On a plane I'm assembling I've finally got around to installing the cabin servos and hooking up controls. The rudder utilizes two cables instead of pushrods and I believe it is my first experience with what I've seen called a pull-pull cable system.

The cables were pre-run down the fuse with clevises installed on the exterior ends. Internally two clevises are attache loosely to the cable by looping thru an eyelet on the end of the clevis assembly, back thru what appears to be an uncrimped crimp sleeve and then looped back thru it again. It is sort of slick in that you can adjust the cable length by varying the amount of cable outside the sleeve as well as the threaded adjustment at each clevis.

But it would seem unreliable to leave it this way. Once the initial cable length is established and adjusted so that the hookup between the rudder and the servo are solid and functioning - should I then crimp the sleeve?

All new to me and hopefully someone out there has seen these and knows what I'm supposed to do.

Thanks in advance,

Clay

Yes...that's [precisely the point of those small "sleeves" or crimp tubes. they crimp to hold the cable (usually at the servo end, though not always...appears yours is done at the rudder end, from your description...and that's fine).

The two major things to make sure of in a Pull-Pull system are this:

1) You are correct..it is insecure to have any point where things could come "undone". So, be sure things are nice and tight, crimped, secure, etc.

2) Cables can and do stretch during use, due to weather/temp, etc. So, first, make sure you can adjust tension at one end or the other without too much hassle. This is usually done by having some sort of threaded clevis at one end attached to a threaded coupler. Second, after the first 2-3 flights, inspect those cables...they will probably have loosened a bit, and need tightening. Do this pretty consistently....every 2-3 flights...until things stabilize for you.

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You'll find, ime, that pull-pull systems can be very precise and slop free when correctly installed and maintained, but they DO tend to require a bit more attention to maintenance than other systems. Just keep an eye on things, and you'll enjoy the results, I think.

The biggest error the first users of pull-pull make is to have the cables to tight. You must also have a little positive Ackerman built in, this is where the cable not being pulled will go slightly slack as the other cable pulls the surface off center. Go to http://members.cox.net/bdfelice for some good advice on pull-pull as well as many other items of interest to the modeler.

Hi!
I use wires wen ever possible. Wires is the lightest control system available to us modelers.
It is much easier and nicer looking if you use a clevis inside the fuselage and just crimp the wire together at the outside control surface.

Sorry I wasn't clear; the crimp sleeves are inside next to the servo. there is threaded rod adjustment at both ends of the cable so I figured on centering it at each end and then crimping the sleeve once the initial setting was established.

From experience how much would yall say the cable might stretch before settling down? Might leave my adjustment more for taking up slack instead of letting line out.

Thanks again,

Clay

I usually do exactly that...set things up so there's more room to take up slack than to let some out.

How much depends on quite a few factors, including length of cable, type of cable, rudder deflection and size (those planes with large rudders and high deflection angles seem to stretch more ime), etc.

My general "rule of thumb" is I want the clevis "3 turns in" when initial setup is done. IME, that's more than enough room to back off if needed due to temp changes/etc, and leaves a large amount of room to take up slack. YMMV, of course.

So far, I have two Pull-Pull planes. One of them has pull-pull on both the rudder AND the elevator. So, there are a lot of cables going back from the radio compartment... 6 to be exact because the elevator is in halves and each half has it's own cable pair.

The other is rudder only.

Anyway, so far, I've adjusted the elevator three times and the rudder twice. On one occasion, I had to replace one of the cables on the rudder. I've probably taken about an eight of an inch out. Again, when tightening the cables, don't make them to tight. Should definitely not be like a guitar string!! Tight enough to take the catenary out of the cables and that's about it.

By the way, I bought some vinyl coated kevlar cables. Tough stuff, all black.

CGr.

A couple points for the first time setup. Lock your rudder centered. I use a couple pieces of 1/8 lite ply and a couple sping clamps. Any thing to keep the rudder centered. Then center your servo. Make sure the sub trim and trim is also centered. Mount your arm as near you can to be 90 degrees to the case, sub trim so it is. Keep power on the servo to keep it moving whild you now tention the wires and make you final adjustments per the above good tips. While making the adjustments, make sure the center point of the servo and the rudder are not moved. A little on the right, a little on the left and so on until you have things as you want.

