Hi,
I am using pull pull setup on my 90 size pattern rudder ( airtronics digital servo around 75oz at 4.8v ).
i have chcked the pull pull cables tension and geometry, and it looks like perfect rectangle but still there is
centering problem i mean when i through the rudder to on side it will return to the center but when i through it to the other side it will return to around 3 - 4 mm from the center. i tried to reduce one cable tension and then the other but in both cases there was no real improvement. i whould like to have 0 devation like i have on my dual elevator setup
do you know how i can achive that ?

Regards,
Shlomo.

Have you checked the centering accuracy on the servo itself?
Also, how is the ratio between the servo arm distance and the rudder control horn distance? A large difference here may amplify the slightest centre offset on the servo..

With the radio on, can you wiggle the rudder? if so you have a slop issue.

Check to see how much movement is in the servo (some have centering issues), and tighten your cables, they should give a satisfying "tong!" when plucked.

Hi,
both servo arm and horn length are the same ( arount 20 -25 mm on each side ), the servo centring is also ok. about the cable tension as i didn't want to put to much stress on the servo shaft i set it so there will need of small force in order to connect the servo arm to the servo.

Barring any issues with the servo a pull-pull system is flawless. As long as the cables are smooth there should be no issue with centering, does not even matter if the geometry is wrong because when you set it up to neutral its centered. Hence, it should come back to the same position. With the wrong geometry perhaps the throws will not be equal but it should not affect the centering. Best to check your servo and ensure that the wires/cables are taught, not too tight please ...

Well,....If your servo is centering and the geometery of the control hardware is good,......turn on the radio and move the rudder in both directions,..check to see which cable is more relaxed with the rudder deflected. You may find out one cable is slightly longer than the other. Also make sure the control harware at the rudder is the same on both sides of the rudder,....from the hinge line back and the height is the same. The transition time and centering of the sevo should not change normally,..however if your cables are not adusted close to the same lenght the transition lenght is not even and one side pulls less or more than the other (slop) and the word "slop" dosen't mean it has to be signficiant.

Don't be affraid to tighten the cables,..the servo will let you know when it's unhappy and you have gone to far,...it will constantly buzz or humm,...back the cables off equal turns on both sides,...move the rudder left and right with the radio on and check center again,..repeat this as many times as it takes to make center in both rudder directions.

I spend more time tweeking my rudder cables than anything else in the beginning during set up,...be patient and good luck with it.

Also,...make sure the servo plate is not moving under a load ( twisting, etc),...have someone put a light amout of pressure on the servo and see if the plate is bowing or twisting. I have seen this happen on several entry level type ARF pattern planes and non-pattern type planes and it will drive you nuts trying to figure out a centering issue in flight.

Regards,
Bill Holsten

thanks alot
i think i will take the following steps :

1) Remove the cables from the plan and check that teh length is equal. also i will check the rudder horns in both side to make sure all measures are the same.

2) Reassmble the pull pull and check the centring

i will update.

You don't really need to dissassemble,....just deflect the rudder in both directions and check the tension on the opposite side you deflect the rudder. Tighten the slack side or the side opposite the direction you have moved the rudder. Do this in small adjustments,...and the rudder will eventually center,...you may have to change your trim a tad to get it to bounce back and center in both directions. If you have any sub-trim in the radio for rudder,..remove it and put it bach to zero,..make sure your trim is centered before you start making any large adjustments. For sure check, the control hardware at the rudder.[8D]



Good luck,

Bill Holsten
Dist2