Leadout Termination

LAST EDITED ON Nov-28-04 AT 10:00 PM (CST)
 
A widely accepted technique for terminating leadout cables at the bellcrank is to put the cable through a short length of brass tubing. The tubing/cable is then bent 90°, passed through the bellcrank arm and bent 90° again. The cable itself is then wrapped per AMA specs or crimped. This is illustrated in the first photograph below.
It seems to me this technique will create stress risers on the leadout cable where the tubing is bent. Bending the tubing crimps the cable within; proof is that the cable cannot slide freely through the tubing. These stress risers are potential failure points. Can someone explain why we don't see (more) failures from this technique?
The second photograph shows how I terminate my flying lines. I form a thimble of 1/16 OD tubing then put the line through. The bend radius is such that the line can pass smoothly through without binding. Hence, no stress riser.
The third picture shows how I terminate leadouts on my .15-.25 size planes. Modified 4-40 blind nuts act as bearings for leadouts terminated with eyelets. This technique works well but introduces a twisting moment on the bellcrank arms- but that's another subject.
The forth picture shows a variation of the third, but using slotted arms to avoid the twisting issue.
Derek Moran

Your method of terminating the lines is the method I prefer too. I have a bender that allows me to make the bends smooth enough that the wires will pass through the tubing.

Have you tried passing the lines through the bellcrank as shown in photo one? If you radius the leadout holes the setup is almost friction free, there is no bending force, and no screws to come loose.

Derek,

I did my line termination this way on the the USA-1. I had never tried it before. So far, so good.

I've been using the slotted ear bellcranks for about 10 years. I made my own until Windy started selling them. I used a few of his and have now returned to machining them myself (finally found some material I like for this).

Hi Randy,
I agree that the slotted arm bellcrank is the best way to go. If I were to make one myself I'd use .250 thick Delrin. What is the material you like?
Derek

Derek,

I'm currently using Delrin, but I like the high silicon aluminum that I got from the scrapyard too. Only problem was he only had it in 3/8" thick stuff. Had to mill it down to 1/4". But it worked fine.

>It seems to me this technique will create stress risers on
>the leadout cable where the tubing is bent. Bending the
>tubing crimps the cable within; proof is that the cable
>cannot slide freely through the tubing. These stress risers
>are potential failure points. Can someone explain why we
>don't see (more) failures from this technique?

Hi Derek,
Beautiful photos! And very nice work, too. No one else tried to answer your question, so I'll take a shot . . .

This is just my "SWAG", but - - Even though that technique does create stress points, there is no flexing of the leadouts; therefore no failures.

They don't fail because there is no flexing of the wire inside the tube- no fatigue failure.

the other factor, each side of the loop is only carrying half the load. The loop will only fail under very unusual circumstances.

Derek,

Your first photo illustrates the way I terminate both leadouts and line ends, although especially with the more fragile lines I will cut tubing longer and fiddle with open ends such that lines do not ride hard on ends of tubing. And cut ends of tubing are carefully deburred, always.

The fourth picture shows my personal preference for a PA model, although I will drill and hack at the 'crank to lighten it some. At one time I did the conversion to cutting ends into forks but no other machining. Then PW related that a setup very much like N.4 picture had been used in his B-17s, the light one and the heavy one, "light" being a relative term here.

My best,

SNIP>>>>The fourth picture shows my personal preference for a PA model, although I will drill and hack at the 'crank to lighten it some.

WHAT FOURTH PITCHER? I only get three. M EYE LOST?

>SNIP>>>>The fourth picture shows my personal preference for
>a PA model, although I will drill and hack at the 'crank to
>lighten it some.
>
>WHAT FOURTH PITCHER? I only get three. M EYE LOST?

Scott,

Yes. See Windy Crank, lower left, third picture.

As long as we're on the subject of bellcranks, I have another burning question. When you use a ball link on the bellcrank, the off-axis force to the pushrod can cause the bellcrank to rock. There are various solutions to minimize this, my own is shown in photo 3 above. Basically I have put the bearings 3/8 inch above and below the bellcrank, effectively lengthening the span of the bearing and limiting the rocking to a small factor times the shaft-to-bearing clearance. Other folks use fender washers constrained by shaft collars.
My question is: in the grand scheme of things, how important is it to limit the rocking movement of the bellcrank? What is an acceptable maximum?
It seems a shame to devote energy and matériel to a stiff and slop-free linkage only to have the bellcrank rock. On my planes I have tried to achieve as close to zero slop (as measured by immobilizing the leadouts and trying to deflect the elevator) as possible (and I get pretty close). I am aware that some people intentionally put slop in the elevator linkage. Am I wasting my time?
Thanks!
Derek

Derek,

Yes. You are wasting your time. From what I can see of your installation, I would advise that you quit measuring rocking of the bell crank with a dial indicator.

There is an answer to your quest for a rock-solid installation, but it entails driving clear down to Rush's house for more trick photography, and I don't think some of the folk here on SSW can stand too much more of that.

LAST EDITED ON Nov-30-04 AT 10:24 AM (CST)
 
If they are as good as the picture of his wife next to the rattlesnake sign, I think you should make the effort.
Derek

Derek,

>>Am I wasting my time?<<

Depends on how much fun your having.

I've used a horn set up similar to the ears for the leadout cables on occasion. It was not that much better since I had to have the pushrod run straight from the bellcrank then go through rather radical bend to get to the flap horn. I doubt it makes that much difference anyway.

Ted Fancher will probably remember my attempt to build a "pully" belcrank in a plane. The smoothest controls you can imagine. Only problem was, the pulleys weren't big enough so instead of a 3 to 1 or so ratio of leadout movement to pushrod movement, I got about a 1.5 to 1 ratio. Couldn't narrow the line spacing to slow the controls down and make it flyable. Shame, that.

It was a good idea though.