Citabria Tailwheel Shimmy

B

Bob Turner

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First, happy that Ed was not seriously affected by Ian - best wishes to our Florida buddies.

So this nice 160 Citabria has had tailwheel problems. I fixed it a couple years ago - wedges, tighten the tension - worked fine. It was a club airplane, and has had a hard life.

A really nice guy now owns it. I borrowed it today to move some parts around, and noticed a horrible shimmy - I mean BAD.

Since I last worked on it it got a new Bushwheel, and a new spring, and compression-type rudder springs. Those new parts are what is shimmying.

I know about the kingpin angle, and have experience rebuilding the Scott 3200, which is apparently the same as the Bushwheel. I have heard rumors that there is a Bushwheel part that dies fairly rapidly in there, but am not sure it has anything to do with shimmy.

So when I got back, I suggested three things: 1- bring tire pressure up to 50-60 psi. 2- tighten the friction nut, and 3 - if that doesn't do it get rid of that big expensive tire and replace it with the proper 2:50/2:80x whatever - Carlisle makes them.

The angle appears ok.

Can anyone think of anything else we can check? This is otherwise a gorgeous aircraft, and has had tailwheel issues for at least the last decade. It is a 3003, I think - GCAA.
 
you got all the points. one other thing I do on these is replace the rubber pad that the tail spring sandwiches with an aluminum block. And yes, the big tire is not helping things. Here in AK, many fight the shimmy issue that arise putting bigger tires on.
 
is the replacement spring the wider one? 1 3/4" I think?

if it's been shimmying for ten years could a frame tube or mounting point be cracked or broken?
 
I don't know about the spring, thought they were all the same width.

As far as a crack, YES. I have found many to be cracked in the cluster of the forward bolt for the spring.
 
as late as it was made it probably already has the wider spring. have you checked the kingpin angle with two adults in the plane?

there was a video posted about the tension in the steering chains being barely tighter than what it takes just to take up the slack.
 
I was wondering about that -they are those Maule compression springs. I kinda like the old kind that extend normally.

I haven't checked back - will do it Monday.
 
Thanks Bob!

I confess I have never experienced shimmy. Perhaps I did and just did not know what it was. Ignorance is bliss, I suppose.
 
Big Ed said:
Thanks Bob!

I confess I have never experienced shimmy. Perhaps I did and just did not know what it was. Ignorance is bliss, I suppose.
Click to expand...

it'll usually happen when you have a full size adult passenger in the back. you'll land, all hell will break loose at the back of the plane and you'll think the tail is going to break off
 
I just replaced the whole tailwheel spring packeage, including bigger tires.
both main steel springs were bent.
we also have the Piper Maule Husky tension springs with chain, and all works fine!

Had shimmy before , now all gone even with 2 persons...
IMG_7272.webp IMG_7274.webp IMG_7280.webp IMG_7387.webp
 
Most of us are suspicious of big tires. I use the McCreary or Carlisle tires. Very common - used on wheelbarrows or something.
 
What is the source of the tailwheel spring? I'd go with one from ACA factory...compare them side by side from Univair, slightly different. You can use the Fabreeka pad and the aluminum pad (ACA Part # 1-105137-1) together.
 
There is a guy who had trouble with the bush wheel tail wheel on his pa12. Bad shimmy. Took it off and went back to the Scott and it went away. I will be back with the Scout later this week.
 
You mean Gary? Yeah, those giant wheels need a tailwheel lock. I am suspicious of the smaller heavy duty 2:50/2:80x4.

But, on the other hand, the Super Cub had one of those and it went flat last week. They are rigid enough that the two guys inside didn't notice until they were back at the hangar. With the regular tire, you almost have to stop and change it.
 
Is there any official solution to getting the Tailwheel assembly to align and not move where it connects to the leaf springs?IMG_2763.webp
Jacked up the rear to replace the leaf springs and mine was visibly loose. Torqued everything down, but I still got shimmy on one landing and I can see if shifted a bit. Maybe bend a thin piece of sheet metal to fill the gaps?
 
I think I answered my own question. The ABI parts list shows a leaf spring adapter that’s not on the Scott parts list.
 

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Derek Lawless said:
Is there any official solution to getting the Tailwheel assembly to align and not move where it connects to the leaf springs?View attachment 8789
Jacked up the rear to replace the leaf springs and mine was visibly loose. Torqued everything down, but I still got shimmy on one landing and I can see if shifted a bit. Maybe bend a thin piece of sheet metal to fill the gaps?
Click to expand...
That's what I did. I also replaced the castle nut with a self locking.
 
The "arm assembly" in the tailwheel has a couple of detents in the ID that engage the ends of a v-shaped flat spring. (It's a ~$400 part!) Shimmying seems to round off the corners of the detent. Sharpening-up the corners with a file seems to help.

Also, when detecting a shimmy on rollout, a quick "water pump" fore-and aft movement of the stick unloads the tailwheel for an instant and usually stops the shimmy.

I've thought many times that Scott tailwheels are "an invention of the Devil" but maybe Maule and the others are just as bad and we're just fighting physics.
 
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