JustErik
Posts: 564
Joined: 7/19/2006
From: Charlottesville,
VA, USA
Status: offline
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I originally posted this on another site, but it's forbidden to post a link it, so I'll just duplicate the whole thing here. There's more than one way to make good rods and surely some will have other methods, but you asked so here's my method. 
Erik
It seems like the topic of CF pushrods comes up every few weeks around here. I don’t recall seeing any kind of pictorial step-by-step thread on the subject, so I thought I would show how I like to make them. Obviously, there is more than one way to make a solid, long lasting CF pushrod and what I describe may not be your personal preference, however it’s worked well for me and unless I’m mistaken, there are at least a few other Bros out there that use the same or similar technique.
Here are the materials ready to begin the assembly process.
I use .188 Midwest CF Tube for the outer sleeve, 4-40 all-thread for the core, Dubro Adjustable Control Horns on the control surface, Dubro Safety Kwik Links to connect the rod to the control horn and finally, Dubro Swivel Ball Links at the servo arm. You’ll also need a 4-40 nut with nylon insert, a couple of #4 washers, a ruler and something to cut the all-thread and CF to length. I like to use a Dremel with a cut-off wheel because it leaves a nice clean edge, but you can use wire cutters on the wire and clean up the cut with a file if you like. Regardless of what you use on the CF, take precautions with the dust generated. I usually wrap the tube with making tape at the cut point and keep a moist paper towel close by to help contain the fallout.
Note that I don’t use any glue whatsoever. The longevity of this pushrod derives from a rigid mechanical assembly that is easy to inspect and resistant to any kind of deterioration. I know the glues we use will outlast virtually any plane we build, but taking the glue out of the equation means less mess and more simplicity.....and it also helps me sleep a little better at night. An added benefit is that should the pushrod outlast the plane, the CF tube and all-thread can be used again, even if the desired length is not exactly the same. It's hard to cut down a glued pushrod without destroying some of the components.
The first step is to accurately measure the distance between the servo arm and control horn with both set to neutral. The pic below shows a ruler being used to measure between the servo arm and the control horn on my Sabre 56. It shows a distance of 153 mm.
Obviously, we don’t want the all-thread to go all the way from point to point. To get the needed length of all-thread, we subtract the portion of the Kwik Link and Swivel Link that extend beyond the end of the all-thread core. This pic shows the links installed on scrap pieces of all-thread and has the segment I’m referring to identified with the red line.
For any given link you choose to use, these numbers are fixed and once you know them, the calculation is easy and consistent. The portion of the Kwik Link that extends beyond the end of the all-thread is 14 mm and for the Swivel Link it is 9 mm. So for this pushrod, we need an all-thread core a total of (153 – 23) mm, or 130 mm.
Second: We need to know how long to cut the CF outer tube. To do so, we subtract from 130 mm the amount of all-thread that extends into both links. This pic shows the links installed on scrap pieces of all-thread and has the segment I’m referring to identified with the red line.
As before, for any given link you choose to use, this measurement will always be the same and once you know it, it’s a simple, repetitious calculation. The portion of the all-thread that extends into the Kwik Link is 14 mm and for the Swivel Link it is 12 mm. So for this pushrod, we need a CF outer tube of (130 – 26) mm, or 104 mm. But there is one more allowance to make. We must also subtract the width of the 4-40 nut, which is about 4 mm. The final calculated length of the outer CF tube, therefore is 100 mm.
It may all sound confusing but the formulas are really simple:
All-thread length = point-to-point-measurement – 23 mm
CF tube length = All-thread length – 30 mm
Once again, this is with the Dubro parts listed above. If you use different hardware, you’ll have to substitute your actual measurements.
Anyway, that’s it; the hard part is done. Screw one end of the all-thread all the way into the Swivel Link, slip on a washer, the CF tube, another washer and finally the 4-40 lock nut. Thread on the Kwik Link and test fit until you have it right. Obviously all adjustments must be made at the control horn. If you need a little extra length, you can unscrew the Swivel Link a bit and snug up the lock nut. However, the more thread you have in the links, the better. Also, I tighten the locknut just enough to secure it. I think if you go too tight, the tube may be more inclined to split and putting the rod under too much tension could possibly introduce other unwanted stresses into the assembly. It doesn’t need to be super tight to be solid.
Here are the loose components, except for the Swivel Link, which I had already screwed on at the time the picture was taken.
And here is a pic of the finished product. It came out 1 mm too long, but there was plenty of adjustment in the Kwik Link to overcome that error.
Finally, here is it installed on the rudder.
I realize this technique yields a heavier pushrod than one with just short segments of all-thread glued in each end. Being curious about the incremental weight, I checked the weight of a 12-inch all-thread rod and found it was 11 grams. That means all-thread weighss less than 1 gram per inch. By my quick measurements, I have an extra 15 inches or so of all-thread in the four pushrods on my Sabre, which means a weight penalty of 15 grams, or slightly over 0.5 ounces. On a 6-pound, 60-size plane like the Sabre, this amounts to about 0.56%. On a 4.5 pound, .40-size bird, this is a penalty of about 0.74%. If you’re really watching grams, there are probably lighter ways to go, but if you can stand an extra half-ounce or so, these rods should serve you well.
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"Toy airplanes are for people with more money than sense."
- My Ex-wife
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Need advice on pushrods/horns for Mojo40 - 
