jester_s1

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.

jaka

Hi!
Why not use epoxy plate instead

jester_s1

Instead of the bolts? I didn't know where to get it or how to install it. The bolts do give you some adjustability though in case you miss the setup the first time, so that's a benefit. Any tips on where to get a plastic material that works well though?

Insanemoondoggie

Bending a bolt in threads makes an excellent fail point , especially on such small bolts. I'm not trying to be a kill joy , just wanted to make you aware bending a bolt in the threads like that , is not a good idea.

TLH101

My Feedback: (90)

Very good point. They will snap eventualy. Well one will, and it won't matter about the rest.

jester_s1

I do know about stress risers and I did do some testing of the bolts' strength after bending. I was careful to put a generous radius in the bend so as to minimize the weakening. My standard servos can put about 3 pounds on these bolts at full stall, and that's nearly a straight line pull along the bolts' length. That isn't going to break them. The wood might rot away or the plastic connectors could photodegrade in the sunlight, but the all-thread rod isn't going anywhere. In a bigger application with high torque servos, the concerns about stress risers in the threads would be well warranted and I'd worry too about the duty cycle of the bolts. But not here by any means.

wjcalhoun

I've used the 6-32 threaded rod / washer / hex nut system for pull-pull as well, and it works well. Coated braided steel fishing leader doesn't rust, and generally is 30#-75# test - way more than any servo will generate.

I agree that bending the bolts or rod is not ideal. I put #6 hardware on the bolts, and adjust the positioning of the #6 rod so that the pivot point is EXACTLY on my hinge line, giving square geometry, and no change in tension with deflection. Obviously, the distance between pivot points on the rudder has to equal the distance between the pivot points on the servo arm.

Alternatively, if the pivot point is behind the hinge line, you can make the linkage at the servo end such that the pivot point is exactly the same distance aft of the center of the servo as the pivot point is aft of the rudder hinge line. So long as the geometry is square, the tension will not change.

dhal22

those plastic connectors will break long before the bolts will.

Insanemoondoggie

I'm not trying to be a bad guy here, just point out that a threads cut into a bolt , make a great spot for failure without bending it . Bending it stress's the metal between the tread and make an excellent point for the metal to fail. I,m surprised an other machinist or mechanic didn't say anything about it too. I admit , after years of working on full scale , I'm a little overkill when it comes to somethings, safety is one of them.
I think you have a good idea and well use it in the future , I just disagree with bending a all thread6/32 bolt .

blikseme300

My Feedback: (118)

Machine screw quality all-thread is fragile when bent. On small aircraft you might be OK, but on bigger stuff use stainless steel all-thread as it is a lot tougher. It is easy to take frugal (cheap) a step too far.

Bliksem

wjcalhoun

If the #6 rod is not bent, it is plenty strong enough; no servo I have can bend it. I think the point has been made that standard carbon steel bolts and rod WILL suffer weakening if they are bent

jester_s1

Oh it's definitely weakened because of the stress risers that the threads create. My point is that it is still far stronger than the 56 oz that these servos put out. If I were flying something bigger then the point would be well made though.

wjcalhoun, In working this out myself, I wanted to put a straight piece of all thread though farther up in order to get the horns at the hinge line, but there isn't nearly enough meat there to work with. On planes you've done this with, did you beef that area up or was the hinge design different to make it possible?

wjcalhoun

So there are a couple of options; you are right that the structure at the LE of the rudder is often not strong enough (for me) to trust.

1 - use a connector that extends forward from the bolt line (aft of the hinge line) and makes the PIVOT point right on the hinge line. This is the approach I ususally take, because it is easier for me.
2 - use an offset arm on your servo (boomerang shape) that sets the pivot points back aft of the servo axle the same distance as your pivot points are aft of the hinge line on the rudder. Usually it is easier to make or buy the arm, measure the aft offset, and drill your rudder bolt the same distance aft of the hinge line.

The square geometry you want is equal moment arms (off the longitudinal centerline) for the servo arm and the rudder bolt, and equal offset of the line connecting your pivot points (2 servo pivots and 2 rudder pivots) from their respective axis of rotation (servo axle and rudder hinge respectively)

Charlie P.

You've also got some funky differential built into the elevator by having the pivot WAY forward of the hinge line.

You have a future as a political speech writer. ;-)

jester_s1

The pivot isn't forward of the hinge line. It just looks that way because of the angle of the camera. The setup has very slight ackerman in it. After flying and tweaking it a bit, I like that I can adjust the positioning fairly easily to get the throws I want and the geometry right too.

wjcalhoun- I've seen a few made up servo arms that are attached to the stock arm with screws. What do you make yours out of?