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 ;)

This begs the question Why doesn't somebody produce dual sided servo arms that are offset? Although (and this assumes perfect alignment and angles of hinges and horns, which is NOT going to happen in my builds anyway except by accident, HA HA) that setup would induce binding, albeit a very small amount, geometrically...

You mean like these... They make them for JR, Futaba and Hitec.

http://www.rcuniverse.com/forum/atta...mentid=1935239

Bob

No I meant one side higher than center, the other side lower than center - so that if you crossed lines they wouldn't touch. Actually I have a couple of those and will be getting more. Way worth the extra cha-ching for the metal arms, IMHO.

I run pull/pull on rudder and elevator on all my pattern planes.

I use nylon coated, braided stainless steel fishing trace and it is available in diferent sizes. The nylon coating prevents any wear through exit holes or if the wires touch.

Kevlar will fray at the exit holes.

Crossed or straight doesn't really matter but in alot of cases, especially on smaller models, crossing the wires gives a better set up.

Well, if your heart is set on that, you can always install one clevis above the arm and the other below.

Regardless, coated cables rubbing on their cross over points has never been a problem. If one uses bare steel cables, it might cause RFI so I would never use uncoated cables. Of course Kevlar eliminates any possibility for RFI and teflon coated kevlar is the slickest there is at fuse exits....I remove a small length of teflon coating when tying the knots and dripping ca on the knots. No need for ferrules

The biggest advantage to running nylon coated steel cables is the fact that connections to theaded rods or eyeloops becomes trivial. What we do (In Pattern circles) is to fold the nylon coated end over and run a match flame over the nylon insulation, remelting the nylon slightly. Many guys don't add any ferrules and swear that none are necessary but I always have (in the distant past when I used steel).

All of my planes have the pull-pull system criss-crossed, it usually does give a better straight shot to the control horn. I run all of my wires through plastic antenna tubing so they do not rub against each other.


Good diagrams Sensei...

Vincent, that's a clean install, thanks for sharing !

I'm liking that install... not quite sure why, but the tailwheel does NOT turn off the rudder, I need a seperate control rod for the rudder... but I will run it off the same servo, just in a hole from the rudder clevis.

Dale, another set of cables can be run from your servo arm directly to the tail wheel tiller for a more responsive action if need be. I've always been happy with how well the response has been with this type of set-up...

McMaster Carr has some amazing cable for this use even though that is not what it is intended for. I use a vinyl coated cable they offer that is seven strands of seven strands, if that makes sense. It is as flexible as string but is steel. The great part is that you can tie it in a knot instead of crimping. After it is tied you heat the knot with a lighter to just melt the vinyl. The result is a permanent knot. I use it on my 30cc and 50cc planes. Have used it for two seasons and it works great.

McMaster item # 34235T23

FYI, they have fuel line of all types and an amazing amount of items useful to us.

Vasek,

Have you ever ordered anything from this company in the Czeck Republic?

www.aerocockpit.com


AEROTEAM, Ltd. Svarov 922
755 01, Vsetin
Czech Republic

They show a really nice custom cockpit for my Skyshark P-40 but I am concerned if they are a good company to deal with.

Thanks for your help.

Regards,

Keith

Keith, yes and no. I did buy this product but from a distributor overseas, believe it or not it was cheaper then buying it here. Go figure...

The product is very good, the company I can't say, but I always get better service from the "western block" LOL.

Hope this helps, V.

I use pull-pull all the time. I have a 1/4 scale vintage sailplane with cable operated everything and I mean everything! Cables from the joystick and rudder bar travel over pulleys to the control surfaces including the ailerons. Some of the cables are routed though ferrules. I use products from Proctor-enterprises.com. Joe has everything you need: cables, swages, turnbuckles and all kinds of nice fittings!
http://www.rcuniverse.com/forum/atta...mentid=1935469http://www.rcuniverse.com/forum/atta...mentid=1935470

ARUP,

I could barely stop admiring the construction and in particular the joint reinforcements.

Not many people believe the considerable strenght increase for very litlle added weight.

Nice work well worth a bit more tme in building and the result difference in a bad landing.

Zor

Ok, I have a good idea of how I am going to proceed...but I'd like to see the connections at the rudder/elevators if you guys that have Midwest SS's have photos or can take photos of your setups. Just got home and mocked up the tail section, and if I want to put the horns UNDER the horizontal stab (which of course I do), there is not much room between the rudder and the elevators for an offset of the size I would need...