pull/pull wire crossing
 

I see many are crossing the wires on a pull/pull rudder set up. What is the reason for this? Advantage or disadvantage? I know rubbing may be and issue and can be taken care of fairly easy. I'm working on a H9 (rebuild almost from scratch) CAP 232 that in reality is a 60 size plane. I have room to run strait from the servo back to the rudder. The cables will exit about 6 inches in front of the rudder hinge line with the current uncrossed plan.

Ken

RE: pull/pull wire crossing
 

I don't like the crossed setup as the lines get slightly tighter and looser as the system is rotated. Some like them crossed because you can get a slightly shorter exit slot.

RE: pull/pull wire crossing
 

It's not because the lines are crossed but rather due to a trapezoid geometry.

Kurt

RE: pull/pull wire crossing
 

Another plus for crossing is that the exit slots can be positioned closer to the rudder. The section of the wire exposed outside the fuse is shorter.

RE: pull/pull wire crossing
 

With crossed wires there is NO geometry that will exactly maintain the same line tension.
With straight wires, you can set the length of the servo arm to the same length as the control horn and achieve a perfect parallelogram. the lines stay the same tightness throughout the range of motion.

If you don't believe me about the crossed lines, draw it up in CAD and measure the lengths of the wires. Draw up something that looks like this attached picture.
Draw something like the first picture with both the servo arm and rudder horn the same length. measure the distance of the "wire" S1-R1 and S2-R2.

Then draw the next picture with the servo arm at 45 degrees and the rudder at 45 degrees. Measure the two "wires". They will not be the same as in the non-rotated picture and the sum of the two lengths will not match the sum of the original (unrotated) wires. No matter what angle you draw the rudder, you will not be able to get both wires to match the original drawing.

RE: pull/pull wire crossing
 

if length(R1,R2) = length(S1,S2), and the pivotal points are at the center of (R1,R2), and (S1,S2), then amount of travel of both wires should be the same.

RE: pull/pull wire crossing
 

crossed lines helps reduce the effects of ackermann....it least that is what I think it's called where one line becomes slack when the servo arm is crossed....someone know what I'm trying to remember?

RE: pull/pull wire crossing
 

Actually, Ackerman is an off-set to help keeping tension in the lines:

http://www.qmfc.org/school/ackerman.htm


I believe that is the real reason.

Note that the servo-surface deflection is reversed for crossed wires.

RE: pull/pull wire crossing
 

If I had my choice, I'd run them parallel as it's 'easier' to set both sides up identical. Ackerman can be introduced with an off-set servo horn if there are issues with tension.

I think my 2 pull-pull planes are setup with the lines crossed - they're ARFs, and the slots were not in the right place to run the lines parallel.

Crossing the lines works, but I don't like the non-symmetry.

RE: pull/pull wire crossing
 

Parallel lines can also have tension problem, if the pivotal point on the rudder side is not exactly at the hinge line. A servo horn with the same amount of offset and direction is useful to address the issue.

RE: pull/pull wire crossing
 

It's just easier to set up crossed in most cases. Fuse design has a lot to do with it, the way the fuse tapers usually makes up my mind for me. As DM showed, your always going to get some slack if you cross the wires. When I can I will run my wires parallel, if not I don't worry about it. I do spend a lot more time on my presision planes making sure my controls are set correctly without any slack but on a weekend sport plane if I have a bit of slack on the rudders pull/pull I'm not going to worry about it. You won't notice a thing.

RE: pull/pull wire crossing
 

It looks like to not cross the lines is the preferred method. I'll have about 6 inches of line exposed so not a big deal in my opinion. I'm using 60 pound test braided fishing leader for lines so they are not very large diameter and I would think drag would not be and issue.

Ken

RE: pull/pull wire crossing
 

You can use a short length of nyrod inner as a fairlead to change the direction of cables provided the angle is not acute. Makes almost no difference to the cable drag and means you can put the cable exit wherever you like, and route it around internal structure. The cable tension in models does not have to be any more than enough to remove slack. Any more will prematurely wear pivots and cable attachment holes.
Evan, WB #12.

RE: pull/pull wire crossing
 

Whether or not the lines are crossed is of no particular importance. As long as you have some positive Ackerman in the installation and that there is no binding at the extreme positions, either work equally well. Lines do not have to be parallel and the horns and servo arms do not have to be of equal lengths from the pivot point to the attachment point, just make sure that when the servo throw is at it maximum, there is no binding anywhere in the system. I've been using pull-pull on all my large models (and many smaller ones) on both elevator and rudder with no problems. I often vary horn length or servo arm length to control throw angles and doubt if they are ever of equal lengths.

