1) You always hear talk of doing a pull-pull system on the rudder, can it be done on the elevator as well?

2) Can someone post pics/graphics on how the hookup is done? The servo end, pushrod tubing (if applicable) and the horn end.

Thank you

I had three pull-pull systems on one airplane. One on the rudder and one on each elevator half. Sorry I don't have pictures. Basically, you put two control horns on the elevator - one sticking up and the other sticking down. Run a cable from each horn to each end of a double-armed servo arm and that's it. Try to keep the cable runs straight.

I've even heard of a pull-pull throttle.

Pull Pull setup is quite simple, but it requires a bit of geometry to make it work well. The basic idea is to translate the movement of one end of the servo arm to an equal movement of the rudder or elevator horn. This requires a couple things. First, the center of the servo arm to the attachment point has to be the same as the hinge line to horn attachment point. 1/4 " of movement on the servo arm attachment point has to equal 1/4" of movement on the horn. If you don't have this setup correct, one side of the cable is going to go slack. This results in fluttering some times.

The second thing is not to build any Ackermond (spelling??) into the linkage. This requires that the center of rotation of the servo and it attachment points or both sides be on a single line, and the same goes for the control surface. Right horn attachment point through hinge line to the Left hor attachmen are all on one line. This ensures that the control surface exactly follows the servo movement and that both cables remain tight throughout the move.

I'm currently fine tuning a Ruperts Dad that I set up with a pull-pull rudder. I made the mistake of putting the attachment points of the horns behind the hinge line of the rudder. The Pull side is tight but the other side is slack. In a netural position, both cables are tight, but as soon as the rudder starts to move one side goes slack. This is not good.

This is a basic setup problem that a rooky, like me, made. All throws have to be equal and all have to be on the same reference line. Everything has to be tight what ever position the surface is in.

Don

Here's a couple of pics of the pull-pull setup on my Bristol. It has pull-pull on rudder, elevator, and aileron, just as the full-scale did.
Rudder and aileron was easiest, the elevator uses a single servo with a linkage to a "crossbar" that pivots horizontally, dual horns on the crossbar go to four wires to the elevator halves (one each, top and bottom). Maybe more complex than needed, but works great. I did it this way as sort of 'proof of concept' for later, larger models so I could put servos in places not conducive to running all those wires, and drive the pull-pull setup from a distance.

Campgems - if the lines are tight at neutral, but get slack when deflected, that is OK. The surface will be under pressure from the moving air when deflected, so it does not matter that the other line is slack.

Thank you for your quick responses. A friend of mine was wondering if it was possible to do a pull-pull on the rudder and elevator of a Piper Cub kit he was planning on purchasing.

According to the above detailed explaination it would be way beyond mine or my friend's understanding or abilities.

It's really not that dificult. Take a ruller and draw a straight line. Put a tick near the center and one 1" on either side of that tick on the same line. The center tick is your servo and the two at 1" out is the pull, pull attachment to the servo. Draw a second line and do the same tick in the center and one, one inch away on either side. This is your control surface, Rudder or elevator. You draw a line from each of the outside ticks, these are your cables. You adjust the length of the cables so the two lines, servo and control surface are parallel. You hafe just setup your pull pull system. As long as the two endpoints of the lines are equal length and the lines are equal length, then you have a tight pull pull where the control surface will follow the servo exactly. The lines can cross each other, in this case the movement of the servo is reversed in the control surface, or the lines can be twisted 90 degrees apart, IE for the elevator.

Where you get into the goofy stuff is if the center point isn't on the same line as the two end points, either on the servo end or on the control surface. In this case, you get an uneven movement on oppsite ends of the control horns or servo arms, resulting in one line going slack. This results in uneven control surface movement and slop in the linkage. Imagine if your push rods in a standard setup allowed for a three or four degree movment of the control surface without movement of the servo. This is what a slack pull line gives you.

Don

There is a company in California that sells "Vinyl coated KEVLAR line" in two or three thicknesses. Very tough stuff. Many of our local Pattern pilots use it on their 2 meter pattern planes. I will try to find the link.

