Everyone knows that saving weight in the tail of your airplane is a good thing. An ounce saved in the tail equals twice that much (or more,) you don’t have to add to the nose. Eliminating a pushrod by using cables is also an excellent way to keep your model more scale like in appearance. Let’s get started
To make a good pull-pull setup you need to have a powerful high torque servo, stiff control horns and plastic coated braided steel cable. Also needed is 4-40 hardware in the form of ball links for attachment to the servo arms, threaded studs designed for pull-pull cables (they have a hole in one end,) and some bronze tubing for use as “swages” (crimps) to lock the cable into place.
(Below) Here you see some of the hardware. Most is available from DuBro and Sullivan Products and often available from the local hobby shop.
I have found the best deal for cable is 0.032-inch brainded fishing leader material. It is often referred to as 60 pound test and it comes with a nice clear plastic coating that almost completely eliminates corrosion when used with RC planes.
(Below) You can get it at most sport fishing supply stores.
To cut the cable cleanly and eliminate frayed ends, use a quality cable cutting tool. I use the Classic Cutter. www.bbtools.net email: firstname.lastname@example.org I reviewed these great tools at: http://www.modelairplanenews.com/blog/2011/05/18/classic-cutters-great-additions-for-your-toolbox/ They work very well.
(BElow) When it comes to servo installation, be sure to center your rudder servo on the fuselage centerline so both cables will have the same geometry relative to the rudder’s hingeline. Also, a stiff double-arm servo tiller is needed. I use the aluminum Hangar 9 brand that comes with a clamping center. I also use a metal gear servo with a metal spline since there is a lot of clamping force applied when you tighten the screws. The ProTek RC Servos available from A-Main Hobbies are great for giant scale applications. www.amainhobbies.com .
To begin installation, cut two lengths of cable that are long enough to reach from the servo to the rudder plus a foot or so, so you can make the loops and crimp the swages.
(Below) Here you see the cable crimped to the threaded studs and the studs are threaded into the ball-link clevises attached to the servo tiller arm. The Protek RC 330T servos running on 6V work very well.
To make the cables I first attach them to the servo arms and then feed them through the fuselage to establish the exit points and to find any framework that will need to be trimmed to clear the cable paths.
Now to make the swaged/looped ends, you first thread on the swage tube and then feed the cable into the threaded stud. Loop the cable back into the tubing and pull the cable to form a small loop about 3/8 inch long. Now bend the cable back along the tubing and slide it back into the tubing. Use a long nose plyers to pull out all the slack. This is why you need extra cable length. When the cable is pulled tight, (see photo below,) use the crimping tool and make two crimps to lock the tubing into place.
Where the cable pass through the fuselage sides, I use an inner-Nyrod pushrod sleeves as fairleads to protect the cable from chaffing. Here the cable has been attached to the rudder control horns to adjust the length. You’ll need to fire up your radio to center the rudder servo and use some sort of scrap balsa shim to center the rudder while working out the cable lengths.
Above you see the Nyrod sleeves guiding the cables through the aft bulkhead. Once everything is working properly, they will be glued into place.
This is my very well used, old school crimper. It works very well by making small crimps in a couple of places along the 1/2 inch long bronze swage tube. Also, bronze tubing is much better for crimps as brass tubing is too hard and can often crack or split when you crimp them in place.
So here is the clevis end of the rudder control horn. I use Robart Horns and clevises. The extra cable has been cut away leaving about a half inch in front of the crimped swage. I use this long screw to make it easy to remove the clevis from the horn during the process. After the model is finished and painted, all the clevis screws will be installed and properly secured with the Robart screws and Locknuts.
For the closed-loop pull-pull circuit to work properly, the distance between the control horn attachment points (left and right) has to be the same as the distance between the tiller arm attachment points at the servo. (Note the plywood shims under the rudder control horns.) Also, the geometry has to be the same at both ends. The pivot points relative to the attachment points also have to be the same. If not, the cables will become slack on one side or the other when the servo moves.
Just like for the elevator pushrods, the rudder cables need an exit plate glued in place to support the fabric covering. For a neat appearance, these have to be flush with the outer surfaces of the framework.
Here’s another view of the finshed pull-pull cable installation. You can see the crimped swage tube in place. After the model is covered and you reassemble the model, you can add a drop or two of thin CA glue inside the swage tube to lock the cables into place.
Shop Note: When installing the cables and working out the length, install only about a third of the threaded rod into the clevises. This will give you 2/3s of the studs length to use to tighten the cable. Also, use a lock nut to set the clevises and prevent them from coming loose. Also one last bit of advice. Do not over tighten the cables. Just remove all the slack. If the cables are too tight, you can add accessive side load to the servo gear train and possibly damage your servo.