Tailwheels, pavement, shimmy Oh My!

DFH65

DFH65

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DFH65
Talk to me about TW shimmy. Have a J5 I fly and it is fine on the grass but on pavement the TW shimmys like mad. I did one pavement landing yesterday and thought I was going for a ride took every bit of rudder work I had in me to keep her tracking down the runway and the shimmy was so bad I literally thought something on the plane broke. We did put a new TW on the plane this winter and maybe now that it has some real tread on it the shimmy is amplified because it bites harder.

I know there are lots of theories and I have read many of them about TW geometry, rebending the springs, looser springs, always doing wheel landings and others.

Any ideas are appreciated.
 
Sounds like you had the shimmies...me and the shimmy, yeah, we get banned...got that going for us.

 
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Tailwheel shimmy can come from a variety of places.

You HAVE to jack the tail up so you can grab the thing, wiggle it and find out what is loose. Grease, tighten and test. If still shimmy, rotate tire 90 degrees (or balance if you can). If none of that works, get a new tire (and perhaps tube if you are wealthy). Its possible to adjust caster with shims. What kind of tailwheel is it?

As for technique, landing on grass, stick forward, wheel landing and keep the tail up all can reduce shimmy. So can landing on different runway surfaces (bumps and those water rivs).

Frequently what sets off tailwheel shimmy is when landing in a crosswind and on some models the tailwheel follows the rudder, when the tailwheel comes down the wheel is crooked to the airplane's forward travel. Ergo, airplane does a lurching turn (away from crosswind btw), and the impact can set off shimmy. A Scott tailwheel follows the rudder. Not all tailwheels follow the rudder though.

There are other kinds of tailwheels. Locking tailwheels(185's), full castering tailwheels etc.
 
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Rake and trail, the same rake and trail used on motorcycle front ends, has a lot to do with it. I'll dig up the article we found that help the TW shimmy in our J3...if you're interested.
 
The biggest and most common factor is the angle of the pivot axis of the tailwheel. If the top is tilted forward even a little, it's going to shimmy. Badly. Re-arcing the springs, or replacing them, often stops it dead. Everybody has had the shopping cart with the bent caster that shimmies like crazy. There you are.

Other factors include loose attachment hardware, a misaligned mounting, and a dynamically imbalanced tire/wheel assembly. That last one is usually totally misunderstood and almost never addressed, yet getting it right can fix a lot of trouble.

Shimmy should never be tolerated. It introduces twisting forces into the tailspring that are transmitted as bending forces to the tailpost tube, which can break. VERY expensive to repair. Been there, done that.
 
So I happen to have a picture of the TW and the pivot axis definitely could be the issue.

The airplane empty looks very much like the picture referenced in the article.
twnow_zpsm0oumeoh.jpg


I then photochopped it and added a 15 degree cant and it ended up more like the second picture. To make it worse the plane is empty in the picture and we are often flying as we were yesterday close to GW which would make the squat even worse.

Great info. Keep it coming.
 
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What tailwheel spring is on the plane? The PA-25 spring is a common replacement, and I believe that Piper issued a service bulletin regarding this. I'm also relatively certain that the late model Super Cubs got this spring as standard equipment. I believe the Pawnee springs were stiffer, thus helping the pivot axis even under a heavy load.

That said, I've never experienced shimmy that took "every bit of rudder work I had" before. Every time I've had shimmy it has been a much higher frequency shimmy, that does not respond to rudder input. It just cleans the tread off good tires. Anyway, this description makes me wonder if there isn't a more serious problem with your tailwheel assembly itself, necessitating a rebuild.

Another factor that may contribute is main and tailwheel tire size. What sizes are you running?
 
On my Citabrias, tailwheel shimmy would start to be problematical as the tailwheel leaf springs weakened over time, changing the angle shown in DFH65's image.

The other recurring failure mode was the breaking of the small spring that holds the tailwheel in alignment. #23 below:

ABI-3200A-2.jpg


I spent a lot of time with these things torn apart on my workbench, cleaning and greasing and repairing. They take a LOT of abuse on a training aircraft.
 
mondtster said:
...
That said, I've never experienced shimmy that took "every bit of rudder work I had" before. Every time I've had shimmy it has been a much higher frequency shimmy, that does not respond to rudder input. It just cleans the tread off good tires. Anyway, this description makes me wonder if there isn't a more serious problem with your tailwheel assembly itself, necessitating a rebuild.
Click to expand...

