Tail Wheel Shimmy

[Helidesigner]

I have mild shimmy on my Iron Design tail wheel unit. I checked all the attachment bolts, shimmed the springs down by the aft bracket for a more favorable king pin angle, and even added a bracket to hold the springs together. No improvement. I took a video which is at the link below. You will see the mild shimmy and how it goes away and comes back with elevator pressure. I was solo with 3/4 fuel and some small amount of baggage, I estimate about 1950 lbs, The tail wheel steering has slop, i.e. there is NO resistance +- about 5 degrees until it contacts the steering mechanism. With no resistance, there is no damping and I believe this is the cause of the shimmy. I think Scott and other tail wheels have some kind of damping set up with belville washers or something else to give some swivel resistance in the slop region.

Is there a way to tighten up the unit to eliminate the slop, or, add some damping? Have others felt this shimmy? At least it's not a full blown shimmy like I have seen on other (non Bearhawk) aircraft.

Thanks in advance,
Mark

PS, the ventral fin is an experiment in work. I'll report on that at a later date. I had shimmy before I added the fin.

[sjt]

I wonder, if you twist the tire left or right, does the tail go up? I believe the geometry should be such that it takes energy out of the system if - any twist forces the tail up. It would be bad if by twisting the tire, the tail goes down.


The other possibility, is that it happens to everyone, and no one else has ever used a camera.

[Battson]

Your springs and chains look too tight. There should be a little slack, or it will try to shimmy.

[Battson]

Hard to see clearly from that angle though.

[Helidesigner]

This is my second set of springs. The first were stiffer compression springs. That being said I did not try either with a little slop in the connection. Is that how you have it set up?

[Battson]

That being said I did not try either with a little slop in the connection. Is that how you have it set up?

Yep.
Tight chains = strong tendency to shimmy. A little slop will mean one less possible source of shimmy.

[Helidesigner]

I still think the problem is the slop in the tail wheel steering but I will give it a try.

[Battson]

It's an undamped system with tight springs, so once you apply the excitation the amplitude increases

Slack chains removes the springs from the equation.

[N942VT]

Mark, it looks like the shimmy starts as the pinion angle decreases with weight. Make real sure everything is tight, try a little more air in the tire, if no joy then re-arch the spring for a little more pinion angle, all that fixed mine.
Hey, where's all the snow?

[whee]

I've always liked my chains tight😄 but it doesn't hurt to try loosening them up. But really, I like mine snug.

I'm my opinion that is a full blown TW shimmy. The BH I flew with that same TW shimmied just like that and went though tires like crazy. We finally removed the grease zerk and replaced it with a brass bolt. We tightened it against the king pin till it reduced the shimmy enough it could be landed three point if you kept a little weight off the TW till slowed down.

[jaredyates]

Mark, I'm running a very similar setup with a 10" wheel. I purchased the whole assembly second-hand from Georg who removed it due to shimmy problems. Mine will shimmy reliably when the elevator is deflected up on rollout. As such, I don't deflect the elevator up on the rollout. It was much worse when the bolt that held the tailwheel body to the spring became loose. It took me a little while to realize this bolt was loose, because of the way the assembly looks when the wheel is in the normal trail position. When the wheel is 180 degrees swiveled from the normal position, I could see a gap between the tailwheel body and tailwheel spring. This developed on its own over time, without the bolt and nut turning relative to each other, and I suspect it was due to inadequate removal of material from the edges of the spring steel to clear the weld bead inside of the mounting area. I removed some material before the first install, but not enough. I think this led to contact only at the high points of the weld beads, which wore down and allowed for more clearance. I took it all apart and saw shiny spots that led me to this conclusion. I removed more material to clear the bead without any doubt, and reinstalled everything with a new bolt. I also put in a split lock washer to help act as an indicator of looseness. The full description of this process is in queue for my building log but I haven't had time to post it yet. Even now though, with everything tight, if I place the stick full aft after touchdown, it will shimmy, likely because of spring deflection as Dave points out.

[Helidesigner]

I will check my attachment bolt. Discouraging that with all your attention you still have shimmy. I'm also going to see if I can reduce the slop.

Mark

[jaredyates]

I don't consider it a problem- I just don't apply up elevator when the tailwheel is on the pavement. I use tension springs for steering.

[Battson]

It was much worse when the bolt that held the tailwheel body to the spring became loose. It took me a little while to realize this bolt was loose, because of the way the assembly looks when the wheel is in the normal trail position. When the wheel is 180 degrees swiveled from the normal position, I could see a gap between the tailwheel body and tailwheel spring. This developed on its own over time, without the bolt and nut turning relative to each other,

That also sounds like a classic situation where stresses on the bolt cause it to strain over time, it only has to elongate very slightly to cause looseness, and that can easily happen if the torque relaxed (due to poor fit as you describe) so the bolt experiences the strain rather than just transferring it to the spring.
It's good that you replaced with a new bolt, the old one was probably damaged.

One solution is to land on something other than pavement. It doesn't seem to matter what I need to do when I'm landing, it will never shimmy unless there's pavement in the equation. But that said, I don't think I've seen shimmy on pavement either. Standard spring, standard BH tailwheel, nothing special / no modifications. It just worked.

[Battson]

I'm also going to see if I can reduce the slop.

I suggest doing that will make it worse.

Think of a motorcyclist who gets the speed wobbles, the more he tries to fight it the worse the problem becomes. Same with shimmy, I bet. The more you turn that into a highly sprung system (and without a dashpot for damping), the more the system will excite when you hit the speeds that correlate to the natural frequency of the spring-system. As a mechanical engineer yourself, I'm sure you know what I'm talking about

P.s.
For that hypothesis, I am relying on knowledge that older more experienced IA's have told me. The advice I've had is that tight steering springs cause problems, not limited to shimmy, but I should add that I haven't run some tests myself to empirically prove it. I take their advice at face-value.