[NSRCA-discussion] JR servo question

Thu Apr 26 21:37:31 AKDT 2007

John,

its likely a worn pot.

The 8411 and 8411SA will tend to be a little less tolerant of a poor linkage setup on rudder. What can happen is a spongy rudder cable setup, and the servos are extremely precise so they will try to fight their way to center. The result of the spongy cable setup will cause it to over shoot each time and it will with a little vibration and pot wear cause the servo fight itself. 

The rule of thumb on these if you can put your hand on it and it stops then air loads will not allow it bounce while in flight. Now I personally look at it as an opportunity to be thinking of a servo change. I don't immediately change it out, but it goes on the list of things to do. I may fly it until next week when I check the other servos but I probably don't put 50 flights on it this way. 

This is a dead band thing. A price that you may pay for a very accurate and precise servo. Now it is my understanding the 8311 is an 8411SA that has an upgraded motor and the deadband is adjusted a bit wider for the Heli guys so the servos don't tend keep fighting for center as hard. We are talking about small changes but they will affect the results. I have heard from guys that use the 8311 in place of the 8411SA and they tell me that the pot wear shows less with this servo. You may want to talk to Eric Henderson. Last I knew he was running these servos 2 years ago. I don't know for sure, Eric seems to have dropped off the map. I know he was moving and such but I haven't heard from him since about Thanksgiving. Eric has some experience with these servos. Another source would be Dave Lockhart, he is always playing with servos, and probably has used the 8311's. 

I switched to 8611's on the rudder a few years ago. Its way overkill for sure but I have not had any servo wear issues since I installed the beast in there.  I run my cables on the rudder pretty tight. As in Willie Nelson could use them for guitar strings. The setup I use is a extremely tight rudder response with no spongy feeling. It will make you re-evaluate your rudder deflections and expo settings as the rudder is as tight as the ailerons or elevator.

This spongy setup is a cause of the bouncing, but there are others as well. Sometimes a longer control horn on the rudder will make a huge difference. I tend to run pretty long rudder horns on the rudder side. Most guys I see put the link close to the rudder and then have little mechanical advantage. They are trying to get a 1-1 ratio servo arm to rudder horn. You don't want that. That will maximize any errors in the linkage or servo pot and gear wear. 

Another thing I have seen is the heavier rudders will be more affected. There was an Alliance that was built by a friend. He'll remain nameless, but he used triangle stock on the LE of the Rudder to get a good bevel. The problem was he made the rudder normally and then added the tri-stock on top original LE facing. This added a bit of weight to the rudder and also it changed the size of the surface about 1/2-3/4" more chord. The model then got sold through a couple of friends and that beast was hell on Rudder servos. I mean it would eat any servo in about 50 flights. Well cut the bad boy off, bevel it correctly, and hinge it a little better than it was and the model went 250 flights on the same servo that was showing signs of the bounce after about 30-40 flights. It was an extreme case and something about that plane, that rudder, and  that setup gave some kind of a harmonic. 

If you continue to have the problem look to change the setup and it will likely solve your issue. Look to control horn length. Most guys use a 6-32 bolt. Replace it with a  longer one. My rudder horns are about 1.25" from the rudder hinge line. Another help is to Bend the bolt so the pivot point is over the hinge line. Adjust the servo arm side of things. Make sure you are getting good mechanical advantage over the rudder. If this doesn't help look to change the mass of the rudder. Cut holes in it and recover it. Do something to change the dynamics. 

The 8411SA and hence the 8311 is an awesome servo for just about any pattern application, ailerons, elevator, or rudder. Its super strong, fairly fast, and is great in the precision which beats out the other two specs hands down. What you have is a servo that is being precise but the system its used in has a property that is causing it to over work itself.

Hope this helps

Troy Newman
  ----- Original Message ----- 
  From: John Pavlick 
  To: NSRCA Mailing List 
  Sent: Thursday, April 26, 2007 9:26 PM
  Subject: Re: [NSRCA-discussion] JR servo question

  Yeah, that sounds like what I remember hearing about but I couldn't remember exactly which servos behaved that way. I think my problem may just be a worn pot as a few guys have suggested. As long as I know there's not something going on with this specific servo (as was the case with your 8411's) I'll just have it rebuilt. I replaced it with a spare servo for now and everything is working fine again. Hoping for some good weather on Sunday so I can get some practice time in.

  John Pavlick
  http://www.idseng.com
    ----- Original Message ----- 
    From: JonLowe at aol.com 
    To: nsrca-discussion at lists.nsrca.org 
    Sent: Thursday, April 26, 2007 11:35 PM
    Subject: Re: [NSRCA-discussion] JR servo question

    In a message dated 4/26/2007 8:10:39 PM Central Daylight Time, jpavlick at idseng.com writes:
      Since I'm a Futaba user, I'll try to be nice about this. <VBG> I have an airplane set up with JR servos and I'm experiencing the classic "oscillating rudder" issue. There's a JR 8311 servo on the rudder and the cables aren't excessively tight. I remember hearing about this a while ago but I can't remember the solution. Any help would be appreciated.

      John Pavlick
    John,
    I don't know about 8311s, but I've had two 8411s do it, one brand new.  Some suggested an oring around the two pull pull cables, to apply a very slight drag..  I cured mine by going to an 8611.  Something in the 8411s is too tight in the feedback loop I guess.

    Jon Lowe

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