Hi Guys
How do you decide weather to cross or not to cross the wires on a rudder pull pull?
What advantages or disadvantages are there to each system?

Falcon

I always cross. It makes a better geometry at the rudder horn. If you don't cross in many applications it will bind.

Crossing gives a more direct route from the servo arm to the rudder horn.
Also, there is less chance of the wire "sawing" its way through the fuz sides over time.
There are no disadvantages that I can think of from crossing.

You really only NEED to cross if you need to to get the right angle from the Rudder horn to the control horns on the rudder and you cannot get the right angle by crossing. In short, if you can get there in a straight line without having to cut a super long slot in the fuselage then there no real advantage that going to amount to a hill of beans in crossing. There may be some sort of mathematical advantage, but in reality as long as the cable does not have to bend around the exit hole or a former then there is not enough to gain to warrant crossing unless that just how you want to do it. 6 one way, half dozen the other.

If you do cross you need to use an offset servo arm. I am not a mathematical geometry person but for some reason when you cross you endup with the not pulling cable going very slack if you do not use an offset arm. The offset arm somehow allows the backside cable to stay tight when it is not the pulling cable.

Also, if you do cross try to install the hardware so that one wire is connected on top of the servo arm and the other cable is attached at the bottom of the servo arm. This will help prevent the cables from touching at the point where they intersect. Touching cables, especially if the cables are wire (I use Kevlar) they can create interference in you radio system.

Hi Mike
Thanks for your input here.
I have tried it both ways on a Fliton Extra.
Not crossed; it looks like the cable is going to eat it’s way through a former at the rear of the fuse, but the geometry looks good, the not pulling cable only has a very small amount of slack on it.
Crossed; the cable goes straight from one end to the other, no obstructions perfect, but the not pulling cable gets a lot of slack.
I use a nylon coated cable so no problem if they do touch and I do have one on top and one on the bottom of the servo arm.
I don’t know if it’s worth getting an offset servo arm for this model?

Thanks

Falcon

It depends on how much slack. If its just a little bit loose then I would probably not worry about it. The transition to the slack cable happens instantaneously( I would say to the average human) and you never feel it go taught because the pushback on the rudder takes all the slack out in a millisecond. If its a lot of slack or if you are not sure, then I might consider an offset cable if for no other reason to keep from having the the cable spring taunt and jar the connections as it transitions back and forth from left to right. If there is enough slack that the cables are banging tight every time they transition back and forth its more tham likely shorten the life of something. Either the cable, the end connector or even the wood to control horn connection at the rudder depending on what sort of rudder horn you are using.

It really doesn't matter weather you cross them or not as long as you get the geometry right. I typically look at where my servo mount is compared to the slits for the cable & go with what fits the plane.

On my BME Edge I went with an offset box design (example C in the picture) because the hard point in my rudder was too far back for the normal box option (example A). Both wires are guitar string tight throughout the entire range of the servo.

diagram taken from http://www.swbmfg.com/rudex.html which is a great read on this topic.

Thanks Guys
I think I'm going to play around with some different servo arms and try to cut out some of the slack

Falcon

Here's a link to Brian Felice's web page explaining all this:

http://members.cox.net/bdfelice/Ackerman/ackerman.htm

Best darned explanation on pull-pull I've come across. I agree with Mike - I only cross 'em when I need to, ans I go out of my way to avoid creating "ackerman". I make the distance across at the rudder post and servo arm the same to avoid binding... and I ensure the rudder horns are situated with the clevis holes on the control surface hinge line. This avoids complication and makes for smooth trouble free operation.

Hi
I've read the info on the link; it would seem to suggest that the hole in the clevis should be some 1/16 to 3/32 behind the hinge line!! Therefore creating a small amount of "Ackerman", much like an offest servo arm?


Falcon

You can set it up however you like - the main thing is to avoid a setup that binds, or where 1 line goes excessively slack as it moves towards full travel. Now you've got the info... you're decision on how to use it :^) Pull-pull is great once you've got a handle on how to install it correctly. It's probably the lightest control setup done properly, but I use direct pushrod using CF tubes myself quite a bit too.

Daryl
Thanks for the link
I think on my next plane I'm going to try it with the clevis a little behind the hinge line and see how it works out

Falcon