Tail wheel shimmy

Tail wheel shimmy

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A follow-up on curing Da Liddle Birdie’s sashaying around on the ground because the tailwheel spring mount was wobbly.  It’s somewhat long (sorry), but if you’re going to do this, detail is handy.
I chased down the aircraft welder recommended by Bob Reynolds and Gary Hendel.  The gentleman’s name is Monty Wells, at (705) 774-8814, for the Stinsoneers in southern Ontario (“We happy few”…… 


).  The people at Lawrence Aviation (recently bought out by another firm) at Parry Sound Airport were most helpful in providing Mr. Wells contact information.  
End result: Da Liddle Birdie now tracks straight on grass (I have to go to North Bay or Parry Sound for some pavement to play on, yet), and I do not have to correct for uncommanded yaws.  The steering is MUCH better – I much need less rudder/brake/throttle to effect a heading change on the ground.  Very much worth doing, and a whole lot less painful to implement than I thought (was afraid) it was going to be.
Eric and Da Liddle Birdie, who now doesn’t waddle around.

The gory, sordid details:
I did as much prep work as I could ahead of Mr. Wells arrival; he came to South River CPE6, which was a good thing, because half of the tools I thought I would need weren’t needed, and the other tools that I didn’t think I would need, I just had to go to my tool chest. 
The tail was up on a bipod support, and after I pulled the tail wheel off, we drilled/deburred a No. 19 (0.166″) diameter pilot hole up and forward (45 degrees) through the cross-tube centerline.  Degrease the cross-tube and the outer bushing thoroughly – you get a better, cleaner weld with uncontaminated parts.
The outer bushing was then set in place using a jig I created, and the bushing was rosette welded via the just-drilled cross-tube hole.  This fixed the position of the bushing, allowing the removal of the jig.  Mr. Wells then showed a very deft hand with a TIG welder, by putting the proverbial circle of dimes weld around both ends of the bushing.  If I weld it, I will NOT fly it – but I know enough about welding to know THIS guy KNOWS how to drive a TIG.  It was kinda a shame to cover up the welds with the spring yoke….    I had to dress out a small bead around the inside edge of both ends of the bushing, but not a lot. 
Best of all, there was NO, ZERO damage to the Ceconite fabric.  No burning, no discoloration, no dimples, no wrinkles, no glued fabric separating from the fuselage structure; a little smoke needed wiping off, but that’s it.  This part really made my day!  
We discussed protecting the fabric prior to starting the welding, and I cut and folded a heat-shield out of some thin aluminum sheet (another item not on the list of stuff I thought I’d need) to cover the rudder post, and the fabric on either side of, and under the fuselage.  All kinda eye-balled, but I could generate a heat-shield sketch as a starting point (your mileage may vary……).
Not having a fancy ream, I cleaned out the bushing ends using a modified chain-saw file (I AM from the Great White North, after all….).  Chain-saw files have fine teeth, a constant diameter (no tip taper), and a tooth-free section at the tip of the file.  Put a small, polished smooth radius all around the (untoothed) tip of the file, and with a little lubricant, the file can then ride the inside surface of the outer bushing without scarring that surface.  Remove material ONLY from the ENDS of the bushing – the central section of the bushing is already factory-fitted, so keep your cutting strokes VERY SHORT and confined to the welding bead ONLY.  You want the file parallel to the axis of the bushing, all around, with the teeth traveling over the bead, ONLY.  The untoothed, cylindrical tip of the chain-saw file helps to set the parallel alignment of the file to the inside bushing surface, so I STRONGLY recommend you do not use a taper-tip file.
Be patient, and take out only a little material at a time – you will be doing a lot of trial fittings of the inner bushing, to gauge your progress.  Do NOT hammer the inner bushing into place – if you force it, then you will distort the bushing out of round, and the spring mounting bolt probably won’t fit through the inner bushing.  You thought you had problems before….?
The inner bushing needs to JUST fit, and to go all the way through the outer bushing (from either side of the aircraft), WITHOUT any wobble.  Any wobble here just means you’ve re-introduced the problem you were trying to fix.  Mr. Wells was in and out in 2.5 hours; I spent the rest of a longish day gingerly shaping the bushing ends with my chain-saw file.  BE PATIENT.  Some wooden dowel makes a good mandrel for fine emery paper, to do a final polish on the inside ENDS ONLY of the outer bushing..
Don’t forget: the welded parts WILL need some scaling, cleaning, and then painting, to avoid rusting.  Best done before putting the tail wheel back together; let the paint cure, then grease the inner bushing and the bolt.   

