I've had so much fun with my 48" 3DHS Demonstrator Edge that I wanted to build another one. I've been threatening to write an article on pull cable set up and this gave me an excuse to build another Demonstrator even though my first one is still in terrific shape. Maybe I'm just greedy.
For those who have never done a pull system, it can be quite intimidating, but it's really just as easy as using a pushrod, just different. Like anything else, you need to know a few tricks and you learn those by doing them, and hopefully, from reading this article.
Setting Up The Pull System
My first pull system had me scratching my head and cursing a little, but I learned enough from that to make subsequent ones go smoother. It's not that hard and there are no mysteries. After you do a couple you will wonder why you were intimidated to begin with. What I'm going to try to do is show you some of the little tricks I learned the first few times.
If there are any hard and fast rules they are that you want a drag free and slop free system. The rudder has to move smoothly with no drag coming from a bad hinge job or ill fitted tailwheel assembly. The swivel connectors on the servo arms also have to have no drag on them, and as little slop as you can manage without getting them so tight they drag. If there is ever any doubt on any part of this, always go for smooth and drag free, slop free operation. This is critical for good rudder centering and makes for a good flying and nice tracking plane.
Cleaning Up The Cable Exits
The slots for the cable are already laser cut into the fuselage sides, and the covering is slit so the wires can pass through. The first step it to tuck the covering into the slot with a trim iron. Get them out of the way so they don't put any drag on the cables. You can either cut them flush with the edge of the slot or tuck them in, but tucking them in makes for a neater and cleaner looking job. Here you want to use low heat to avoid the iron sticking and pulling the printing off. Take your time, do a neat job. I think you'll agree this looks a lot cleaner than leaving it loose.
The Cable Ends
Now, thread the ball link onto the threaded connector piece. You can either hold the threaded connector with a pair of pliers and spin the ball link on by hand, or chuck the connector up in a small drill and spin it into the ball link. If you have arthritis, using the drill will make life a lot easier.
The connector will be easier to handle this way because it gives you something to grip. I suggest threading it on all the way, then backing it off five turns or so. This will give you some additional adjustment should you need it later.
With any operation, the cleanest and most tidy job is going to be the best solution. The least clean part of any cable installation are the cable ends and crimp pieces, and I've done my best to make that neater. Here is how the manual calls for the cable ends to be assembled. This works really well, though it's a bit messy for my taste. We are still going to do it the way the manual calls for, though we are going to tidy it up a bit.
Follow the manual and you will get what you see below. Again, this works well, but we're going to clean it up a little. I've put a battery on the ball link to hold it down and aid in getting a better photo.
Once you get it all lined up. take a pair of pliers and smash the crimp piece flat. Just to be sure, put a drop of thin CA on the wire and hold the piece so gravity helps it run into the crimp piece. Snip off the loose end of the cable and the whole thing looks a lot better. It's still pretty ugly. but we're not quite done.
Finally. I use a piece of heat shrink tubing to cover the whole thing up and make a nice presentation.
Setting Up The Servo Arms
Now we take care of the other end. Here I am using the standard Hitec double arm that comes with the servo and the 3DHS swivel connectors. These are nice because they afford easy cable tension adjustment.
First, drill the outer holes in the servo arm with a 5/64 size drill. I use a little hand drill from the Hobbico set, but in a pinch you can get away with hogging it out by spinning #11 Xacto blade in the hole. Drilling can leave behind some flash around the hole, and this can interfere with the swivels moving smoothly. I use an emery board to lightly sand the top and bottom of the arm. Good, smooth operation will help with proper servo centering.
On the newest 3DHS swivel connectors the threads on the end are a little tighter so the nut spins on a little harder. This is a good thing because it makes it harder for it to come loose. Get the nut as tight as you can get it without introducing any drag. Again, you always want smooth, drag free operation on any control system.
As you can see, the threaded end is a little long, and you have a few threads sticking out. The reason for this is you want to put to put CA on these threads to lock the nut on. I like to use a T pin to apply some thin CA to the exposed threads, and after that sets up, a dab of medium CA. This assures it's not going to come apart.
When you put the arm onto the rudder servo, use the sub trim in the transmitter to center it.
