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Pull-pull setup

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Old 12-09-2014, 09:42 AM
  #1  
Roo Man
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Default Pull-pull setup

I am setting up my first pull-pull rudder control on a Yak 54 70" from ARF mfg. sold by General RC.

Based on my initial review it appears that the cable will rub on the slot that the cable goes through in the fuse. I am using the widest dual servo arm that fits into the fuse and am crossing the cables. I thought of lengthening the slots but there is structure just forward and just aft of the slot. I used a pc of rod to check the alignment and don't see any way around the problem.

I tried to get info through search tools using pull-pull setup without success. If you can send me to a forum that covers this I will appreciate it.

Any suggestions.

Gary

Last edited by Roo Man; 12-09-2014 at 09:52 AM. Reason: left out info
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Old 12-09-2014, 09:58 AM
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Bozarth
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Just put some outer pushrod tubing in your fuselage and let the cables run through it. Any small tube (antenna tube, Gold'n'Rod housing, etc) will work.

Kurt
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Old 12-09-2014, 10:10 AM
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I hadn't thought of that, good idea.

Thanks,

Gary
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Old 12-09-2014, 10:55 AM
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speedracerntrixie
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Originally Posted by Bozarth View Post
Just put some outer pushrod tubing in your fuselage and let the cables run through it. Any small tube (antenna tube, Gold'n'Rod housing, etc) will work.Kurt
+1 I have done this several times. The hard part is getting the geometry correct so one cable does not go slack.
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Old 12-09-2014, 12:58 PM
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Depends on the setup but i see a lot of people chasing that and in fact, when there's tension on the pulling side, the air load will keep the slack cable from causing any issues.

The Golden Rod sleeve is an A+ idea, and for a few bucks at the local fishing store, you can replace the cable with 60 lb coated leader.