I had prepared the bipod to hold the wheel-less tail off the ground, when I previously replaced the sagging tail wheel spring (a necessary part of the initial efforts to fix Da Liddle Birdie’s tail wheel problems).  Made out of 2 x 4 lumber (I picked the best wood I could find), the bipod is basically a wooden H, with the H uprights on the floor, running fore-and-aft.  At the ends of the H’s crossbar (also on the floor), I have two hinged arms that swing upwards towards the stabilizer attachment bolt heads over the center of the H crossbar.   Don’t go cheap on the hinges – saving a few bucks on flimsy hinges won’t begin to pay for patching holes in the fabric…..  I’m a big fan of glue-and-screw, build-for-a-thousand-years, especially for construction projects that don’t have to fly.
The hinges are deliberately not quite square to the H, although they are both centered on the H upright/crossbar intersection.  This allows the bipod to collapse flat, with the (skewed) swing arms lying flat beside each other – it’s easier to slide the (collapsed) bipod under the fuselage.
A short piece of 2 x 4 is secured to the aircraft-inside of each swing arm, just below the top of the swing arm, to form a step.  There is a large-diameter flat washer screwed on the top of the step, to act as a load-bearing surface (instead of the 2 x 4 end-grain).  Two stacked eye-bolts are secured above the washer on the step – these serve as a guide for the short lengths of copper tube that catch on the heads of the stabilizer mounting bolts (acting as sockets, so the stabilizer can’t slip off) and carry the load down through the eyebolts to the flat washer on top of the step.  Copper tube will carry the weight, but is soft enough to not damage the bolt heads.  The pairs of eyebolts ensure the copper tube stays aligned in the bipod.  
You do have to be a little careful to ensure the eyebolts do not catch on/drag the rudder cables while lifting the aircraft on to/off of the bipod.  One person can install/remove the bipod (I have, several times), but the process is faster with two people.  
You will have to lift the aircraft a little higher than the resting height on the bipod, to ensure the swing arm arc clears the bottom surface of the stabilizer, especially if the copper tubes are already in place.  I use a scissor jack, sitting on a hollow box made of 2 x 6 lumber (glued and screwed together) to do the actual lifting; I find a bottle jack doesn’t have the necessary lift, but is too tall to fit onto my hollow box AND under the aircraft tail. 

I also prepared a fixture to hold the OUTER bushing in place for the initial (tack) welding.  I’ll make a drawing of this, but essentially, a threaded rod goes through the cross-tube, with the outer bushing supported on Swage-Loc conical tube seals, to center the bushing on the threaded rod; washers and nuts hold the conical tube seals finger-tight against the bushing ends.  
The jig is assembled to also bind on the ends of the cross-tube; this won’t guarantee the bushing’s alignment, but it will hold the bushing for tack welding.  The threaded rod needs to be straight and long enough to establish the squareness of the bushing in the yaw and roll planes; I used the stabilizer rear spar as my datum.  Finger-tighten the jig, then adjust the threaded rod for squareness in both planes, and CAREFULLY snug up the nuts, WITHOUT moving the rod.  Do the initial tack weld before anything gets moved.

On Monday, February 11, 2019, 02:05:14 p.m. EST, ‘Jim Custis’ jimcustis@livewirenet.com [Stinson] <Stinson@yahoogroups.com> wrote:


I’ll be following your progress as I’m facing the same thing. I’ve already got the new bushings and bolt but haven’t quite figured out how to make it all work.. Good luck and let us know how it all works.

Jim Custis   N97626  

From: Stinson@yahoogroups.com[mailto:Stinson@yahoogroups.com
Sent: Monday, February 11, 2019 10:18 AM
To: yahoogroups
Subject: [Stinson] Tailwheel mount repair

I’m looking for advice on repairing the tail wheel mount on Da Liddle Birdie (1947 108-2 Voyager, s/n 2252) 

The cross tube for the bushing that accepts the long AN4 bolt has deformed out of round over the years, so the tail wheel tends to flop side-to-side a little, resulting in a yaw on the ground. Da Liddle Birdie actually had the original one-piece bushing, but replacing this with the Univair 108-5311000-12 / 108-5311000-10 pair didn’t solve the problem – the original bushing was still perfectly cylindrical (THICK walls); it was the THIN-wall tube welded across the longerons that distorted.

Right now, I am piecing together a jig to ensure the larger (108-5311000-12) bushing sits in the (loose) tube level WRT the aircraft and square to the aircraft long axis.  The jig is using the mounting bolts under the stabilizer as the datum.

What I am proposing to do is:

  1) jack the tail up (I built a stand to support the aircraft), 

  2) pull the wheel assembly, 

  3) drill a small hole UP through the bottom (only) wall of the distorted tube, dead center, 

  4) jig the replacement -12 bushing into place

  5) fire-proof the fabric in the vicinity six ways from Sunday….

  6) rosette weld the -12 bushing into place

  7) remove the jigging

  8) finish weld both ends of the -12 bushing to the distorted tube (ensuring the -10 bushing still passes through the -12 bushing readily)

  9) repair the fabric

10) re-install the (Scott 3200) tail wheel

Has anyone ever made this repair?  Have I missed anything?  Is there a better way?  Has anyone built a jig for this task?

I need to wobble the aircraft while taxiing, just to see over the nose, but (especially on pavement), I’d like the wobble to be MY idea, not the tail wheel’s….

Eric and Da Liddle Birdie (who has her own idea of “cross-country” on the ground….  )