The absolutely cheapest and most secure control rods for smaller planes are piano wires with double z-bends like these ones.
There are only two problems with double z-bends.
- The first is that you only get one chance to get the length of the control rod right. When you make the second Z bend, you better make it in the perfect spot. There is no going back or fine tuning.
- The second problem is that it is very permanent. In order to remove the control rod you have to unscrew the servo arm. Not a problem if the servo is easily accessible, but a pain if the servo is built into a wing.
When I can remove the control arm, this is the solution I use 9 times out of 10.
To make the rod removable it is possible to make a so called modified Z bend on either side of the wire. In this bend the point is bent upwards instead of backwards. Using pliers it is possible to twist the wire enough to get it out of a control horn. I do not use this solution since my Z bends tend to be less than perfect and I suspect that the wire might twist enough under stress to release from the control horn without any pliers.
A while back I was completing a plane with servos recessed in the wings. I would not be able to get them out again easily. I decided that I had to make some sort of detachable piano wire control rods.
My idea was to form one wire into a standard Z bend and another as an L bend. When the wires are placed along side each other the Z bend side is adjusted to protrude on its own, while both wires run along side each other at the other end. This way the Z Bend wire serves as a safety alongside the L bend wire.Something like this.
One option would be to use some glue with shrink tube around to fasten the wires together, but I happen to know of another way of fixating piano wire that is even lighter and stronger than shrink tube bonds.
I entertain two very competing hobbies; fly fishing and flying RC planes. Both are weather dependent and are best performed in fair weather without wind. The negative aspect is that there are simply not enough suitable days per season to devote to each hobby in the northern part of Europe where I live. The positive side is that some of the techniques I have picked up over the years for making rods and tying flies (both hobbies in their own right) have come in very handy for building model aircraft.
One such technique is using fly-tying silk to fasten things to a metal wire. This is naturally used to make fishing flies, but it might just as well be used to make control rods out of piano wire.
A list of materials and explanations of the advantages of using fly tying material rather than e.g. sewing thread can be found at the end of the article.
First some training
It is probably a good idea to practice laying down some thread on a single piece of wire before you try joining two wires together. Fasten the wire in your clamping device of choice and spin the thread around the wire so that it locks down the start of the thread. Never release thread tension. If you have to release the bobbin you roll thread back into it and leave it hanging. Spin as much of the thread as you like onto the wire and make a whip finish knot to secure the thread.
At the end of this article there is a video that describes how to make finishing knots with just your fingers.
Make a control rod from two pieces of piano wire
Start by cutting pieces of piano wire that are a bit longer than the control rod has to be. Make a Z bend in one wire and an L bend in the other.
Center the servo and position the wires as the finished control rod will be installed. Use a sharp object to create an alignment mark over the wires. Remove the wires and trim off the L wire at about 10 mm from the Z-bend,
Make a 45 degree Z in the Z wire and form it so that there will be enough room between the wires to let the control horn move freely, like in the picture below.
Place the wires along side each other and use the bobbin to spin thread over them to keep them together. Make some adjustments after the first cm or so to adjust the wires so that they form a perfect control rod and that all bends are in straight angels to the servo arms and control horns.
Spin the thread over the rest of the wire. Take care to never release the pressure on the thread. After a while it is easier to loosen the wire from the vise and hold it in one hand while spinning thread with the other. Do not spin all the way down to the L bend. You must be able to pull the wires apart enough to open them for the control horn.
Fasten the rod in the vise or clamp again and make some knots at the end of the thread using your fingers. It is not the knots that keeps the thread locked into position, it is the glue or varnish you use.
Double check that the control rod will fit by mounting it. If its not perfect you can easily cut the thread using a sharp blade. Thread is cheap so see to it that you get everything just perfect.
Finish the control rod by dripping CA glue on the threads or applying varnish with a needle.
The finished result in place.
Fasten piano wire to a carbon rod
Carbon rods make perfect control rods. They are far lighter than piano wires of the same stiffness. The only problem is that you need to attach clevises or Z-bends to the ends. Using the same technique as above you can make a strong and light bond.
Start by laying down a thin layer of thread on the rod. Please note that this is essential. Without this initial layer the metal wire will not be held in place as firmly.Place the wire on top of the thread layer and spin a dense layer around the wire and the pin.
Finish with varnish (or glue)
You will need the following to join piano wire with thread in a permanent way.
Fly tying thread, often called silk, although it is no longer made out of silk.
Uni-Thread is my personal favorite, but any kind should do fine. The difference between regular sawing thread and fly tying thread is that the latter will form flat layers on top of the wire and will not expand under pressure. It is simply the perfect thread for tying metal wire together. But if you do not want to buy fly tying silk, you can get by using nylon sewing thread. Cotton thread is cheaper but is to fragile that I do not recommend even trying to use it.
Fly tying bobbin holder. You really must have one of these, there is no replacement. Do not cheap out and buy the simplest sort that has an all metal tube for the thread.
These are 100% guaranteed to cut the thread when you put pressure on it.I only use this kind of bobbin holder for metal threads. What you should optimally get is the cheapest bobbin you can find with a ceramic tube at the top. Something like this:
The kind with a ceramic bead at the tip is also fine and often a bit less expensive.
Lacquer or glue to make the bond between the thread and the wire permanent. Thin CA glue works OK. Just drip it on the thread and let it soak in. The best alternative is to use a needle to spread fly tying varnish over the thread. The varnish is water and UV resistant and just perfect for the application.
Veniard Clear is an old favorite. Take care using it. The conveniently quick hardening comes from using toluene as a solvent. Inhaling toluene for extended times will cause brain damage. But as a model flier you probably have CA glue but lack fly tying varnish anyways.
Non essential equipment
The following items are specialized tools used by fly tiers that you can probably do without. But if you have them they are handy:
Whip finish tool. A specialized tool for making knots. Looks something like this
The problem with whip finish tools is that it takes a while to learn how to use them. If you do not plan on taking up fly tying as a hobby you might be better off doing manual knots.
Fly tying vise. The best possible tool for holding on to a piece of wire and allowing maximum access.
If you want to tie flies you have to get one of these, but otherwise you can get by using a regular vise or some sort of clamp to hold the wire.
I have made a video of me making a control rod. It might be interesting to watch to see the techniques in practice.
This video describes how to make whip finish knots using only your fingers. I use a whip finish tool since I have never mastered the technique.