Installing a rudder pull-pull system
 

When setting up a rudder pull pull system, just how tight should the pull pull cables be so as not to preload the servo output shaft bearing :confused:

Karol

About this tight!!!:) OK, got the smartass over with. With digital servos I tighten them up on both sides and sometimes get a bit of that servo hummmm but not tight enough for the servo to be jumping or centering. I allow no slack at all but it's a feel kind of thing. I set it up with the plane turned on so I can hear and feel what the servo is doing. When I think I have it I try to move the rudder by hand then I can feel the tension on the servo.
Does that make any sense at all. I want the servo to hold the rudder in place during flight without moving. You can get the digital humm with just the added weight of a control rod so it's more of a feel thing.:confused:

Like Grey Beard said its a feel thing.. I just take the slack out.. If you make it too ,tight you have a hard time hooking up your clevises..

Like Gray said it is a feel. I hook the whole system up, turn the radio on and try to move the rudder. If you look at the cables they will look like they are too loose so don't worry about that. Just ad tension until the rudder will move 1/16" or less.

David

Thanks guys, and Gene if you never made such a crack I would think you were not feeling well :D

The only good reason to have pull, pull is if you have a tail heavy condition, otherwise servo direct in the tail is the best way to go. If you must go forward with the servo's then you can build or buy a setup that places 0 tension on the servo in a pull, pull setup. This setup comes from a company called SWB manufacturing and sells for $90.00, or as I stated you can build one for just a few bucks if your willing to do the work yourself. 1xtray-1__82722_zoom.jpg

Bob

"The only good reason to have pull, pull is if you have a tail heavy condition, "

Not at all.

Scale appearance, or internal fuselage construction that makes for impractical or difficult straight line push rods.

That set up you show looks likes an awfully lot to go through, versus a string from one side of the servo arm to one side of the tail, then a string from the opposite side of the servo arm to the opposite side of the tail. A lot less complicated.

Les

Your right no matter what others have to say.

From an operational standpoint a servo to pro link and pro link to control horn through ball ends is the cats meow, it doesn't get any better then that. For those that need to use pull, pull due to a tail heavy condition, the setup I show is great way to keep the load off the servo's and the cables tight so there is no slop in the system, especially for those that have larger airplanes with long cables and wish to possess a precision feel in flight.

Bob

Pull-pull is by far the best way to go for precise control. Installation can be a problem though. The main thing is to be sure to NEVER have any negative Ackerman. If you do not know what Ackerman is, there are several discussions of it in the forums. A bit of positive Ackerman is okay, in fact IMHO somewhat desirable as it is far easier on the servos. With positive Ackerman or neutral Ackerman, you just tighten the cables tight enough to have no slop or sag at neutral. If you get a bit of slack in the non pulled line as you move off neutral, you have a bit of positive Ackerman but that will not detract from the ability for precise control and has absolutely no negative effect. I set up all my 1/4 scale and many of the smaller ones with pull-pull on both elevator and rudder as it is so reliable and easy to do once you give it a try.

I have made up a system like Bob has shown and it really is slick and easy to set up. I'm a bit surprised at the $90 bucks for the one shown but it is very clean and positive. Ackerman Ackerman, how did we ever do it before we heard of Ackerman??? The idea is and was to make sure you had equal tension on the pulls without an slack in the line. If done correctly it is very positive without any delay in the rudder control. Getting it set up 100% correct is a pain though. I forgot all about the system Bob showed but wish I would have thought of it before I did the set up on my new plane. Brain farts are happening more and more. I recall a system just like it with springs installed between the linkage so there was no servo shock on landing if you had the tail wheel set up directly on the rudder. So many ways to do something this simple. Karol, what plane are you setting up with a pull/pull?? If room isn't a problem or weight then two servos set up as push pull is also a good way to do it. I haven't done that in ages either but it doesn't get any stronger or or positive then that.

To answer the OP's question; you should be able to strum a bass note on the cables when the rudder is centered. If your servo has ball bearings, there is no need for the "Johnson Bar" setup shown above, but it does add to the cool factor.

Jim

Might I suggest what I do too. I hang at least a 10# wt on the wire for about a week prior to installing. This takes the stretching out that will happen. You want to make them as tight as possible, due to they will stretch even more over time

I use nylon coated steel fishing line for the cables and have never had a problem with the lines stretching over time. I've also use the nylon fishing line (no steel innards) on the smaller models (little electric jobs) with no problem with stretching. I suggest you use just enough tension so there is no sag at neutral, to tight is very hard on the servos and causes excessive servo gear/bearing wear and higher current drain. This has proven to work for me for tha past 20 years with no problems.

That statement is completely incorrect, take a look at IMAC competition today, most all that have been competing for over a couple of decades have gone through the pull, pull era and reverted back to servos in the tail as close to the control horns as possible, why? Precision thats why, even the high end kit manufacturers like Carden, Dalton and a few others design around this concept. At least for me a good reason for going Pull, Pull is a tail heavy condition as I have already stated, but if I were to come out tail heavy then you can bet I would go with the SWB pull, pull setup because it is a properly engineered setup to keep the lines tight and the strain off the servo's, not because of the cool factor LOL... By the way SWB also sells a gray coated cable guaranteed not to stretch, it was the only cable I ever found that did not stretch when I was flying my 141" wingspan Yaks. just saying...

