anyone explain (in 10 words or less ) how the trim cable works? I have two BC12D projects of which one used a spring and 2 clamps and the other has a turnbuckle to adjust trim cable tension. Any advise will be greatly appreciated!

[M Towsley]

Re: How does the trim cable work?

Too convoluted for ten words, but....The cable goes around the front pulley, through guides front and rear, to the back pulley. I have only seen the spring and clamps used to keep tension on the cable. The spring on mine is on the left side of the system.

[3Dreaming]

Re: How does the trim cable work?

Ok can anyone explain (in 10 words or less ) how the trim cable works? I have two BC12D projects of which one used a spring and 2 clamps and the other has a turnbuckle to adjust trim cable tension. Any advise will be greatly appreciated!

Really well if set up right.

[Pearson]

Re: How does the trim cable work?

Every one I have ever seen had a spring. It is hard to say how much tension it has. I would guess between 5 & 10 pounds. I have never seen a turnbuckle used.

[Ragwing nut]

Re: How does the trim cable work?

In ten words or less only by magic they work.


the tension mechanism is the two clamps and spring, no turn buckle and is located between the cable guides piot side which is the longest free length the cable passes. the spring holds slack in the cable between the two clamps. when force is exerted on crank, it loads the spring forcing tension to the cable to allow it to turn the tab pulley without excessive cable tension. the major issue with this design is when the pulleys get worn or the spring does not have enough tension, they slip horribly and can be very frustrating to deal with. Somebody like Terry needs to get piper double wrap pulleys approved to eliminate this most crap design on the entire airplane. hope that helps although was alot more than ten words.

[Mark Julicher]

Re: How does the trim cable work?

The pulleys have a V shaped groove for the cable to ride in. The V makes enough friction to move the cable. If the pulleys become worn, the V becomes U shaped and the pulley slips. Several tribesmen have used violin rosin and other substances on the cable to increase the friction with more or less success; however, it is usually sufficient to use a pointed instrument to sharpen the V groove. You can do this operation by turning the pulley whilst holding a knife tip in the groove.

Note that whenever the elevator is full up or full down, the cable binds in the guides located at the horizontal stabilizer. The trim was designed to be actuated with the elevator OFF the stops. The quickest way to ruin a forward pulley is to set the trim for takeoff without holding the elevator in neutral while doing so. The aft guide is a post made of phenolic with two guideways lined with copper. It is not difficult to make a new phenolic post, and it is not too difficult to take a bit of annealed copper tube and work it into the guideways... just in case you needed a good project during the winter months.

The aft pulley wears out quickly when the elevator screw jack becomes jammed. Jamming occurs when the ancient grease packed in the screw jack becomes hard. Sometimes the screw jack must be carefully opened and cleaned and re-lubricated. Some in the tribe have even had a new screw jack machined as an owner produced part.

It sounds like someone got frustrated with a slipping trim system and added a turnbuckle to force it to work. Hopefully the pulleys are not damaged beyond repair, but they are available NOS from Forrest . I also have a few aft pulleys that I made using a water jet, but I'm out of front pulleys.

[Guest]

Re: How does the trim cable work?

I have had good results by 1) cleaning the pulley grooves with lacquer thinner to remove grease & oil, 2) do not lubricate the trim cable, 3) operate the trim only when the yoke/elevator is in the neutral position.

Dave

[Forrest Barber]

Re: How does the trim cable work?

Mark, Rude and the others are correct; operate in neutral only. Never go to full travel ; 2 turns nose up is plenty ; "V" the pulleys ; keep cable clean ; yes the spring is for tension ; lube the screw with light oil ; if you feel her slip ; STOP and service on the ground. I ahve NO trouble with the ones here! 10 words ; hmmmmmm can't be done. Screw it up and screw it down . WTF that was 7 ....

[George Bassett]

Re: How does the trim cable work?

On a BC12D the front pulley for the trim cable has a shaft through the center of the pulley. This shaft shows a washer between the pulley and control crank brkt. This seems to put the shaft to far up, the end of the shaft is above the strings. Do I need to install a thiner washer? The picture I have shows a D-9091 washer. Need some help!!!!!!!!!!!!!1

[TimHicks]

Re: How does the trim cable work?

Not sure if you can see much in this picture. But here is my installation. '46 BC-12D

[George Bassett]

Re: How does the trim cable work?

Tim If you put a straight edge across the top two stringers, above the trim shaft, how close does the trim cable shaft come to the straight edge? Mine is higher than the stringers. Your picture looks just like mine except mine is higher.

Thanks
George Bassett

[barnstmr]

Re: How does the trim cable work?

10 WORDS

Turn crank clockwise nose up.
Turn crank counterclockwise nose down.



Sorry.. couldn't help myself.

Getting the trim to work is only half the battle. Wait until you try to figure out the indicator. Haha..

[TimHicks]

Re: How does the trim cable work?

Unfortunately, after I took these pictures, I dissembled the mechanism. And I didn't measure the clearance. It's close.
For what it's worth, I always had an inspection cover directly over the trim mechanism. I don't know if it added any clearance.
I don't remember seeing any evidence that the crank screw ever was rubbing the inspection cover.

[Steve Pierce]

Re: How does the trim cable work?

Thanks for the explanation. I received a BC12D with the trim tab in the baggage and was asked to fix it. The system was bound up somehow. I disassembled and lubed and seems to work fine now. I wondered what the deal was with the spring and thanks to you guys I now know.

[Steve Pierce]

Re: How does the trim cable work?

Since the tab came to me in the baggage sling I am curious if this is correct installation. The hole in the hinge pin is only large enough for one side of the cotter pin to be installed. Do I need to use a smaller cotter pin or is there something else I am missing? Thanks.