Just a note here on Pull-Pull configurations.

My Excelleron 90 is set up with both elevator and rudder pull pull. The advantages of this system are that it is lighter, stronger, and provides very positive and consistent control. The only real limiting factor for repeatability this setup is the correctness of servo centering.

The downside is that it can be somewhat challenging in the setup phases and that you will have "cables" running the length of the fuselage from the servo trays to the rear control surfaces (if two are used). Many use pull-pull for the rudder only with rods or individual servos for the elevator.

Here is a source for the Vinyl Coated Kevlar cord that you can use. The cord I got, well, I got it several years ago, and got it from a different source, but this one is as good as any. Note that at the bottom of the page, they also have a 4-40 connector that will screw into a typical 4-40 clevis that makes this installation much simpler than the older crimp method.

This is yet another method of controlling the elevator and rudder as compared to those mentioned above (GoIrish got this Pull Pull going.. great idea to present it, Irish!!!)

Anyway, here is the link to the cord and the linkage.

http://www.acpsales.com/Kevlar-Tow-and-Cords.html

CGr

Reading through this thread made me curious about the amount of thermal expansion we really might encounter using a plastic push rod. I did some homework and came up with the following:

Assume the pushrod is 30 inches long (typical for a 40 sized plane)
Assume there is a 30 degree F increase in temperature

The CTE of plastic varies widely with the type of plastic. The lowest I found was 15 parts per million per degree F (for polyimide) and the highest was 110 ppm per deg F (for polypropylene).

Doing the arithmetic, the pushrod would grow in length about 0.1 inch for the high-CTE plastic and about 0.01 inch for the low-CTE plastic.

have used the golden rod lots of times on many applications, long and short.... aint enuff heat growth to worry about might be 2 clicks of trim thru the day.
jeeze people try to make this harder than it really is.

What he said. They may expand, but one or two trim clicks and it is forgotten. Not a big deal.

Hi!
Exspansion isn't that much to worry about. What's worse is the lack of stiffness in the system. With wires (Pull-Pull) you get both a lighter system, no vibrations to the servo that can (will) destroy the pot/electric motor and a more stiff set-up!

I had a lot of trouble with the trim changing on my Hog Bipe. I replaced the plastic push rods with Sullivan composite rods and the problem went away. At the time I blamed it on thermal expansion of the plastic rods.

Now I wonder if the plastic rod was flexing. Maybe the stiffness of the plastic rod or the sheath it ran through changed with temperature so the flexing changed with temperature?

I still haven't put my pushrods in...LOL... I can not make my mind up on which style to use..LOL

For that airplane and where you are on the learning curve 'Smart money is as per plans'

Absolutely. That's a no-brainer. Go with the original plans and save the exotic stuff for later builds.

CGr.

When you layout your bend to go through the rudder's pushrod exit, do you have the rudder centered, hook your clevis to the horn and bend dead center of the pushrod's exit slot? Seems like this would let the throw go both ways..right?

edit: I'm talking about the bend so the threaded rod ( 2-56 ) can go through the exit.

Yes The idea is to keep them straight no matter what you use...I have often used the wooden dowl type as well...I support them a bit though just by putting lite ply cross braces under and over them at a former or two..Just so they cant move around too much...You dont want them to bow or bend if possible...I think they are one of the best type you can use I would only use balsa if it was fairly hard and strait...I like hardwood dowl or arrow shaft or carbon fiber tubes . I have never had them do any damage to a servo or anything else..It seems like they are are one of the old ways of doing things..You dont see them in use as often as you used to... I would just use whatever your kit came with , follow the plan