The last pavement landing I made late last year (the plane is based on grass) the shimmy was as you describe more high frequency and no big deal other than the shake. My wife was in the back so less weight than the other day. We also had an older tire on the tailwheel and we replaced it this winter, wondering if the new tread has more tendency to "bite" the asphalt rather than just scrub the rubber off. So I think there are probably several things going on here. TW geometry, possibly springs, and might be worth just pulling the whole assembly apart and going through it. Again everything works fine on the grass but that seems pretty typical.

Also my "every bit of rudder work" is probably different than yours based on my limited TW experience. I have only been flying TW for a few years and only have 42 hours in TW time. It just felt like I had my hands feet full at the time.:D
 
DFH65 said:
So I happen to have a picture of the TW and the pivot axis definitely could be the issue.

The airplane empty looks very much like the picture referenced in the article.
twnow_zpsm0oumeoh.jpg


I then photochopped it and added a 15 degree cant and it ended up more like the second picture. To make it worse the plane is empty in the picture and we are often flying as we were yesterday close to GW which would make the squat even worse.

Great info. Keep it coming.
Click to expand...


That.

Also I've found the maule twilwheel (single fork arm) just sucks, a properly rigged and flown Scott is smooth as glass
 
Grummans can have a similar problem with the castoring nose wheel. The maintenance manual says to tighten the pivot it so that there is a specific sideways force needed to turn the wheel when applied at the axle. From memory, around 15lbs I think. A tailwheel that can pivot with very little sideways force sounds ideal, but it my be at the root of the problem being experienced.
 
creweite said:
Grummans can have a similar problem with the castoring nose wheel. The maintenance manual says to tighten the pivot it so that there is a specific sideways force needed to turn the wheel when applied at the axle. From memory, around 15lbs I think. A tailwheel that can pivot with very little sideways force sounds ideal, but it my be at the root of the problem being experienced.
Click to expand...

Never noticed that, but that's got to get really bad to do that, I mean that nose wheel doesn't touch till you're dead slow in a Grumman, heck touch and goes they don't even touch
 
If it's a Cub the spring has probably lost some arch. VERY common problem. A Pawnee spring isn't the answer unless your old spring was too soft. A standard replacement spring will likely fix your issue, or you can get you spring re-arched.
 
creweite said:
Grummans can have a similar problem with the castoring nose wheel. The maintenance manual says to tighten the pivot it so that there is a specific sideways force needed to turn the wheel when applied at the axle. From memory, around 15lbs I think. A tailwheel that can pivot with very little sideways force sounds ideal, but it my be at the root of the problem being experienced.
Click to expand...

This highlights the primitive nature of some aspects of aircraft manufacture. Using friction to discourage shimmy. Really?? Can you imagine having stiff steering on your car or motorbike to discourage shimmy? Of course not. Shimmy was fixed decades ago with dynamic wheel balancing, and even before that wheel alignments that adjusted caster and camber were used.

Aircraft wheels sometimes get static wheel balancing. Sometimes. The equipment is simple and its use is simpler yet. We still don't have an affordable electronic balancer designed for nosewheels or tailwheels, though there are units for motorcycle wheels that could be adapted for nosewheels. I built a mechanical dynamic balancer based on an old design, and it works. Takes all the shake out of that nosewheel. Need to make a smaller mandrel for the tailwheels; I still do those dynamically by hand. The bigger problem is convincing people that it's a dynamic balance problem on their nosewheels and sometimes the tailwheels, though most tailwheel shimmy is due to beat-up tailsprings that hold it at the wrong angle. A lot of mechanics try to stop nosewheel shimmy by tightening wheel bearings or servicing the shimmy damper or replacing torque link stuff; some of it might help but it's only masking the real problem, and any new stuff is only going to get beat up again.
 
Stewartb said:
If it's a Cub the spring has probably lost some arch. VERY common problem. A Pawnee spring isn't the answer unless your old spring was too soft. A standard replacement spring will likely fix your issue, or you can get you spring re-arched.
Click to expand...

If it's a cub you're nuts if you put a old school leaf spring back there, T3 baby!


 
How do you attach the weights to the wheels when you balance a wheel? Its fairly simple to statically balance a wheel. Just put it on a oiled steel ball and see which way it leans. Then put a weight on the opposite side until it doesn't lean anymore. But how do you attach the weight?
 
I had the same issue with the Pacer. Replaced the spring with one from Airframes AK and no more shimmy, beefier, stiffer spring. Noted in logbook as owner provided part.

Rich
 
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