Pull-Pull system advice needed...
 

I'm putting a pull-pull system on the rudder of my Midwest Super Stearman. I have the Sullivan S-521 kit which is Kevlar. I'm thinking that will be strong enough. I also have a 3" aluminum dual-sided control arm on the servo. Any tips or ideas on how to run it out? I do have control rod exits I intend to use, will they be ok? How taught should the cables be?:confused:

As long as the kevlar line doesn't rub across a sharp corner it should be OK. You need to get all the slack out of the system, but you don't need to put any extra tension above that.

+1
just be aware that one cable (outer) will go a little slack when rudder is deflected & that's normal. Don't over-tighten the system.

Cheers, V.

Hello DaleJEckart,

Unmentioned previously is that the cables should not have any elasticity. They should not be able to stretch in length.

The differential motion is obtained by having the cables at the servo attached about 5 to 7 degrees ahead of the servo axis of rotation; Closer to the front of the model.

Tension has already been explained.

Zor

Here is an option for you from SWB Manufacturing, I have used these on my giant scales in the past and they are vary nice quality with all the geometry done for you and everything needed for the installation in a neat little kit... Just google SWB Manufacturing for further details if your interested.

http://www.rcuniverse.com/forum/atta...mentid=1934649

Bob

The Kevlar apparently does not stretch... not quite sure about differential motion...

Whether one line goes slack or not depends on the geometry of the servo arms/rudder horns. It's called "Akerman" geometry. Here is an explanation: http://www.qmfc.org/school/ackerman.htm

As radfordc said, the Ackerman factor is very important. You definitely do not want any negative Ackerman. IMHO a bit of positive Ackerman is desirable. As for tension, you want just enough to not have any slack in either line when the surface is a neutral.

I am familiar with Ackermann from setting up race cars. I'll give it a shot on this plane, maybe mount the control horns 1/8" back.

I believe that what you wrote will give you negative Ackerman.
In pull-pull we want the wire doing the control under tension to travel a bit less then the other wire.

What I referred to as differential.

Zor

Kevlar in this application does not in fact stretch to make any difference in control. But it does relax over time and will probably need some minor retightening. It will work fine. I've used Kevlar (braided material that I built and covered myself with teflon (not for sale so don't even botherto ask)) for at least 25 years.

In a pull-pull rudder application, any Ackerman effects have been negligible......the pulling side always is tighter than the other side. Tension is very light, just enough to be snug, but not tight. No guitar strings needed.

I use pull pull on elevators too because it is the lightest arrangement and allows 4 points of adjustability.

No, if the horns on the rudder is aft of the hinge line (and the servo arms are on the center) you will get slack in the unpulled line as desired for positive Ackermann. Eckart is correct.

As MTK suggests, a little slack on the wire side that's NOT pulling is not a bad thing, so why bother ?

Remember the KISS principle. I tend to over-think everything I do @ the beginning, but luckily my "lazy spirit" keeps me well within the KISS parameters @ the end, and I'm glad it does ;)

Cheers, V.

I used Kevlar on a few installations and though it doesn't stretch, attaching the end fittings seemed to to be a pain at times. These days I just use wire cable from either hobby sources or fishing tackle suppliers. The cable seems much more tolerant overall than Kevlar. You can get anything you want from the large fishing tackle suppliers including bare steel cables in several different alloys, coated steel cables on through Kevlar and other composites. They also have the sleeves, crimp tools and most anything you'd need for a control cable in a variety of strengths and sizes. Much cheaper than hobby sources!

Rodney,

I agree that doing it at the rudder or elevator will work fine.
I used to do it at the servo because it is easier to install a new servo arm if the amount of differential has to be changed.

I find it easier than moving the horns at the control surfaces which have already been drilled in position.

Thanks for bringing this up.

Zor

TR,

I use eyeloops for the clevises. These are standard McMaster Carr eyeloops, the smallest they carry, which are made from .078" wire. The threaded section is long enough for just about any type of clevis we use. The loop section has no sharp edges to ruin your day. Almost impossible to wear the set-up out. Very cheap, about 3$ for 100 if I recall

What I do with kevlar is simply tie it to the eyeloop and drip one drop of ca on the knot. Then a short piece of shrink tube of appropriate color to match my paint scheme....QE...DUNN

I can't help with the pull-pull but your going to love the Super Stearman.


Tom

Dale,
I see that nobody has explained to you that you should cross the lines-in essence the left side servo arm should deliver the pull to the right side control arm and visa-versa...just my two cents and may the airplane gods find your stearman in favor. Good Luck.

It really depends on the setup...

Bobhttp://www.rcuniverse.com/forum/atta...mentid=1935109

Regarding crossed lines, some of my planes have crossed lines, some not. It all depends on the airplane, fuselage width, servo location, bulkheads in the way, plus a bunch of other factors. Probably the main factor would be whether the airplane was an ARF or one that I built. Seems that ARFs always require compromises where ones you build yourself allow you to set up the linkages / lines before the fuselage is closed up. I have some nifty 1' and 2' long drill bits that help with setup but they don't always solve all the installation problems in ARFs. Crossed or straight can work equally well if installed properly.

Sensei beat me to a response and posted some really nice illustrations. Good job!

What advantage does crossing the lines have? I would think you would want the lines as straight as possible to avoid any kind of rub wear... and this is the first I've heard of this, nor have any drawings showed it :(

Because usually if I don't cross them they chafe at the fuselage sides. Crossing them (For me) seems to work better and gives them a straighter shot to the control horns. I'm pretty sure that in the case of a Midwest Super Stearman that this will work as a benefit. As far as a reference as to where I heard or read about this I cannot remember-I wish I could. I do however encourage you to do your homework and research this if you haven't already :)

Sensei,

Can you post the link for that website where you found the diagram - I have been to that site and read the article but I forgot to save it. I am also setting up my P-40 with Pull Pull and I would like to read the full article again.

Thanks

Keith

I pulled it off of SWB Manufacturing's sight. (images)

Bob

If crossed, the exposed part of the wire is significantly smaller & the exit hole in the fuse looks better (better angle for drilling) it just looks better IMO

Cheers, V.

P.S. if lines are metal, I use a piece of tubing where they cross. Even better, if possible attach the wires this way to the servo horn : one on the top, the other on the bottom. Of course that depends on the attach point type used. I use the ball link type that screws onto the s. arm so I can put one on top & one on the bottom, wires never touch ;)