Ignition causing servos to twitch
 

I'm setting up a Great Planes Giant Yak 54 with a Fuji 43. I have the ignition battery located all the way up on the firewall and a Sullivan gold-n-rod running from the throttle back to a standard futaba servo mounted in the default location in the built in tray. When idling I get some twitching out of the aileron servos. I moved the receiver and the receiver battery as far back as I can without messing with the CG but I still get a twitch... running the Futaba 10CAG. Thoughts or ideas as to what is causing or how I can get rid of the interference? I don't want to put this bird up unless I know for sure that everything is solid!

Thanks.

Get rid of that golden rod unless you want to constantly be trimming the throttle. Are you using HiTech servos? How about optic kill?

What radio system? Is it 72MHz or 2.3GHz? Are there any metal to metal contacts anywhere in the plane: if so that could be causing problems when any vibration rattle the connections. Do you have the spark plug shield making good contact with the motor? As to using Golden Rod, I use it a lot with no problems at all, just make sure the guide can not flex or bow under vibration.

If you are using a golden rod with metal cable on your throttle, and choke if you have that set up to be servo operated and you are using metal clevis's you may be having an issue from that. I use a Nylon push rod so that there is no possibility of any current from the ignition traveling near the servo from the engine. It also depends on what servos your using as it may just be the fact that the wing is vibrating at idle which means your ailerons will be vibrating causing a bit of servo twitching as they are centering from the vibration.

Make sure the plug cap is FULLY seated

I'll have to see if I can get some pictures of the set up to better show but the setup is...

Futaba 10CAG 2.4 system with the battery and receiver moved beyond 18" from the firewall.
Hitech HS-645MG's in the wing and all in the tail.
Throttle is just a standard Futaba 3003.
The nylon Sullivan rod has the blue sleeve and yellow nylon center all anchored and does not bow out. I have a nylon ball link on the throttle linkage and the only metal there is the threaded ball end into the end of the pushrod and the ball cup. On the other end is another ball link on the servo arm.
Kill switch is the great planes switch.
Battery, kill switch, ignition are all mounted either on the firewall directly or the firewall box to keep everything as close as possible to the engine to make it easier to separate the rest of the electronics.
Ignition is grounded between the washer on one of the engine mount bolts and the front face of the engine mount where it is anchored.
The throttle servo is the closest to the ignition and I should have clarified about it being in the default location which is in the center of the fuse after the receiver tray behind the wing tube.... definitely more that 11' away.
I have noticed that the only time the servos twitch is with the engine running. When its off everything is perfectly still.... even with the ignition switch on.

I thought about finding a resistor plug cap to help contain the interference but was worried that I may reduce the spark too much? I have found another article talking about changing out the ignition module completely to resolve this?

All that keep stuff two feet apart is 72mhz stuff.

The two most likely causes, are 1. the plug cap isn't fully seated, 2. Vibration is causing a loose connection to act up.

Is there a better way to mount the engine other than directly to the firewall? Like use a soft mount??? Not sure on the loose connections... I went through and any extension I added I tied the connectors together and then put a piece of heat shrink over them to make sure they wouldn't come apart. I've padded the ignition module along with the receiver and batteries too. Prop has been balanced but I do notice a good amount of vibration. This is my first gasser so I've been trying to double check everything going together and setting it up with what I believe is the right way based on the manual and posts I have been reading around here too.

I just thought of something, make sure the plug cap is completely and fully seated.

If that doesn't fix it unplug all but the throttle servo and add one servo at a time till it starts acting up again, then check that extension/servo.

The stock Fuji ignitions leak RF like crazy. Do yourself a favor and get the RCExcel conversion from Milton at RCExtreme.
http://www.rcextremepower.net/FujiIgnitions.html

I had a similar problem using hitech servos. I called hitech and was told that twitching will stop with airflow over the surface. It was caused by the weight of the control surface and the servo correcting for the weight.
ive flown the airplane lots with no issues

Out of curiosity... would this be the case with the engine off too.. that it would twitch under the weight? or the vibrations magnify it? I wonder if you were able to support the aileron if the twitch would stop? just to see anyway.

I have found that these issues are almost always related to the plug cap. Start there, and do one thing at a time. If you make multiple changes, you will never know what the real problem was. Golden rod with nylon clevis is perfectly fine. Start with the cap.

Thanks for the tips. Will definitely start with the plug cap and work out from there to track the changes.

APG, If your spark plug does not have a resistive element in it, replace it with one that does. Start unplugging your aileron servos one at a time. Dan.

It sounds like a similar set up to my 2 Pitts, at idle the servo's twitch a little and it is due to the vibration of the motor not an electrical problem. When its flying there is no issue.

Are the aileron servos connected by a "y"? I had one of those cause the problem. Replaced the y and the problem went away.

Yes there is a soft mount system for gas motors, just not sure you would be converting one issue to another, soft mounting motors come with other issues.

Some digital servos will twitch when on the ground, but analogue ones shouldn't.

Are you sure it's interference and not vibration?

Three likely causes. 1. Loose cap on sparkplug or not seating properly 2. Non resistor plug 3. IG module defective. I had a similar problem and I traced it down to the IG module.

Wire the battery directly to the ignition. It could be the switch, I had a similar problem, it was the "hitec" switch that I bought. Wired ignition directly to battery and the servo twitching stopped. Bad switch, needless I don't buy hitec switches anymore!

Use "well nuts" in lieu of "T" nuts at the firewall. Adjust for the proper resilance & use " nyloc" nuts on the back side to lock the adjustment.

http://i76.photobucket.com/albums/j3...ps0a1469d5.jpg
Given the set-up you describe, I would suspect a bad ground on the plug lead. Maybe a lose cap or a bad ground at the module.

With the engine turned off and if you get buzzing or twitching on analogue servos is usually caused by dirty servo pots. But that doesn't seem to be your problem . With unbalanced rotating systems you will get some surface "flutter" and when throttling up that nay diminish.
Some good suggestions what to look for and some not related to you problem. Suggestion on going to a resistor type plug is good but would also make sure the high tension wire from the module to the plug is shielded at both ends on the braided portion; both at the metal plug cover to the shield and at the ignition module. Is the electronic ignition built inside a metal box or plastic ? The ignition module change , as suggested, may be the way you have to go.

Yes if I remember correctly it twitched with engine on or off but was worse with the engine on. To troubleshoot I placed my finger under the aileron just to support the weight and the twitching stopped

I am going to have to go with a dirty pot on this .....assuming there is no ignition problem. Two stroke , four stroke and gas motors especially raise hell
with electronics especially servos due to high vibration levels. Is this an older used servo ?