talon58

My Feedback: (125)

Anyone got any input on how tight the pull pull wires should be on a pull pull setup?

Just want to make sure mine are not unnecessarily tight.

Thanks.

DMcQuinn

My Feedback: (13)

Guitar string tight is too tight. I have allowed mine to get pretty loose at times and there seems to be no problem. So I don't think it is really that critical. If they visibly sag, then that is probably too loose.

Rodney

Keep as slack as possible without loosing control. Tight lines will increase servo current and servo wear.

the-plumber

Actually, in the Ackerman system favored by some, one line indeed goes slack when the other line is under tension.

Tight lines won't do anything to the servo current, but without a tiller bar tight lines will certainly accelerate servo wear (and failure).

NOVAflier

My Feedback: (21)

I disagree, tight lines will put tension on the servo, putting it under load (just like binding will in a pushrod). If the servo is humming or buzzing in a stationary position, it is under stress and eating at the battery.

Stick Jammer

It's interesting how many different opinions there are on this subject. As post #2 mentioned, they don't need to be "guitar string tight".

In reference to the above term "tiller bar", I'm assuming that a bellcrank is what we are talking about? It's doubtful that the bearings of the servo output shaft will see any excess wear without a bellcrank. As soon as the servo moves from neutral it experiences the exact same load with or without a bellcrank if the geometry is correct.

Tight lines won't put a load on the servo unless the rudder is binding. The pressure of the tight lines will be equal on both sides of the servo arm resulting in more tension on the servo output shaft but the servo motor won't be drawing any more current at neutral.

NOVAflier

My Feedback: (21)

Thank you for the correction, after thinking about it, you are right..Regardless, still not an optimum condition for a servo.

Stick Jammer

I agree, excessively tight is not necessary and should be avoided.

the-plumber

Two different loads.

The primary main load is that of moving the flight control surface, and except at neutral one of the two lines will have a load; at neutral both lines should have equal loading.

The secondary load is the shear loading on the servo output shaft and bearing (or bushing), caused by excess tension on the lines, and that tension will be relatively constant unless the geometry incorporates the Ackerman principle.

Brian Felice describes his method of implementing Ackerman at http://members.cox.net/bdfelice/Pull-pull/pull-pull.htm

In a straight pull-pull setup, sans Ackerman, the side load derived from line tension is constant, hence accelerated servo bearing wear.

In the case of ball bearing servos that wear may not evidence itself for a considerable time, if ever. In bushed servos, and there are a _lot_ of those things in use, the bushing wear will become evident in fairly short order depending on the amount of excess line tension.

Yes, 'tiller bar' is the old term having been supplanted by 'bell crank' more recently.

Personally, I avoid undue loads on servos because I work them hard enough as is.

That a lot of builders have line tension pulling on the servo output shafts and bearings is a given, but the question is how long will the servo tolerate that abuse ? I don't want to find out in a model costing several thousand dollars.

Stick Jammer

Are you saying that a given servo will experience a greater side load (flight load) without a bellcrank incorporated into the pull-pull set up? As far as anyone using a bushed servo for pull-pull (or for any surface) in a GS, well, not the best choice for sure.

Diablo-RCU

My Feedback: (1)

And now for the other side. I have a Wild Hare Extra with a single 5735 Hitec on pull-pull to the rudder. Cables attached directly to the servo arm. They have tension at neutral. Some planes will hunt or tail wag in the air if you allow the rudder cables to go a little slack at neutral. This can also be a problem using a bellcrank or tiller bar. I test for this by turning on the receiver switch and finding out how much free play is in the rudder. How many degrees of movement before the servo stops it? If your plane doesn't tail wag, then your cables are tight enough. I have used the same rudder servo with original parts for several hundred flights in 2 different planes.

the-plumber

No, I'm saying that a given servo in a pull-pull setup sans bell crank will be subjected to a static side load which has nothing to do with the operation of the flight control surface. Excess line tension causes that static side load and contributes nothing to the operation of the system.

Again, _excess_ line tension. Since there are few, if any, tools in the sizes we need for measuring line tension, the variations run from full slack to banjo tight, and the ideal is obviously somewhere between those extremes.

The objective of using a bell crank is to shift any pull-pull cable tension loads, not flight loads, from the servo to the bellcrank. The effect is to reduce the line tension to a "don't care" with respect to premature wear on the servo.

Diablo-RCU

My Feedback: (1)

Hmm. What part of the servo is going to wear with excess cable tension? The ball bearing can handle the load. If you use a tiller bar, you just move the static load to the ball bearing in the tiller bar. Adding parts, weight, complexity, slop, and cost. What's the benefit?

Stick Jammer

So in essence what you are saying is that a pull-pull with direct connection to the servo will prematurely wear out the servo bearing while it is sitting in the neutral position?

