pull-pull setup (again, sorry)
 

Sorry to bring it up again. i know it's been beaten to death here but still, (i looked everywhere and read everything)...

i have my cables crossed, servo arm straight (no offset), rudder horns on hinge line (no offset), both arms are same in length,

but still i get noticable slop on non-pull cable and some (maybe neglegible) extra tention on pulling cale.

after reading everything here and other places as well - i still can't find the reason or a reasonable solution.

this is my last cry for help on this matter.

attached is a simple drawing of my setup

i thank you all for being supportive and helping out

RE: pull-pull setup (again, sorry)
 

FWIW, you need to make absolutely sure that the distance between the horns at the rudder is the same as the distance at the servo arms.
For a non-crossed setup, and everything straight and on the hingeline, this is ideal. But, in your case, the lines are crossed, and I read somewhere that the servo arms need an offset arm if you cross your lines.

Check out this very invaluable info from SWB [link=http://www.swbmfg.com/rudex.html]http://www.swbmfg.com/rudex.html[/link]

Hope this helps.
Spar

RE: pull-pull setup (again, sorry)
 

yes, i read it before,
but that example B is talking about differend arm lengths in servo and in rudder, so - not the case.


as much as i try to think about it i can't figure why will i need an offset for crossed cabels if all is in good giometry...

thanks anyway

RE: pull-pull setup (again, sorry)
 

Well, I would recheck the geometry, as something is not right. Whether one side of the rudder horn is longer than the other, or the servo is not sitting exactly on the centerline, but, I have yet to see a crossed pull-pull setup work yet that didn't have an offset.

Some further info to digest: [link=http://www.andreyko.com/html/ackerman.html]http://www.andreyko.com/html/ackerman.html[/link]

Good luck,
Spar

RE: pull-pull setup (again, sorry)
 

I have had the same problem when my rudder control horn linkage isn't on the hinge line.
I personally think that a lot depends on where the control horns are located on the rudder.
The servo has the screw as it's pivit point it is in a fixed location.
The rudder control horns need to be directly over or very slightly aft of the hinge line. Any further back throws things out of wack because the pivit point is in constant change. It is moving slightly to the left or right each time the rudder is moved.
Try e-mailing RC report (Gordon Banks). He may be able to shed light on it and if he can't, someone on his staff will have ideas too.

RE: pull-pull setup (again, sorry)
 

You need different arm lengths and some offset when you cross the wires to minimize slack.

RE: pull-pull setup (again, sorry)
 

Offset which way? If you were to offset the rudder end, would the pivot be behind or in front of the hinge line?

RE: pull-pull setup (again, sorry)
 

See the link on SWB site above, it clearly shows the offset in the case of the rudder cable pivots being inline with the hinge line at the bellcrank or tiller arm toward the rudder the rudder post.

If your rudder cable pivots are not inline with the hinge line this does not apply.

If you want to use a straight bellcrank/tiller arm I beleive off-setting to the rear behind the hinge-line would suffice.

RE: pull-pull setup (again, sorry)
 

The cables are going slack because the length of the cable is longer than the length from the servo hub to the hinge line... if the servo and rudder could deflect 90 degrees you would have slack equal to the difference in these two distances. The way you fix it is to shorten the length of the cable to be equal to servo-to-hingeline distance... you do this with servo arm offset.

Here's what you can do...

Set your rudder control horn on the hinge line.

Now, measure the distance from the servo output shaft to the hinge line. Call this measurement "A". Lets pretend it's 35".

Now, call the control horn length "B"... lets say it's 4".

Now, do this math: Offset = Sqrt(A^2 + B^2) - A

So, for the example, Sqrt(35^2 + 4^2) - 35 = .228", or about 7/32". When rounding figures, round down or you will get cable tightening.

Now, call Nelson and order a servo arm the same length as your rudder horn but with the amount of offset you just figured. Install the servo arm with the offset facing aft, giving you the effect of shortening the cable length.

If you want, post your A and B dimensions and I'll tell you what servo arm to order.

SWB does its arms, which work very well, by using a combination a little less offset than the above equation provides, but also uses a shorter servo arm which also serves to tighten the cables somewhat. Figuring those dimensions requires CAD. You can call him and give him your "A" and "B" and he will tell you if he has an arm that will work... most are designed for a given set of dimensions though. The advantage of SWB is that he can tell you EXACTLY where to place your cable exit so it's a straight shot to your servo. The advantage of Nelson is that he can make you any arm you want. You choose.

RE: pull-pull setup (again, sorry)
 

sillyness,
Would you please post a picture of your hypothetical setup?
Bob

RE: pull-pull setup (again, sorry)
 

I don't have a picture. It looks just like the picture in the first post except the servo arm is offset toward the tail a little.

You can make a setup at home with scrap to try it... I've given this to others and they said it worked flawlessly. Or you can order a servo arm and try it in your plane. It's a $10 experiment.

RE: pull-pull setup (again, sorry)
 

Ackerman says the offset should be behind the hing line. Otherwise the non pulling cable will tighten up and thet isn't good.

RE: pull-pull setup (again, sorry)
 

You can't go using the term Ackerman loosly... there are too many cases. Generally you are correct... for straight run cables. You want no offset,or you may want to error slightly on the side of the control horn being behind the hinge line.. this causes the cable to loosen slightly when deflected.

We were talking about crossed cables in previous posts. They loosen when deflected if offset is not used. You WANT an offset that will take up the slack when deflected... hence a servo arm with offset toward the tail. Millions of flyers and companies can't be wrong!!! Another way to take up the slack would be to set the control horns so they are in front of the hinge line, but this is not practical because they will hit the fuse before they reach full throw... not a good practice.

TEST out the formula above... it works. Don't use someone's opinion as a basis for arguement. I don't want this thread turning into the crystal taping thread or EF Yak threads. 'Nuf said.

RE: pull-pull setup (again, sorry)
 

Now that's good for a chuckle! :D Yes, lets not beat this to death. The best thing you can do is make up a jig and test it out yourself. Take a board, draw the rear outline of your plane on it, and set up a mock pivot point for the rudder, etc and figure out what offset you need on the servo arms. How long will that take? About an hour, so get on with it.

Spar

RE: pull-pull setup (again, sorry)
 

sillyness, you were right on the money, thanks alot.
and finally an explenation i could understand (for why offset is needed for symetrical setup).

RE: pull-pull setup (again, sorry)
 

No problem man... glad I could help. I sat down one night frustrated with a pull-pull setup and started drawing pictures on graph-paper... it becomes pretty obvious when you kill a couple brain cells working through it. The beer helped too.