Tailwheel tie down ring - winch point

[RobfromME]

Good morning,

For a couple months my airplane was in a hangar with a gravel apron and fairly steep up slope. I would pull it in as far as I could with the BAS handles, then attached a rope through the tie down ring on the tailwheel and used a winch (low speed) to pull it the rest of the way into the hangar.

I recently had the tailwheel shimmy on landing for the first time since owning the airplane (about 18 months). I experienced shimmy once a couple weeks ago, and once last Sunday.
Is there a chance I provoked this by pulling the airplane up into the hangar with the winch?

One additional item; since installing the Sportsman STOL kit, I usually touch tailwheel first when 3 point landing.

Thanks in advance for any thoughts.

All my best,
Rob

[cessna170bdriver]

Check the angle of the pivot bolt on the tail wheel. It should lean back slightly (the top should be slightly aft of the bottom). If using the winch has bent the tail spring such that the pivot is leans forward, that could cause shimmy. It’s also possible that the main leaf of the tailspring is just tired and in need of replacement anyway. Another thing that can cause shimmy is that the pivot bolt and/or its bushing is worn.

Hope this helps.

[GAHorn]

MIles is describing the correct set-up angle of the “Kingpin” ….

To avoid “shimmy”….the Kingpin angle should be as illustrated by the GREEN line… not the red lines. (one of the few times Green is Good on a 170)

This angle is established by the tailwheel Main Leaf Spring. If yours is sagging, then it should be replaced with a new one from Spruce or Univair. (only the Main spring is usually necessary, and should be changed every 500 hrs or so anyway to avoid a common breakage issue that can ruin your rudder when it breaks upon landing and the captive tailwheel beats the smithereens out of the rudder.)

CLICK to ENLARGE

[n2582d]

Rob,
I think it would be much more likely that the tailwheel mainspring was tweaked if one was pulling from the tailwheel axle than from the tie down ring.

Bruce has posted Scott’s tailwheel shimmy checklist here. I’d never noticed that they say, on item seven, that the center spindle should be “perpendicular to the ground.” Alaska Gear Company agrees that the kingpin should be vertical but with the important detail that it should be vertical “at maximum gross weight”. The Installation Instructions for their ABI 3200 tailwheel assembly say, “The center pivot spindle should angle back away from the airplane 5 to 7 degrees when un-loaded and be perpendicular to the ground at maximum gross weight.”

[RobfromME]

Thanks so much for the information!

I did have the angle of the kingpin on my list of things to check, and the spring. The additional information and explanations in your responses was extremely helpful! (Especially since I'm a visual person!)

I'll be up at the hangar Friday or Saturday so I'll have a look then and report back.

Once again, thank you for the help!

All my best,
Rob

[n2582d]

I’ve calculated that, at gross weight, to go from the forward C.G. limit to the rear C.G. limit would amount to a change of 54 lbs. at the tailwheel. I wonder how much 54 lbs. would change the kingpin angle? I doubt it would be more than a degree. Do any of you notice the tailwheel more likely to shimmy when you have rear seat passengers?

[RobfromME]

Good afternoon,

I looked over the tailwheel assembly and found that the kingpin is angled forward (top is further aft than the bottom), however the spring is definitely less contoured than what I'm seeing on the examples seen here, and appears to be bent near the junction of the lowest spring and the rest of the stack.
Thus, I've ordered a new lower spring and new hardware to install. There's no damage from the next lowest spring cutting into the lowest spring, but the bend there is a bit alarming so I'm not flying it until the new spring is installed.
I'm considering larger tires to help with the three point landing always being on the tailwheel first. Even with just me and full tanks, the 3 point landing results in tailwheel touching first.
Thanks so much for all the input and help! The information on how to change the spring out that I found here is also incredibly valuable!

All my best,
Rob

[GAHorn]

A full-stall landing will almost always result in the tailwheel touching down first,..at least it does when I attempt it with full flaps.