Citabria Play between rudder leading edge tube and tailwheel control horn

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kubark42

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When pushing the rudder, my 1998 7ECA can have a bit of play where the tailwheel steering horn enters into the rudder leading edge tube. It's not loose per se, but it doesn't take a tremendous amount of pressure to get it to snap back and forth between the two positions. This will certainly be happening every time I apply enough rudder to break the caster loose.

(Be sure to turn on the sound to hear the click/thunk.)

https://imgur.com/a/xb0Aw1S

Every now and then I had a bit of shimmy when I was learning to fly the plane, but-- likely due to improved control on my part-- it hasn't happened again.
  • Is this play a likely source of the shimmy?
  • Does it need to be addressed?
  • If so, is that as easy as tightening the bolts or removing the control arm and adding tight-fitting bushings?
 
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I would take that out and have a look. I have seen horns crack and break. I think bushing it should be fine.

Shimmy will transfer to the horn and wear it out quicker now that it is loose. The shimmy is likely caused by the caster angle, common problem on citabrias.
 
Thanks for the response. Do you know where in the service manual this is described? The closest I can find is in the tailwheel reinstallation section:

Screen Shot 2021-05-10 at 3.47.58 PM.png
My takeaway is that the correct way to access the steering horn is to drop the tailwheel spring so as to provide clearance for the horn to slide out of the rudder post.
 
If you are putting a new one in there, remove the rudder. When you get to the drill press, get your girlfriend to help you hold it steady.
Note that the rudder main tube can be accessed from either end - you can drive the arm out from the top.
 
Update: I tightened the bolts a smidge and that seemed to be the ticket. No more play.

There's no specific torque spec for them but they were not very tight for an AN3/10-32, as evidenced by the fact that I tightened with a screwdriver handle. I lent out my [0-100] in-lbs torque wrench but once I get it back will tighten them up properly (accounting for the lock-nut).
 
No more play, but did not resolve the shimmy problem. My tire has gone from having deep grooves to almost being bald in two months, and all that with probably no more than 40-50 pavement landings.
 
The horn would not have fixed the shimmy. Shimmy is a caster angle issue. Viewing your video, it appears that you have the rubber shim between the tail spring and the rear pad. do away with that and insert a 1/4 aluminum block. I believe ACA sells them, or fabricate your own.

Ensure your tailwheel mount bolt is tight, and check often. They work loose.

And, ensure you have the proper tension between the tailwheel assy head, and the fork. It should be just loose enough to rotate the fork, with very-little to no endplay (wobble)

I would start there. These 3 things where 99% of the issues of the planes I have worked on.

The next steps will involve taking apart and repairing the tailwheel assy, and/or re-arching or replacing the tail spring.

The image below illustrates the caster angle. You want a positive to neutral (bottom of head horizontal to the ground) at gross weight.

1626552565821.webp
 
Axle, yes. Pivot, no. Shimmy is damped by friction.
Some of you may note that shimmy starts after an annual, when they grease that zerk. Then you have to tighten the shimmy damper nut.
 
Bob Turner said:
Axle, yes. Pivot, no. Shimmy is damped by friction.
Some of you may note that shimmy starts after an annual, when they grease that zerk. Then you have to tighten the shimmy damper nut.
Click to expand...

I'm going to respectfully disagree. There are wearable parts in there that are meant to protect the castings, grease is necessary for everything to move freely. Mine seems to let me know it needs grease by shimmying. If something is binding it will catch, release, catch, release....which will feel to us like shimmy.
 
You are definitely in the majority. Folks come to me all the time with shimmies right after greasing.

I agree - that little spring and the thing it cstches in there need lubrication, but that clutch-like affair in there works on friction.
 
And while you're back there checking out your tailwheels don't forget the wheel part...
Prop up the tail, spin the wheel and look for an out-of-round tire. These things (solid or balloony) are cheaply made and take a hell of a beating...it doesn't take much of a flat spot (solid) or a broken sidewall belt (bias ply tire) to send a tailwheel into convulsions.
Chris
 
In the big scheme of things, I think more grease vs. less grease is less a factor than whether or not there is forward lean to the pivot bolt. imho
 
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