I have installed a pull-pull system in my ESM Corsair. The cables are driven by Hitec digital servos. Neither the rudder or elevator consistently return to the original zero position - sometimes being off by 1/16" to 1/8". Both surfaces moved freely before attaching the cables. How taught are the cables to be adjusted on pull-pull systems?</p>

If they are not binding then at rest the lines should be taunt but not tight at all. So some give. Make sure both sides are equal and the same length. good luck

It may not be the cables. A servo with a worn feedback pot will give you the same symptoms especially around neutral. Try new servos, right out of the box, and see what happens. Dan.

These are new servos. I will try to relax the lines slightly.

Well, there goes that theory. Here's an image of a set up I just got completed. I haven't flown the plane yet but the servos are working fine. The cables are not that tight. Dan.

Are they Hitec 5645? If so I've never had much luck with them returning to center.

I might be seeing it all wrong, but your installation is not typical for a pull-pull setup. Their only needs to be one servo, and the distance between the holes in that servos arm needs to be the same as the distance between the arm on the rudder post. The servo should be on the same centerline as the rudder. Or, are just putting us on with the setup shown?

Oh sorry, that reply was for All Day Dan on his setup image.

Stoneke,

What is the distance from the servos to the control horns on the rudder and elevator? Also are the horn holes on the hinge line? Also do the cables cross or not?

If the distance from the servo to the control horns is too short this could cause tension and centering issues. If the horn holes at the control surface are ahead of or behind the hinge line this could cause issues.

While a set up for say a 3D rudder is a different situation. For one I did that I bought from SWB had very specific instructions. It called out a minimum distance from the tiller arm on the servo tray. Said the longer the distance the better. Also called out an exact distance from hole to hole for the control horns and said these had to be on the hinge line.

Sorry not to provide exact info to resolve your issue. Hope this info is helpful.

John B.

Spaceworm, I understand what you are referring to but I did not have that luxury due to space and function. I was using two servos because I had to leave room for a retractable nose wheel to come between them. A single servo would have been too difficult to implement. I haven’t used it yet in real life but it is acting all right in the house. For now, the servos are centering properly. It would be a real nuisance if they don't when I am taxiing. Maybe it's not suitable for a control surface. I would probably have to do something different for a rudder. Dan.

Responses to questions:
The cables do not cross and they are approximately 32" in length.
The elevator and rudder horns are on their respectivehinge lines.
The servos are new Hitec 5625 digitals.
The paired cable lengthsdo not vary more than .062 to.125" of each other.

The control surfaces (rudder and elevator) do return to neutral when I manually return the radio gimbal sticks to center, but I see the issue when I simply release the gimbals from a deflected position and hope that they center. I think that I can live with the situation, but won't really know until some flight time. This is on the ESM74" Corsair.

If you have your cables to tight it may create a bind at the hinge or pivoting surface. I don't get my cables very tight very barely vibrate if plucked. Can you loosen your cables just a bit and retest? IMHO it is most important to have equal bellcrank distance at both the control surface and servo, otherwise a bind or slop can develope.

Personally, I would cross the cables, it seems to keep the cable run straighter and makes it less likely that they'll rub on the exits......

On my setups, when I have the cable properly tensioned, it will make a dull 'twang' when plucked, very similar to a guitar string before tuning it. Basically tighten just enough to take any slack out, then its just a couple more twists from there. A little on one side, then a little on the other. Shouldnt take much.