A while ago I experimented with pull/pull rudder cable geometry simulating different scenarios (parallel cables, crossed cables, servo arm offset, etc.) in AutoCAD. I started with drawings from the Great Planes 27% Extra 330S ARF that has 27” cables. I used 50-degree servo throw (each way) to simulate maxed ATV’s for 3D flight (even though this may be way more rudder throw than needed). Below are my findings;

Example 1 illustrates perfect geometry; the cables are parallel, the mounting holes on the rudder horns align with the pivot point (leading edge) of the rudder and the distance between the cables at both ends is equal. The servo arm has no offset. As the drawing illustrates, even at full rudder deflection the distances between the rudder horns and the servo arm do not change, so cable tension remains the same and neither goes slack. This is the only scenario that provides perfect geometry.

I’ve heard it said that as long as the rudder horns are in line with the leading edge, and as long as the servo arm is the same length as the rudder horns, you can cross the cables and the tension will stay the same all the way to full deflection. But this isn’t true. In Example 2 the “idle” cable acquires approximately 3mm slack at full deflection. Most pilots might find some cable slack acceptable. But given the same scenario on an even larger plane, the amount of slack could be greater.

On some models (that have traditional bolt-on control horns instead of threaded torque rods or custom-made imbedded horns), the horns must be set back from the leading edge. On models like this, I’ve heard people say that as long as any offset in the servo arm is the same as the offset in the rudder horns, then cable tension will stay the same all the way through the throw and you won't get any slack. But this isn’t true either. In Example 3, the horns are 5mm back from the leading edge and there is 5mm offset in the servo arm. But the idle cable acquires approximately 3mm slack. Again, 3mm of slack may be okay for most pilots, but it isn’t perfect and would be even worse on a larger plane (about 10mm slack on a 35% model).

The idea of offset on the rudder servo arm caused me to think about Example 2 again (where the cable connection points on the rudder align with the pivot point, but the cables are crossed—probably the way most 35% and 40% kits/ARFs are set up). So I made another drawing (not shown), only this time with 5mm offset in the servo arm. Now, at full rudder deflection, any existing cable slack was immeasurable. This means that if your rudder cables cross, some offset will be needed in the servo arm even if the rudder horns are not set back from the leading edge.

Look at Example 4A. The offset on the “rudder end” is still 5mm, but now the offset on the servo arm is 10mm. (This is the exact setup as on the GP 27% Extra.) This time the idle cable has no measurable slack (.1mm), so the cable tension remains virtually the same even when the rudder is fully deflected.

This might lead you to believe that the formula for servo arm offset is double the offset on the rudder horn. But it doesn’t work. I drew another illustration in Example 4B with the servo located closer to the rudder. But with the same offset as example 4A, the idle cable now has 1.5mm slack, so you can’t simply multiply the cable mounting point offset on the rudder by some number—I guess there are just too many variables.

There may be a formula that describes the relationship between horn offset, servo arm offset, cable length and the width of the cable connection points on both ends, but I’m not even going to try to figure it out!

So here are my conclusions;
1. In order to get the pull/pull cable setup you want, you have to determine how much cable slack you can live with. I don’t know if there is a consensus on this—some seem to think a small amount of slack is just fine, but some don’t.
2. If your cables are crossed and/or if the cable connection points on the rudder are not in line with the pivot point, you will probably need an offset rudder servo arm—especially on larger models.
3. There isn’t a formula (that I know) for determining servo arm offset that can be applied to all situations. Practically, any offset needed would have to be determined through experimentation or the use of computer-aided design tools such as AutoCAD. (Or hopefully, your kit/ARF manufacturer has already done this for you!)
4. The only PERFECT scenario is when there is a parallelogram (where the cables are not crossed, the cable connection points on both ends are the same distance apart and in line with their pivot points). Otherwise, a rudder servo arm with some offset may be a good idea.