If you really want to save time and aggravation, hit up Aztec Aeromodels for a set of Seacraft cable tensioners
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Old 12-09-2014, 01:11 PM
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Andy, if the load was constant for any particular deflection I would agree but we both know the load is not a constant. I have flown airplanes that were set up correctly and airplanes that were not. I can tell the difference. Some airplanes such as my CARF Extra 330 will let you know in a hurry, if you are hard into the rudder with one cable slack with a fair amount of speed the airplane will oscillate badly. An example of this would be during a rolling circle.
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Old 12-10-2014, 07:16 AM
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Rodney
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Andy is correct, it is always best to have a little positive Ackerman in the pull-pull set up where the non pulled line goes a bit slack as the surface is moved from center. It is nigh on impossible to get any flutter or oscillation from this occurrence.
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Old 12-10-2014, 07:59 AM
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Originally Posted by Rodney View Post
Andy is correct, it is always best to have a little positive Ackerman in the pull-pull set up where the non pulled line goes a bit slack as the surface is moved from center. It is nigh on impossible to get any flutter or oscillation from this occurrence.
I would like to know how you came up with this? I have experienced this a few times. What I believe happens is that during a rolling circle or a flat turn while hard over on the rudder as the airplane is basically in a skid the buffeting airflow off the stab makes the rudder oscillate. having both cables taught solves the issue. This is well documented, try looking up CARF Extra 330 tail wag. It happened so badly to one pilot in the 2002 TOC that he actually lost his rudder. True, it's not common but it can happen. The best prevention is to set up the pull pull correctly from the get go.
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Old 12-10-2014, 08:05 AM
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Thanks to all. With my flying I don't think a slight amount of slack will be a problem..
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Old 12-10-2014, 11:31 AM
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Rodney
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Originally Posted by speedracerntrixie View Post
I would like to know how you came up with this? I have experienced this a few times. What I believe happens is that during a rolling circle or a flat turn while hard over on the rudder as the airplane is basically in a skid the buffeting airflow off the stab makes the rudder oscillate. having both cables taught solves the issue. This is well documented, try looking up CARF Extra 330 tail wag. It happened so badly to one pilot in the 2002 TOC that he actually lost his rudder. True, it's not common but it can happen. The best prevention is to set up the pull pull correctly from the get go.
How did I come up with this? Easy, about 20 years of using pull-pull on mostly 1/4 scale models but also on many smaller ones. Simple physics will confirm this. The only way that the surface could move further in the direction the tight line is pulling it is to have some external force trying to further push the surface (an impossibility unless you are in a tail slide at which point you would not have any control anyway). I'd suggest that the problems you reported on were more probably caused by flutter due to structure imperfections rather than the pull-pull system. Such things as bad hinges, flexable surfaces, etc.
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Old 12-10-2014, 11:49 AM
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So does writing your reply in red make it more factual? So let's talk about how I came up with my viewpoint. 17 years of competing in IMAC, starting with the sportsman class and working my way into unlimited. One of those airplanes you claim to have had bad hinges, structure imperfections or flexible surfaces was the advanced class points leader in 2006 when the TAS applications went out. At the end of that season that airplane finished second in the SW region championship. Competing at this level will highlight setup issues that sport flying will not. Does this matter or apply to the OP? Maybe not but should we suggest models be set up correctly or should we dumb it down because we deem it " Good enough " and not let the OP make the decision himself? Moot point as the OP has made his decision and I wish him well.
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Old 12-10-2014, 07:26 PM
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Well, with an ARF it's a moot point, you get what you get and the geometry is going to be off. When I first started using pull/pull I hadn't heard of ackerman and always had slop on one side, like a lot!! The planes flew just fine but I did notice that when I was on rudder turning one way there was a lag when going back to neutral or the other direction, stands to reason there is no tension pulling the rudder for a while. Setting up the geometry is an art and time consuming but worth it on a good stunt plane. If there is slack in the line then the wind is just blowing the rudder back to neutral, not servo tension.
The OP just asked about keeping the wires from rubbing and the best thing I found to keep my fuse from getting sliced is the yellow Nyrod. I make up my systems using braided stainless steel wire fishing leader, it comes in a lot of different weights and I buy rigging couplers to attach it all together.
I start setting up the ackerman during my builds while I can still get at everything, something you just can't do well with an ARF.
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Old 12-10-2014, 08:52 PM
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Geometry is the key to a pull-pull system , the length of the control horn and the dual servo horn have to be the same width at the attachment points with the rudder attachment on the hinge line and a straight servo horn. If there is off sets in one there has to be an off set of the same amount in the other and the cable tension will always be equal. Simple as that.

Leroy
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Old 12-11-2014, 04:34 AM
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If the cable rubs that effectively shortens the cable if it's any more than just a touch. It's the same effect as when you grab the cable with your fingers and pull. So a rubbing cable can foul up a perfectly good pull pull setup. When I got into IMAC, I played around with control horn lengths until I got the tension darn near perfect. I can pluck the cables and hear a low bass note at neutral, and then pluck them again and hear a slightly lower note at full deflection. That gives me the smallest amount of Ackerman but not enough that the cables go slack. It wasn't hard to do with a threaded control horn on the rudder, so I really have a hard time seeing what the big deal is whenever these threads come up.
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Old 12-11-2014, 05:27 AM
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Originally Posted by jester_s1 View Post
It wasn't hard to do with a threaded control horn on the rudder, so I really have a hard time seeing what the big deal is whenever these threads come up.
I don't get it either. It takes so little additional effort to do this correctly. Like Gene said, sometimes you get an ARF that requires additional work above a kit that someone had the foresight to set up correctly from the get go. I'm going to climb up on a soapbox here for a bit. I try to help guys here on RCU on setup issues a lot. some times it goes smoothly sometimes not. If guys want to be lazy on their setups and have a poor flying airplane then so be it. What kinks my fuel line is when someone comes around and try's to invalidate good advise because of their low expectations. Maybe it's just a case of " I don't get it " on my part but when I was a young man when an experienced competition pilot offered help I listened. I was fortunate to have these guys around. With the internet it's much easier to reach out and find help, however it's much easier to be offensive without accountability as well. I'm not a big name pilot so when I give advise I tend to give reasons for the advise. The only person in this thread who has seen me fly in person is Gene ( Greybeard ) I think he would agree that all the little setup tricks I take the time to perform are well worth the effort.
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Old 12-11-2014, 07:18 AM
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Originally Posted by Leroy Gardner View Post
Geometry is the key to a pull-pull system , the length of the control horn and the dual servo horn have to be the same width at the attachment points with the rudder attachment on the hinge line and a straight servo horn. If there is off sets in one there has to be an off set of the same amount in the other and the cable tension will always be equal. Simple as that.

