I have a 40 size model that I'm building and because of compartment space, am considering using pull-pull control for the rudder/elevator. Never used this method so there are a lot of ????, eh? I do understand that the control horns (on top and bottom) must be mounted mirror image on the respective control surface, but..........

1. Do the control cables on each side have to be the same re:length or is this not critical?

2. Any restriction (or advice) on routing the cables, ie, no sharp bends, bushing material, etc.

3. Is it a desireable modification (pro/con)

Again, thanks in advance.......

GDJ

No, but the cables need to have the same spacing at the servo arm as they do at the control horns. If they are 1" apart at the horns, then they need to be 1" apart at the servo as well.

No real restrictions, but make sure they don't rub against anything or each other. You can route inner nyrod all the way back to the servo but that negates the weight savings of pull-pulls. I just glue 2" pieces of inner nyrod into the exits and fish the cable back to the servo. It's more trouble the way I do it because you absolutely have to make sure it's not twisted or tangled, but it's also much lighter.

I will use pull-pulls on almost all my models from here on out. They are far superior to pushrods in that they create a tighter control setup that is much lighter. The only con is that if a cable breaks which is more likely than a pushrod failure, kiss your plane goodbye. Be sure to deburr every fitting that cable goes through. Take your time and triple check everything.

Hi!
First question: No, But naturally they will have nearly the same lenght. It is not important that the servo arm is the same lenght as the arm at the rudder...in theory yes, but in practise with the short arms we are mostly talking about here the arm lenght is of no importance.It is vital though that you allways make the servo arm as short as possible ..this for getting the least slack out of the system.

Second: You mostly don't need any bushing material...just use the thinnest cables available from sport fishing stores. I do bush the fuselage were the cables go through..just a short peice og thin plastic tubing (1cm), not heavy stuff like nyrods.

Third: I do use cables whereever I can because of weight reduction and that no bad vibrations is transformed through them to the servos. There is also much stiffer set-up with cables.
See pictures of my GP GEE BEE R-2.
Regards!
Jan K
Sweden

Great response - thanks fellas!

Here's the best site I've seen on the subject:

[link=http://members.cox.net/bdfelice/Pull-pull/pull-pull.htm]Pull Pull[/link]

GOINSTRAIGHTUP............pukka gen; that Ackerman stuff could take a person unawares...thanks!