Wouldn't the tension at neutral be equal on both sides of the servo arm? How does that create a static side load? Again, we are not talking about the cables being tight enough for dueling banjos.

Tired Old Man

My Feedback: (1)

Back to the original question.

Having the cables too tight will not provide any room for minor changes in the aircraft that take place as temperature and humidity changes. I've seen a rudder tray get pulled out of a fuselage after a plane left from a cold and dry climate and arrived at a hot and humid climate. The cables were too tight.

Too loose and the cables will permit the dreaded so called "tail wag", indicating less than positive control at all positions or settings.

A low pitched "thrum" when you flick the cable is usually ok. If the cables are not hanging loose, or causing the servo to howl due to over tensioning, you will be just fine.

the-plumber

I'd have thought that rather obvious.

The benefit is moving the line tension side load from the molded plastic servo ball bearing >mount< over to the hard wood bell crank mount, which is ever so much cheaper to replace if need be.

the-plumber

Alas, only one silver bullet left for this issue.

I only know of two ways to implement a pull-pull system where line tension is not absorbed by the servo. The simple case is a bell crank, and the line tension is isolated between the bell crank and the control horns on the flight control surface; the servo does not 'see' the line tension. The complex case is to attach both cables to the same end of the servo output arm, with one cable running direct to one flight control surface horn. The other cable departs the servo arm in the opposite direction as the first cable, runs around a pulley, and then to the other flight control surface horn. In this case the servo doesn't 'see' line tension, it is absorbed by the pulley.

Anything else means that the servo is having to absorb line tension, and that's a side load because the cables are (preferrably) in the same plane as the arm; since both cables are on the same 'side' of the arm (the side toward the flight control surface) the cables when under tension are trying to pull the servo arm toward the flight control surface equally.

Pull-pull line tension, how ever high it is, should be removed from the servo if possible. Servos are expensive; bell cranks aren't.

I dunno what line tension you use. I don't even know what line tension I use, 'cuz I can't find a tensionometer small enough to work on models. I know what "feels good" to me, but you might think it sloppy and the next guy might think it too tight. The solution is to remove line tension from the servo altogether and let a much less expensive bell crank absorb the line tension load.

I'm not talking about banjo tight either; somewhere between saggy lines and humming lines, if you get my meaning.

Lastly, I am also not talking about bushed bell cranks, I'm talking about double truss double ball bearing >zero slop< bell cranks as shown in the photo - yes, the model isn't complete yet and the cables aren't attached, but the bell crank is for the rudder on a 1/3 scale L-4.

Cheers,
Fred McClellan

RickP

My Feedback: (2)

If they are too loose you will know it because the tail will wag. Obviously the looser you can tolerate and maintain control the better. I have found no references to how tight they should be. I am struggling with this on my 3 meter because I am having some tail wag and the cables are as tight as I dare make them....
RickP

Rodney

I agree whole heartedly with The-Plummer. I always use a pulley instead of a bell crank just aft of the servo so that all my pull-pull lines go from one horn, around the pulley and back to the second horn on the flying surface. The pulley is then connected to the servo with a very short heavy duty 4-40 arm and ball links. This makes it easy to change servos, easy to change throw angle as the pulley doesn't care how long the control horns are (they always let out as much cable as they reel in) and does not put excessive loads on the servo bearings. I keep my lines just tight enough to not slip off the pulley tracks and have never had any tail waging. Oh yes, I use the pull-pull on the elevators too.

JoeAirPort

My Feedback: (41)

Maybe you don't have enough servo or the mechanical advantage is not good enough. Just an idea.

Stick Jammer

My feelings as well. Here's what it boils down to guys. It doesn't matter in the least what method you choose to hook up a single servo pull-pull, the servo is gonna see the exact same flight loads if the ratio is 1:1. If the cable tension at neutral is reasonably tight, the pull side is gonna get MUCH tighter as soon as the rudder sees a flight load. It stands to reason that if the servo can handle the tension under the flight load, it sure as heck can handle the small amount of tension when it's sitting at neutral.

RickP

My Feedback: (2)

I've got four 8411's and I think that should be enough. I'm going to consider using a seperatly supported bellcrank and 8611's if I can't fix it this weekend.
RickP

CDignition

My Feedback: (9)

Rick, make it even tighter...My 2.6M did this same thing, and I just snugged em up...it wont hurt nothing..

cloudancer03

My Feedback: (22)

I enjoyed the conversations as I am about to install pull_pull..can you tell me a good assembly to purchase..It will be in a giant stinger and I am going pull -pull to keep weight out og the tail..I am using a single servo of high performance and am comfortable.. do the wires cause any concern with respect to radio interference..do you use kevalar or steel wires..