Bob

Gene, I am not setting up a model, but observed recently a buddy's setup in his new 50cc Extra and commented on how slack the pull pull wires were, but he said they it was okay to have them that slack to which I was not in agreement, hence my posting here.

Having them too loose can lead to flutter and we all know that is all bad. Bob is correct about the performance difference between pull pull and direct control. Back a few years I was heavily involved in IMAC and one particular season I had my spot at the TAS all wrapped up ( decided not to go though ). The airplane I was flying was set up with pull pull and it flew well but I had a 12" piece of 1/4" music wite in the stab tube to correct CG. I decided to move the rudder servos into the tail so I could drop that ballast. I went from 3 servos to 2 and had more positive rudder control. Now fast forward to my current 50cc sport airplane, a 33% Laser that was originally designed for a G-62 and I was going to put in a DLE 55. The CG solution was to put both rudder and elevator on pull pull. The system works just fine but there are a few rules that I follow. The first is that I always use a nylon coated Kevlar cable. It is the lightest thing available and will never stretch. Second if that the cables must retain the same tension through the entire travel. I am currently bringing a 40% Extra and am saving the rudder servo setup for the last part in hopes that the servos can go in the tail. I also think that for scale appearance pull pull is a viable solution as well. My current Dalotel project is pull pull on the rudder but has a separate tiller in the fuse that a single pushrod from the servo drives. Either way the key is to have a free moving surface that is tight and centers well.

Ok, I am late to the party again. All this talk of "Ackerman" is really confusing and then I remembered what it was all about. IF you do pull-pull, there are two different ways it is commonly done.

One way is from the outer hole in the servo arm to a pull station on the control surface. This is where the "Ackerman" issue generally makes itself noticed. What it means is that the control surface pull station mounts need to be the exact same distance apart as the powered (from the servo) end. Otherwise the geometry will allow a slack cable when NOT at neutral.

The other way is to use a tiller bar (several different vendors sell them - Dubro, Sig, and others) and make the pull station on the control surface exactly as wide as the attach point to the tiller bar to avoid "Ackerman" problems. The tiller bar gets its 'power' from the servo.

As for CG issues, pull-pull CAN help a lot if you are careful. Right now I am working on a Gere Sport. I have flown another version of this bird before and all versions I have seen needed between 5 and 10 pounds of lead in the nose to correct the CG problem. So on this build I have moved from servos behind the cockpit with long carbon/glass rods to the control surfaces (where someone else had already installed them) to servos in front of the GG with short carbon/glass rods to just behind the cockpit where I have installed tiller bars. The difference in CG correctional added weight should be substantial. The control rods and (1/4 scale) servos weight over a pound and much of that weight is being moved over 12 inches FOWARD. I estimate the CG corrective weight change to be in the neighborhood of 3 pounds. We shall see...

The reason I spoke of this is that the tiller bar takes all the pull-pull load and does not transfer any of it to the servo. That means I am going to (probably tomorrow unless the carb rebuild I am doing right now goes well) make those lines as tight as I can get them while holding the tiller bar at neutral. As someone else pointed out that should help avoid flutter (can you imagine flutter on this beast?).

I can not repeat often enough, the non pulled cable going slightly slack when the surface is pulled off center will have NO deleterious effects; i.e. will not cause flutter nor any other problem as long as it does not sag so much it comes off track or snags on something. Neither does the length of the control horn have to be the same as the length of the servo arm. As long as, at maximum deflection, no binding of the moveable surface occurs, no problem. These two items are the most abused myths of pull-pull systems. The most common mistakes I see on the beginning users of pull-pull is negative Ackerman and to tight control lines. These comments are the result of some 20+ years of successfully using pull-pull systems. When I first started using the pull-pull systems I made the same errors but logic and experimentation soon sorted out the myths from the facts.

I apologize for posting in a thread you own.

For most people, Rodney is correct. At center, the cables are both tight regardless of the setup. When pulled to one side, the airflow from the plane's forward motion and/or the prop blast keeps tension on the pulling cable and the offside cable has nothing to do.

However, if your flying style involves tail slides with the power off, the reverse airflow on the rudder could cause the rudder to pull up the slack on the offside, thereby slightly increasing the rudder deflection. So there is one (admittedly unusual) situation where you might want to have both cables taught all the time.

Jim

There is actually a third myth... Pull-pull is by far the best way to go for precise control, and that is what my 45 + years of successful design and applications tells me...

Bob

That would be an extreme case however having the control system loaded the same on both sides really does give a more precise control response if you are flying an airplane of high enough performance level and have the skills to match. For the average guy flying his Sunday favorite it may in fact not really matter but some of us do push things to that next level or two. Rodney, try flying an airplane with over 300 sq in of rudder area through a 20' high 8 point roll with one cable going slack and you will soon find that it does indeed make a difference.

I want to try it Speedracerntrixie! Can I borrow something in the 40% range?? :-)


david