RE: pull/pull wire crossing
 

"Positive" meaning the non pull side has a small amount of slack? I assumed you didn't want any slack. It seems if you have slack there is an issue when wanting to move the surface the other direction. I realize air pressurw will be pushing it the other way until slack is gone but seems odd to not have full control at all times.

I;ll be doing my first reall pull pull soon on a WWI job.

RE: pull/pull wire crossing
 

Yes, with positive Ackerman, the line not being pulled will develope a bit of slack as it is moved away from neutral; at neutral the lines are just barely taunt. One of the biggest mistakes made in setting up a good pull-pull system is to much tension on the lines when they are at neutral, you want them just snug enough so there is no droop or slack. When pulled and the line not being pulled gets a bit of slack, no harm as the air loads keep things working properly. You will NOT get any adverse results from this slacking off of the unloaded line.

RE: pull/pull wire crossing
 

This slack issue has been beet to death for years

I had a guy once insist there could be no slack at all any time because he could feel the lag difference when in a stall turn,, he insisted he could notice the micro second of lag as the line went slack then tight again .. Yes he told me he could notice the rudder not pushing the tail quickly enough,, not the rudder redirecting the wind over it, but the micro force of the rudder moving the static air..

I think there are some guys that way over think things, just about all my pull-pull systems have been crossed for easy of set up, and a little slack is no biggie ;)

RE: pull/pull wire crossing
 

I don't think so. I have drawn it up in CAD and I have also calculated it using trigonometry. When the servo is deflected 45 degrees, the rudder goes the opposite way (when the lines are crossed), but does not end up at 45 degrees (if you keep the line from the servo arm to the rudder at the same length. And the other wire must get either tighter or looser than before it was rotated. Not a whole lot, but a few millimeters different.

RE: pull/pull wire crossing
 

OK, I'll flip a coin as to crossed or uncrossed. If the coin lands on it's edge, I'll go back to a push rod:D

When I get to that point I'll try both ways and see which works best.

Ken

RE: pull/pull wire crossing
 

I do know some good {very GOOD} 3-D pilots that probably would be able to tell if there was a bit of slack in the lines. As a sports pilot with a little pattern training I'm not one of those people. When I'm flying a pattern plane and making small corrections I don't want any slop in my controls, I want the wire to pull, not allow the wind to do it for me. I would also like to live in a perfect world, what ever that is?? I try to keep my controls tight but if there is a little slop in them I'm sure not going to loose any sleep over it. I have planes that have a perfect pull/pull system, some kinda almost and some not so good. I'm not good enough to really feel it if I'm off a bit and all my planes fly just fine. I can only impress the guys at the field just so much, no one has ever noticed if I have a plane set up with perfect Ackerman or not.
Just an FYI, I do use my fair share of the Nyrod to run my wires through. Not always but on planes I can't get my hands into the fuse very far. I can take photos on my new planes set up, the pull/pull is crosses but I don't have a big pile of loose wire but it isn't perfect. I'm sure I won't notice it though.:D;)

RE: pull/pull wire crossing
 

The ability to keep the wires tight relies upon having equal distances from the pivot of the servo to the control attachment point, and from the hinge line out to the attachment points on the control horn.
It doesn't matter if the lines cross or run parallel.

Lastly, do NOT use a horn on the control arm that moves the attachment point forward of the hinge line. Often, you'll see where someone wanted the precision of a pull-pull setup and then threw it all out the window by using the wrong hardware creating a mis-geometry at the control surface horn.
The actual attachment point must be in line with the hinge line.

RE: pull/pull wire crossing
 

Yes - the amount of travel is the same, but both change from the original length by the same amount. The distance decreases, giving you slack in the wire.

RE: pull/pull wire crossing
 

The one and only reason I cross the cable wires on a pull-pull rudder setup is so that I don't have to use a servo reverser if the nosewheel turns in the opposite direction.

Just keep it simple and don't try to dazzle us with BS about geometry.

RE: pull/pull wire crossing
 

Is the decrease linear to the angle of rotation? Could one measure and install the wires at the max decrease to have a slack-free setup?

RE: pull/pull wire crossing
 

I created an Excell spread sheet to calculate the offset needed in the servo arm tocompensate fora crossed wire setup. It shows you graphicly what is happening to the wire tension (lengths). I just keep poking in values of offset ( the run, servo arm length, and rudder horn length are known ). It assumes the rudder horn pull points are on the hinge line. Here are couple of plots. The first has no offset. At 45 degrees the loose cable is almost 300 thousandths too long. The second shows what happens with proper offset. The loose cable is only 10 thousandths too long at 30 degrees and actually tightens back up again at 45 degrees. Notice that the cables become too short after 45 degrees. If you want more deflection than that you would use a different offset. I hope this helps.

Whizbang