My Excelleron 90 has pull-pull elevator and rudder. When I first saw this, I was realy intimidated. However, after playing with it a bit, I realized that it was not going to be as difficult as I thought. It worked out just fine.

CGr.

Campgems:
Graphics would be helpful, or a link showing details and graphics on setting them up. Thank you for the info.

Another source for wire to use in the pull-pull is coated stainless steel fishing leader line. Available at tackle shops that supply salt water fishing, comes in several test weights. I used 25 pound test in the Bristol, and you can get it up around 50-75 pound test if needed.

I do the same thing as Khodges and have wire in several weights. I buy rigging couplers from my LHS in both 2-56 and 4-40 everything else most people have in there building box. I do run my wires through that yellow push rod tube most the time but it isn't always needed.
If you can beg/borrow/steel or subscribe to RC Report the July issue has one of the best articles on pull/pull I have ever seen. Bad Shirt shows the correct way to set one up and what it should look like and do when you are finished.
Not many of us do it correctly but it always works.
I can't remember the last plane I built without A pull/pull rudder. I do it because it's easy!!! The other reason is it works very well!!

DISREGARD THIS POST, I SCREWED UP. [:@]

I like this Tettra Pull-Pull Assembly #2204 package shown in the links below from Central Hobbies.
1) Is this a good product?
2) The product assembly details (LINK 1) and the easy crimping details (LINK 2) makes it look so easy to do. Is it really that easy or is the details deceiving?

[link=http://www.centralhobbies.com/control_linkage/pullpull/tet2204.htm]LINK 1[/link]
[link=http://www.centralhobbies.com/control_linkage/pullpull/crimp.htm]LINK 2[/link]

Yep, really that simple. With the coated fishing leader you can wrap the wire and hit it with A flame so the plastic melts and adhears to itself. I have done it that way but I now make my own crimps. What you see the line going through is just A rigging coupler.
Pull/Pull is A very simple set up, nothing to over think about.

I decided to run a fully pull pull system on both elevator halfs and rudder. I ran all my wires and I got to looking at it and I am noticing that with 6 wires all running back to the tail they half to be touching and rubbing together, and they are I can see. I was wondering if that is ok to leave it be or should I separate the wires by running them through plastic tubing as they cross through the formers. That was one of my other problems, In order to do the top half of the elevator, there was no other way but to curve the pull pull cable up so it lined up on the top of the elevator. That particular wire is rubbing against the back formers and up through the top of the plane, I was definitly going to put that portion of the cable through a piece of tubing. What are your suggestions?

Thanks,
David

As you thought, cable rubbing cable is not good. It will cause some nasty glitching. The tubing routing is OK, as long as there isn't a lot of drag built up by doing this. If you could run the cables as straight as possible to the surfaces, never mind the cables rubbing at this point, and with power off measure the pull needed to start a surface moving while attached to the servo. A spring loaded gramm gage or a fishing scale woud be a couple posibilitys of what you could use. Just establish a base point of what it take to get the surface to move. Now route your cable through the tubing and do the same measurements. If there is not a significant difference in the required force to start a surface moving, you don't have a problem.

Wear in the tubing and the resultant increase in drag would be and issue, but if you can pass the first test, then lubing the cable can reduce the problem of wear.

Perhaps a couple well placed pulleys woud be better than the tubing. Just a thought.

Let us know what you work out. This is and area of intrest for a lot of us who like pull-pull setups.

Don

I understand how the pull-pull system works with the rudder but I would really love to see some setup photos for the elevator. Particularly the servo area. How do you set up the servo arm so that the wires attach to the elevator from the top and bottom. Can you put the servo in the cabin sideways?

I have never gotten any glitching from my cables rubbing but that could have just been luck?? I have done it A number of times though. Wires rubbing formers is something you should not do at all though. Added drag on the servo and the wires will cut through wood.
A servo doesn't know if it's sideways, upside down or at an odd angle as long as the controls are set up straight. You can put A servo anyplace you want. That's one of the reasons for pull/pull, long runs to A control surface. A lot depends on the plane though.