Pull - Pull system.

[Ross Clarkson]

Hello,

 

I am currently finising off my Yak 54's rudder pull - pull system.

Simply, how tight (tension wise) should it be??

 

Thanks,

 

Ross.

Edited By Ross Clarkson on 25/11/2010 18:20:34

[Garry Pollard]

Got the system in my Extra 260 Ross, rudder control  and I tighten up so there is no slop. Has worked ok for two yrs  doesnt need to be drum tight

Garry

[Biggles' Elder Brother - Moderator]

Ross, there should be no hint of slack, but equally you don't need them bar tight either.

 

Perhaps the best way I can explain it is;

 

 if you pluck the wire, like  guitar string, you should get a definite "note". But it should be "plonk" rather than a "ping"!

 

BEB

[Pat (rActive) Harbord]

Spot on BEB, that's a great description.

[Stephen Grigg]

Ive only done 1 pull ,pull,system on a Tiger moth,it was already in the fuselage,but still made it a  bigger job than need be,but it worked out fine in the end.Only flew it once,grossly underpowered  so sold it.Took longer to do the pull pull than I managed flying it.Ive a Red Zephyr that needs the rudder changing,so if you can put lots of photos on I can get some ideas

[Bert]

Ross

 

With a pull-pull system I cross the wires, this has the benefit of moving any slack from the neutral position to the extremes of travel.

 

You must have the holes in the horns in line with the hinge obviously.

 

Having the slack at the neutral position is obviously not a good idea but at the extremes it doesn't give any problems.

 

Ali at Avicraft put me on to this when I bought my Panic kit from them at Sandown many years' ago.

 

Bert

[BB]

Ditto Bert, crossing the wires does exactly as he says.  And no, there isn't any wear at the cross over.

 

Opinions do differ but anyhoo below is a link with some info etc...

 

Pull, pull

 

BB

[ken anderson.]

ross-i've used the pull pull on a few model's......apart from the plonking/plinking and pinging.......i make sure that there is as little slop(movement)...when the servo is at the neutral position as poss.......that does me......

 

 ken anderson    ne..1.

[Danny Fenton]

I use pull pull, closed loop whatever you call it all the time for rudder and elevators on my models and provided you mount the control horns exactly on the hinge line and the distance between the centres of the holes on bot the servo arm/Bellcrank and the control horns then there will be no difference in tension throughout travel.

 

[r6dan]

When I did it on my Acrowot if it was too tight the servo "buzzed" as said before cross them too, also make sure you have enough adjustment to take up any slack that occurs from the stretching if the wire during use.

Edited By r6Dan on 26/11/2010 10:09:36

[Ultymate]

Only reason I would cross the cables is on some models it makes it easier to exit the cables through the side of the fuselage, avoid slack at neutral but not tight as all that does is load your servo up unnecessarily. Don't get anal about slack anywhere other than neutral as only one cable works at a time ie the one pulling and you won't notice any affect in flight

[Ultymate]

Sorry double post 

Edited By Ultymate on 26/11/2010 10:17:52

[Biggles' Elder Brother - Moderator]

I agree with Ulty. Slack on the non pulling side during operation is no problem. The pressure of the air on the control surgace sees to that. We you re-centralise the other side isn't required to do all the pulling back as again the air pressur helps.

 

So just avoid unecessary slack, and plonk not ping! If you ping, as Dan says, you just put unecessary side load on the servo bearings.

 

BEB

 

PS I personally think closed loop cable is the best system for moving control surfaces, and it is my method of choice. Its more positive, its easy to adjust, its light weight, etc.

[Ross Clarkson]

Wow, cheers for the responses guys. I have set them up at the mo, without crossing them and I think i will leave them like that at present. I understand what you mean about one being slack whilst the other takes the load. I have secured the wires to the horns by looping them through the wire retainer then back through again! This is how the manual suggests and it was a super tight fit, but I was wondering. Do you leave it like this or do you secure it some other way as well so the wires dont slacken? Really appreciate the advice guys, and I have to agree, first time I have used this technique but it seems to be very good. Nimble fingers needed though!

[Garry Pollard]

Leave it like that Ross, I did and mine has never mooved

Garry

[Malcolm Fisher]

I have used fishing trace with the matching crimps to make closed loop systems. The crimp makes sure that they can't come loose.

 

Malcolm

[Chris Bott - Moderator]

 

 

Posted by Ross Clarkson on 26/11/2010 19:49:28:
by looping them through the wire retainer then back through again! 

 

Ross have you "crimped" the brass ferrules too by squeezing them with some pliers? 

[Biggles' Elder Brother - Moderator]

Yes Ross. You should crimp the ferrule with a pair of pliers for security. Do you have either threaded end clevises or a threaded joint in each run for fine adjustment?

 

BEB

[Ross Clarkson]

Cool, cheers guys. I will crimp them. Yes, the wire loops through an adjustable buckle which is then screwed into a clevise. Cheers again guys.

[Danny Fenton]

This is how I do mine, nylon coated steel trace. a drop of thin ca once its crimped and I have never had one slip.

 

Note: this shot is taken before being crimped

 

Cheers

Danny

Edited By Danny Fenton on 27/11/2010 07:50:29

[Peter Miller]

I have used the closed loop several times.

 

I always use an idler bellcrank just behind the rudder servo. This has several advantages.

 

1) There is no side load on the  servo bearings if the cables are tight.

 

2) by connecting the servo to the bell crank with a threaded rod and quick link you only have one easy connection to adjust the rudder position.

 

3) You can remove and refit the servo very easily without disturbing the rudder connections.

 

4) It is no longer vital to locate the rudder servo in the middle of the fuselage.

[Danny Fenton]

I completely agree with Peter, especially on larger models.

 

Pushrods to bellcranks moves the load away from the servos, and makes adjustments/servo swapping easy. Though on small models I must confess to going straight to the servo

 

D

[Ultymate]

That won't take any of the "workload" off the servo/s just any load that's applied through over tightened cables, sorry I just don't see the point  I've have plenty of models up to 140cc with pull pull straight onto the servo/s and never had any issues, obviously I do use servos suitably specified for the job in hand. 

I have heard of cases of multiple servos ganged up on rudder etc where one servo fails , jambs up and burns out others in the gang but there are systems to avoid that (see photo) with this set up should one servo fail you will still have rudder all be it with a reduced amount of rudder throw 

 

 

[Danny Fenton]

Yes,  Ultymate you are quite right it doesn't remove the loads, I probably wasn't clear in what I meant. Bellcranks remove the constant side load that is exerted through the cables being under tension. Larger servos such as those you show above probably have decent bearings and can cope. The cheaper servos that some of us may be using will benefit from being protected from the side load.

 

D

 

 

[Biggles' Elder Brother - Moderator]

Clever set up that Ulty - not seen that before.

 

I sometimes use an idler bar but I must confess that is usually more to do with routing the control cables around something else - like the cockpit area in a scale model. It also has the convience advantages Peter spoke of - ie being able to remove the servo without disturbing the closed loop system.

 

I don't fly very big models so I can't comment fully. But I don't think the side load issue is particularly a problem even of large models - provided: you don't over tighten the cables and you use a suitably sized ball-raced servo.

 

BEB