I started to call this the poor man's pull pull, but I'm not poor. I'm just cheap. After looking at the kits available commercially I figured I could do just as well myself. Here's how to put together a pull-pull system for less than $5 per plane.
First, the supplies. you'll need a 6-32 all thread rod or bolt. It needs to be at least 1.5 times the length of your servo arm if you set it up like I did mine. Two nuts and two fender washers finish the hardware store trip for about $1 per control surface. Then you'll need some 60 lb steel fishing leader and crimp connectors, about $6 at any decent outdoor's store. Last go to the hobby shop and get some 6-32 control rod connectors, rigging couplers, and clevises. You'll need 2 of each for each surface.
There are so many threads on pull-pull geometry and discussions on it's merits and drawbacks I'll refer readers to the search button who aren't familiar with using this method or the advantages of it.
Put the washers on a soft piece of wood and whack them several times in the middle using a 1/4-20 bolt to make a depressed center. A log of half rotten firewood works very well for this. Then flip them over and whack them lightly with your hammer on some flat concrete just enough to straighten out the rim so they will sit straight. The purpose here is to be able to spread out the torque over a larger area without having to worry about the washer flexing and getting loose.
Start by reinforcing the outside of the control surface with some thin plywood. I really don't know how important this is, but considering the number of threads here on RCU about pull-pull setups that have gone loose because they wore out the balsa they were mounted to it seems like a good idea. Then drill a hole and thread the 6-32 rod through the control surface, putting a washer and nut on as shown. Bend the rod so that the rod connectors are in line with the hinge and also spaced the same as your servo arm. You can then tweak the bend to get the geometry and spacing right.
You might notice that the rudder horn is bent straight, while the elevator horn is reflexed. The reason is that I couldn't get enough spacing on the elevator horns without inducing excessive Ackerman into the system. I wanted to keep the cables tight throughout their motion, so reflexing was a good solution. With about an hour of working on this, I have nearly perfect geometry, with the slightest of cable slackening at full deflection. I could keep tweaking it to be perfect, but it's good enough for my purposes. Once I had it right, I tightened the nuts just until the washers started to bite into the plywood and doused them and the washer edges with CA to keep them in place. The servos' deadband is the only play in this system and it is far stronger than the torque of the servos.
Any comments would be appreciated, especially from those with ideas on how to make it better.