Cable Length
 

I asked this question at the end of a thread in the Clubhouse and didn't receive an answer so I'll try here. I am building a kit that will use a pull-pull system for the rudder. Becauseof the positioning of the elevator servos just ahead of the tail, the rudder cables could end up being of different lengths. I don't visualize that being a problem but thought I would ask folks who have used pull-pull systems in the past. It seems to me that if one cable were 5 feet long and the other 10 feet long, as long as there were no slack in either cable, moving the control horn 1/8" either way would move the rudder the same amount. Am I making sense?

RE: Cable Length
 

I meant moving the servo wheel or arm 1/8" either way would result in moving the rudder the same amount.

RE: Cable Length
 

With cables of different lengths the becomes a angularity problem and one cable will become slack at extreme travel

RE: Cable Length
 

I understand that the cable not being pulled is supposed to become slack. That is the point of positive Ackerman. Wrong? My question was whether cables of different lengths would pull the rudder the same amount given the same amount of rotation of the servo wheel/arm.

RE: Cable Length
 

What makes the cables of different length? Is the rudder servo centerd? Is the cable passing over and touching something (another servo for instance?

Depending on what you indend to do with that plane, the different length cables could make a difference (pattern and 3D for example).

CGr.

RE: Cable Length
 

The pull is going to be different in one direction than the other

RE: Cable Length
 

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In my sketch I show two cables going from servo to rudder. The bottom cable travels in a straight line, and is as short as it can be. The top cable takes a path around four pulleys, then on to the rudder. As you can see it makes no difference how long the upper cable is compared to the bottom cable. If the servo arm moves 1/2 inch, the whole cable will also move 1/2 inch, therefor the rudder horn will move 1/2 inch. The geometry doesn't lie.


Lm

RE: Cable Length
 

Just out of curiosity, why the pulley arrangement? That is certainly not a normal routing for a rudder pull-pull setup.

CGr.

RE: Cable Length
 

It will probably work OK if you have the arms the same length and they are parallel

RE: Cable Length
 

The system lordmeren pictures in the above post will work very well and give the same results as a staight run would give. The problem might be in making sure the lines do not jump off the pulleys if there is a lot of positive Ackerman. I often use guides to route the cables around obstructions resulting in different lengths with no problems. There is also no problems having different length between the horns on the moving surface being different that the length of the arms on the servo as long as, at extreme throw, no binding occurs. What is important is that the length of the horns on each side of the moving surface be equal and that the arms on the servo be equal.

RE: Cable Length
 

CGRetired, yes, the cables will have to be routed around the elevator servos which are located just ahead of the horizontal stabilizer. The opening for the servo on the right side of the fuse is up high and the opening on the left side of the fuse is down low in order that they don't butt against each other. Routing the cables around these two servos could have resulted in slightly different length runs. However, I believe I can route both cables in a straight line from the servo (the servo is centered) to guides just above the floor of the plane and from there to the exits which are about 10mm above the floor. That would result in only one significant change of direction and one very slight change of direction and the cables would be virtually the same length.
Lordmerren, your sketch illustrates my point very well. No matter where that upper cable is routed, as long as it is supported at each change of direction, a given rotation of the servo wheel either direction will result in the same amount of movement at the rudder. That is assuming as Rodney says, the horns/arms/wheels/whatever are at equal distances.
This is the last piece of the puzzle. I am waiting now for the elevator servos and some Y harnesses to arrive. When these get here, I can start covering this thing. The aileron servos are mounted in the wing and the throttle and rudder servos are in the fuse. However, I can't cover the wing until the Y harnesses arrive and I get one of them in the wing and then get the elevator servos and Y harness in the fuse. And then this puppy goes up for sale. It is a 3D plane and I have no interest. I'm simply building it because it's winter and I love to build. I saw this kit on Ebay. The plane is beautiful and the price was right, so here I am.
BTW Rodney, where are you buying little bitty pulleys for these cable runs? That sounds like an interesting idea for my next build. Now that I have run across this pull-pull thing, I will probably use it on every build from here on. Everyone has one phase of a build that he hates most and I guess mine was routing push rods. This pull-pull thing certainly appeals to me more than push rods.

RE: Cable Length
 

CGr, you are right ,no one would ever route cables to a rudder like that. I simply used an exaggerated scenario for demonstration purposes.

RE: Cable Length
 

Ok... I hope you didn't take that as challenging this setup, just that I've never seen something like that. ;)

Hey, whatever works!! I have two pull-pull setup aircraft but they are both direct. One has both elevator and rudder pull pull (Excelleron 90 pattern plane) and the other has the elevator servo under the wing so the rudder pull-pull is direct.

CGr.

RE: Cable Length
 

JollyPepper, if you need small pulley and hardware, check out Proctor Enterprises. I have found that nylon guide tubes work as well for less than 90 degree turns so have never used pulleys on any of my later builds. The only time I did was on the ailerons on a 1/4 scale cub but found that the wing flexed enough under high G manuevers that the cable tended to jump off the pulleys.