My first Pull-pull attempt left me with almost no room left for adjustment. I centered the thread in the clevis and I needed the full amount of travel to take up the slack. Now, I get enough thread into the clevis, IE almost fill the threaded part, and then make my cable lengths so they are just long enough to have a very slight slack in the lines. Remember that the big loop through the threaded rod will pull out straight and give you more slack, so manage that when you get ready to crimp. Last, if the threaded ends to clevis are a loose fit, put a lock nut on each clevis. Chances are that the cables will not unscrew, but it is a good precatuion. Also, when adjusting the clevis for length, remove it from the control surface and make the adjustment and then put it back.. Don't be tempted to just screw in the threaded end as you are building up tortion twist to the cable.

All of this sound a bit intimidating at first, but once you set back a take a look at the setup, it will start to fall in place quickly. Oh yes, make sure the cables are running straight. What ever they have to bend around will eventially be cut by the saw motion of the cables.

Don

Hey CG, do you happen to have a pic of you elevator pull pull setup, id love to see one?

Its been my experience that most cables stretch alot. I'd recommend you start out by hooking up your cables to allow for maximum adjustment toward tightnening them up instead of just in the "center" if you know what I mean. By the time their done stretching you might be "centered".

Good tips everyone. I'll keep this all in mind before I crimp the sleeves!

I use Berdley stranded steel nylon coated fishing line for my pull-pull systems and have never had any line stretching or length changing problems and some of mine have been in service for over 8 years never needing adjustments. I also did an experiment using fishing leader material (the clear plastic type) and it also performed well for over a year with no stretching problems. If you are having problems with stretching, you are probably using far to much tension in the lines, they need to be just tight enough so you have no slack at the neautral position.

Sorry, but time got away from me.

Here is the setup I'm using on my Ruperts Dad. The shrink tubing on the cable ends is just to tidy up the looks and keep the stub from snagging on stuff. The mechanical strength is in the crimp. Make sure you get it really tight.

Don

After I posted the photos, I saw that you are asking about the elevator pull pull. I haven't used that setup as yet.

Don

First, I'd like to thank everyone for offering me their insight on doing this. While I can see where the steel wire is lighter I find it inconvenient to say the least. The crimp sleeves on my ARF at the rudder control horns appear to have already been crimped. The ones inside next to the servo weren't. Yesterday evening I finally set down to it and worked the slack out, centered the rudder and servo and once everything felt and looked good I attempted to crimp the sleeves.

The sleeves are semi-square and are not the same as the old steel leader crimp sleeves I used to fabricate my own leaders as a boy. First I tried using needle nose pliers and couldn't do anything but mess up my adjustment. In the end I pulled out a set of lineman pliers that I keep around because they are large and cut control rod off cleanly. One of the sleeves I could actually feel "give" and crimp. The other I couldn't but I applied all the force I could muster.

Once finished I activated the rudder a few times and some slack is now evident on one side more than the other. Will try the threaded adjustments this evening to see if I have enough play to true things up.

To be honest I'm not impressed favorably with the cables and have to wonder why I wouldn't just use some light (2mm) control rod instead of cable. Is this a no-no on pull-pull systems?

As always, thanks for the guidance,

Clay

Clay
Are you threading the cable through the crimp and then loop it back around and through again. So what you should end up this is three lines going through the crimp. That way when you are getting ready to do your crimp it won't change adjustment. So you will be putting the crimp on first, the thread the line through the rigging, back through the crimp and then loop from front to back through the crimp again. Pull the loop snug and then crimp. You probably are already doing it that way, but this is just a reminder. Good luck

I have never had a problem doing the crimps. Go to your sporting goods store and pick up some ferrules (they come in different sizes so pick one that will fit your lines) like those used in fishing line. You can crimp them using dikes (diagnal cutting pliers) a kind of pliers that electricians use to cut wire. Just use enough pressure to nicely crimp the ferrules tight. To much pressure and you will cut them in two.