There is a ton of crap out there regarding the setup of a pull-pull rudder system. The best setups I have seen to date have been in planes at pattern contests, so I want to ask you guys how to do it.

I want a setup that remains tight throughout the whole range of motion - none of this Ackerman business. I know it can be done as I've seen it.

So what do I need, and what are some pointers? Do the wires have to be parallel, or can they be crossed? Do I need identically sized servo and control horns? Should I use aftermarket rudder horns? Should there be any offset in the control horns, or perfectly aligned with servo pivot and hinge line?

This is going on my Sebart Wind 110 and I want to do it right. Thanks!

Joe,

I've always had good luck with pulleys. You can get them from Hangar 9. They stay tight throughout the whole range of motion, and I've set them up crossed as well as straight. If the exits on the side of the fuselage aren't 45mm (which is the width of the pulley), then I usually cross them. If they are close, I prefer to keep them straight. The attach point of the clevis/MK end/ball link, is always in the center of the hinge line in my setup. I try and keep the same width at the attach points on the control horn as I have on the pulley, however if you're a little wide, it won't hurt anything. Try and get it as close as you can though.

My pull-pulls have always been guitar string tight, and I've not broken one ever. Get a pulley, threaded couplers, and your choice of connector from your LHS, and go to Gander mountain and get some 60lb test braided steel fishing leader and you're in business.

Easy to install, easy to setup, and easy to maintain. For lazy people like myself, it's heaven.

If you need any pictures, I can take some for you.

Measure the horn and get an arm that matches the distance spread between the holes. I use a Du-Bro HD arm ([link=http://www.shopatron.com/products/productdetail/part_number=670/101.0.1.1]HD arms[/link]) and 4-40 pull-pull cable system ([link=http://www.shopatron.com/products/productdetail/part_number=518/101.0.1.1]Cable system[/link]), parallel and it stays tight. Make sure the horn holes are inline with the rudder hinge line.

Joe, with your permission...
Jason, do you have a picture in this series that shows the SA 4 lead servo extension running
back to the dual elevator servos?
Thanks!
JLK

Ryan, I guess I don't see the benefit of the pulley, probably because I don't fully get how it works. Brain is pretty crammed with finals week.

Jas, do you ever have issues with the cable chewing up the servo horn since you don't have links at that end? I like the minimal, light weight setup you have, though it links pretty permanent. I was surprised to see stock rudder horn and basic servo horn

Since most of my planes are electric I haven't found it to be a problem. The glow planes haven't shown wear but I haven't kept track of them to notice any wear. I'm just a basic set-up kinda guy. Keep it simple and light.

When I get home I'll snap some pics of the dual extension.

I've run exactly the same setup with no links that Jason shows here and never had an issue with the cable chewing into the arm. And that's in glow.
-Ron

There's not much to see going down the back of the Valiant.

G'day Jason,

Where are you getting those extensions from? Looks like it would save the weight of two wires...

Cheers
Jason.

Jason,
Thanks for the pix of the extensions.
I've heard of them but I've never actually seen them.
I think it's a slick idea!
You sell them right?
JLK

Joe,

I'm not an expert by any means. The pulley keeps the cables parallel. If you think about a servo arm, the cables will actually get slightly closer when you go off of center. The pulley is a circle, so the cables remain the same distance apart. The pulley keeps the cable that's in tension as well as the one that's not tight. When it's setup right, there aren't any issues. It's also a conversation starter for someone looking in your airplane.

What is this extension? What does it do?

The pulley system, to me seems to be a bit of false benefit.

let me explain with the power of pictures!



As I see it, having a set up the same as JAS's (which is the way I do it) seems the most logical. to fixed points on the rudder and the servo arm. As the arm travels the lengths stay the same right?

With a pulley, they don't, calculating the distances between the horns mathematically, the long side would need to GROW to keep the tension on the lines the same, so essentially what i think occurs is the tension on the cables is balanced, but the OVERALL tension increases, which I'm not so sure is a good thing.

Anyway, some food for thought.

There's nothing wrong with a full arm, I've just used the pulley with great success on several airplanes. I guess real world applications don't trump mathematics...

Yes, SA has them, but they are not listed on the site. We are actually in the process of redoing our site, again, and going to be doing it in house. In the meantime I've been adding to my weebly site in the SA section.

Ryan, true, the mathematics can't be beat, and on such a short run the % elogation is quite high, but when in place in the plane where your lengths are over a metre, you'd barely notice it I think. Come to think of it who acutally tests the tension on their cables apart from giving them a "twang" every now and then?

I'm just cheap and can't stand paying a lot of money for a pulley I guess. [:@]

It looks to me like the distance between the servo wheel and the rudder horn would change an equal amount on both sides (and at the same rate), as long as the rudder horns are centered around the hinge axis. What I believe the effect of the pulley would be is to create more sensitivity near the end points - it would take some of the sinusoidal response out of the system and make it more of a linear response. The cable should remain the same tension throughout the full range of motion.

Ooh, I hadn't thought of that.

I guess it would take 50% of the sinusoidal travel out of it, the only true way to get a linear travel would be to have another pulley in the control surface.

That being said, do we want linear travel? having expo in the system would say not.

In this case the effect wouldn't be noticed around neutral, as it would be identical to a straight rod at the neutral point. However the straight rod would become less sensitive as it moved closer to the endpoints, giving your more response at full rudder deflection.

Actually it's starting to sound like a pretty cool setup. Fun to think about, probably more so if I wasn't getting my brain kicked by finals this week.

Dude, don't stop firing those neurons!

Are you thinking what I'm thinking? Tracked pulley style control horns?

Just in case. If you need to make it simpler and save some weight you could thread the wire as shown in the picture.

VB

Way back.....my EMCs used pulleys on both the servo and rudder ends.....looks cool....works....no real advantage in practice. Since I started flying Wistmodel stuff, I use the horns they supply....slight ackerman on the servo end.

Recently, I switched from the servo end shown by Vince to clevises on both ends...because I was breaking cables after 200-300 flights as they frayed on the servo arm. Lots more pressure and wear on cables now than when the 2M stuff was smaller.

Regards,

Dave

Hi Dave,

I think I got the idea from you. I never had problems. I remember that I cut the sharp edge of the horn hole with a Xacto knife. Probably this is the reason I never had problems. This sharp edge effect is also present using clevises. In those cases, I grind the sharp edges in the connecting rods to reduce the chance of cutting the cable. Well in FAI you are doing the knife edge loops. That puts a lot of pressure on the cables.

Thanks,

Vince

I thread servo arms for foamies at the like, (using kevlar thread) the same way Vince does, but I have to admit I can see this being a soruce for failure, with metal cables, radii is just too tight for metal to go around without a kink, and as anyone who flies CL knows, kinks are no-nos!

Vince,

I think you are right on both ideas....I think we did discuss this many years ago, and it was before KE loops. I am using the vinyl coated Kevlar from ACP, and it has no problem "weaving" through the servo arm, but even after radiusing the holes in the arm, the cables still fray with the current planes/schedules - wasn't a problem from 1990 - 2003 <G>.

Regards,

Dave