Leroy
+1
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Old 12-15-2014, 07:23 PM
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Please excuse duplicate post that I deleted.

Thanks,

Gary

Last edited by Roo Man; 12-15-2014 at 07:34 PM.
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Old 12-15-2014, 07:32 PM
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It' me again, the OP.

Came back to see if any new info was available. After reading the additional input, I am going to try and set up with no slack on the non pulled cable. Let me make sure I understand some things:

The horn on the servo must be the same length as the rudder contact points are wide.

Maintain as little contact of the cable to the structure.

Please explain what Ackerman is.

Thanks,

Gary
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Old 12-15-2014, 08:02 PM
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Gary, the servo arm does not actually have to be the same width as the rudder horn but doing so simplifies the process greatly. The easiest way to set it up is to have the two the same span. Then the rudder pivot points need to be right on the hinge line. If they are not and are say 1/8" back from the hinge line then you have to have the same 1/8" offset aft. Ackerman is using offset to the correct side of the pivot to either increase or decrease the tension through the travel. I personally run a touch of negative Ackerman so that the cables actually get a little tighter through the travel. I however use a ball bearing supported tiller arm that the cables are attached to. This keeps that additional tension off the servo output shaft. When the tiller or servo arm has the same length as the control horns and you do not cross the cables then your cable exit will be farther forward on the fuse. Sometimes you need to think function over looks. I hope this and the pictures help. Feel free to ask questions not only on this subject but anything else about aitplane setup.
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Old 12-15-2014, 08:54 PM
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That is one beautiful install!

Kurt
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Old 12-16-2014, 04:24 AM
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Dat's purdy!
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Old 12-16-2014, 06:02 AM
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You can make things very easy on yourself by using a threaded control horn on the rudder. The DuBro kit is good quality and is what I used on my 50cc Extra. I used a 3 inch long servo arm for direct pull pull control with the cables not touching anything on the way out to the rudder. Yes, I wound up cutting a really long slot down the sides of my fuselage which guys at the field will comment that I could have avoided by crossing my cables, but you can't see them in flight! With the threaded horn, you can do a fine adjustment of the length to compensate for any geometric issues that you have. If you see that the cables get tighter when you deflect the surface to the left and looser when you deflect the surface to the right, you shorten the right horn and lengthen the left one and try it again.
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Old 12-16-2014, 06:08 AM
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Speedracerntrixie

Thank you for the very informative reply. I do however have one question, You stated below that you have to have the same 1/8" offset "aft", where is aft, at the servo?

"If they are not and are say 1/8" back from the hinge line then you have to have the same 1/8" offset aft".

Gary
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Old 12-16-2014, 06:14 AM
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Gary, yes that was a bit vague. If the rudder control horn has the pivots 1/8" behind the hinge line then the pivots on the servo arm would need to be 1/8" behind the center of the servo output shaft. The offset has to match at each end in dimension and direction. Unless the servo arm is shorter and/or the cables are crossed then it starts to become a trial and error sort of thing unless you are a math wiz ( I am not so I keep things a symmetrical as possible ).
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Old 12-16-2014, 10:18 AM
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Thanks to Jester_S1 and Speedracerntrixie for the additional indo,

I am using a DuBro 3" servo arm and must cross the cables due to aircraft structure. Will be able to keep both points on center line of servo and the rudder hinge line. Will use tubing at cable slot.

Thanks again,

Gary
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