Hooking It All Up
With the cables attached to the control arm on the rudder and cables run forward into the fuselage, now it's time to hook them to the servo. Here I run the forward threaded end pieces halfway into the swivel connector, and this gives me adjustment both fore and aft, and remember, we have some adjustment in the rear if we need it.
Now you want the rudder to stay centered so you can get the cable tension close, and for this I usually tape the rudder counter balance to the fin. Since the Demonstrator uses printed covering, I don't want to risk damaging it, so I have the wife or a friend pinch the two together.
I like to install one cable at a time because it's easier that way and you have less chance of mixing the cables up. You want the cables to cross over top of each other inside the fuselage so they line up straighter to the servo. By doing one cable at a time, you simply start with the left cable and attach it to the right side of the servo arm and the right cable to the left arm. I don't know why, but I always start with the left cable.
Make sure you slide the crimp piece onto the cable first. Starting with the left cable, run it through the right side forward threaded connector piece. Pull the cable tight. You want to be careful that you don't pull so tight that you start breaking things, but you want it tight enough that you won't run out of adjustment later. Usually the cables end up not being as tight as you thought you had them, so get the slack out. If it's either too tight or two loose, we left ourselves some adjustment by centering the forward connector pieces in the swivel, and remember we also left ourseves some emergency adjustment at the rear with the ball links.
One neat trick here (see above) is to get the cable tight, then bend it backwards at the connector. This will help it stay tight and not slip while you are running the wire back through the crimp piece. You can see I pulled the wire tight and then folded it over the connector. You can pinch the cable down on the connector with one hand and it's not going to come loose. Then you slide the other end into the crimp piece with your free hand (which has slid back out of sight in the photo).
Now you do the loop, which I did not get a picture of, but it's the same as we did on the other end and the same as in the manual. Again, I like to pull the loop tight, but also again, be careful you don't slip and start breaking stuff. If it's all good and still tight, smash the crimp piece down to lock that part of the adjustment in. Apply some thin CA to the crimp like you did on the rear and it's almost done. Put a paper towel in the bottom of the fuselage just in case you spill any CA. From there, snip off any excess wire, remove the threaded end pieces from the swivel and use heat shrink on the end and crimp pieces.
Do the other side exactly the same way and you're almost home. Yes, I am well aware the rest of the radio installation looks like a rat's nest, but we'll clean that up later
You may notice that my connectors did not even need adjusting. I got the centering and tension
perfect just by pulling the cables tight. Like I say, they normally come out a little looser than you planned, and keeping the slack out to begin with made it perfect. After you do one or two of these you can expect the same kind of results. None of this is a mystery. You just need to do one or two to get used to it, and then it's easy as pie.
Now all the hard stuff is all done now. All that is left is the final adjustment.
Odds are pretty good you have it almost perfect. You want the cables tight enough that there is no sag in them, and there is no slop in the rudder. On the other hand, you don't want to get the cables guitar string tight either. That will kill the centering. Just adjust the cables so there is no sag in them, and no tighter. Assuming the rudder and swivel adjusters move smoothly with no drag, the rudder should center perfectly. if not, you probably have it too tight.
Slide the forward threaded connectors in or out of the swivel connectors to get the rudder perfectly centered, then tighten them down. Generally, you don't want the cables so loose that you can move the rudder, and you don't want them so tight that the rudder doesn't center properly. The best rule of thumb is to get them tight enough that they don't sag, and no more. If you get them guitar string tight the rudder will center poorly and eventually the servo will burn out. If you get them too loose the rudder can move when the servo doesn't and it will center poorly to boot, which is never good.
Another good rule of thumb is that if the rudder centers well, you are pretty close. Finally, as long as the rudder will repeatedly center, leave it alone! If you get it right now, you may have to adjust (usually tightening) the cables once or twice over the lifetime of the aircraft, but outside of that, it's pretty maintenance free. A little patience in getting it right now pays the dividend of you not having to mess with it later.
Generally I try to take a little pride in my writing, but this procedure has been challenging to put into written word. I tried to explain this to a friend out of state over the phone and I did a poor job. Later he visited me and brought his new Demonstrator. It was finished except for the pull system, and I showed him how to do it in five minutes. Showing was a heck of a lot easier than explaining, and infinitely easier to understand, and I tried to keep that in mind when I wrote this. I hope you will find this clear and helpful.