Tail wheel shimmy

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”…… 

Emoji

).  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:

Eric,

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….  )

Fuel sending units

The sender is grounded to the tank by the mounting screws but the tank needs to be grounded to the fuselage by a wire that goes from a tab on the tank to a screw on the fuselage root rib. Check for this ground between the tank and any good ground on the fuselage with an Ohm-meter. If the sensor is bad (eg. 2-4 ohms empty, 24-30 ohms full) you will need to drain the tank. It often takes a bit of worrying to get the sender, gasket, and float in place through a misplaced inspection hole. Been there, done that!

The Stinson fuel gauge is fairly easy to test: an open circuit to the sender shows full ++, a short to ground shows empty, a 15 ohm resistance to ground is 1/2 full.

The fuel level measurement system is basically identical to what General Motors used in automobiles from the ’30s until digital and computerized autos came along.  It consists of a gage which has a bimetal strip with some wires wrapped around it connected through a direct current source to a variable resistor that is in the fuel tank with an arm and a cork float to change resistance in proportion to the amount of fuel in the tank and the change in resistance results in a change in current through the circuit and through the wires wrapped around the bi-metal strip and the bi-metal strip is heated in proportion to the current through the wires and the bimetal strip which bends as a function of temperature and is attached to a lever or gear system that moves the visible pointer in proportion to the amount of current through the gage which is in proportion to the resistance of the variable resistor in the tank which changes according to the level of the float.
Unless someone has connected the two terminals on the gage directly across a battery and melted the wires inside, the gage itself rarely fails.  Like someone else mentioned the wiring from the gage to the resistor in the tank goes through a connector at the wing root and then is under a terminal on the sending unit in the bottom of the fuel tank.  The circuit is completed through grounding of the wing to the fuselage.  There was originally a grounding strap in the upper wing root accessible under the metal strip between the top of the fuselage and the wing.  
But, the most common failure is the sending unit itself.  Stinson uses a single gage switched by a switch above the fuel selector valve handle to determing which sending unit in which wing it is connected to.  Page 107 of the General Service Manual shows the wiring diagram for the fuel measurement system.
It is all pretty simple and a cheap volt-ohmmeter from Harbor freight is all that is needed for trouble shooting.   
Larry Wheelock  `

  

I am not Larry, but I am sitting here with an original fuel gauge in my hand. It is so original that it still has a small cardboard note on the left terminal (as viewed from the back of the gauge). The note reads: “Connect sender wire to this terminal. Warning- If hot wire touches, sender will be damaged.” The right hand terminal has a 1/2inch diameter device connected to it with a second connection at the end of the device. I believe this device is a resistor to allow a 6 volt gauge to be used with a 12 volt system. The 12 volt supply connection would be made to the end of this pellet.

Aux Fuel tank

Fuselage tank

From the Archives, a fuel tank that fits in the baggage compartment that contains 19.144 US Gallons of fuel, of which 18 gallons is usable. 18 gallons of fuel in the baggage compartment that is limited to 100 pounds of weight is a pretty significant weight shift,( I estimate the installation at about 122 lbs.when the tank is full) so if you do this, consider the effects of all the extra weight. I have no 337 approval for this installation, anyone who has or gets a 337, please fwd to me or email to the webmaster. A fuel cap that fits the filler neck of our airplanes is an Ideal #46146, or   from any Napa auto parts dealer.

The aux. tank is fabricated from 5052 .032, the baffles are .032 and the end plates are .040.

The tank is assembled with 1/8” self-plugging Aluminum “Pop” rivets, and then welded, all joints and rivet heads. Welding such thin metal requires a lot of skill and should be done by a competent welder, or the tank will buckle and not lie flat. Fuel Cap and Filler neck, use style No. 2  0513-0018, (all part numbers refer to the Aircraft spruce Catalogue.).

Dia 1:

 Weld a ¼” Hose nipple into the side of the weldable filler neck, this is for a vent line. 6 or 8 feet of 3/8” O.D. x ¼” I.D. Vinyl tubing is run from the tank in an overhead loop, then out the bottom of the fuse. The fuel pick up is One Alum. AN833 Bulkhead elbow size –6D. Weld fitting and a 2” Alum. Washer to the top of the tank as shown below.

Dia 2:

No drawings are to scale. Recommend you cut patterns out of stiff cardboard, then Aluminum.

The Pickup tube is fitted to the elbow after the washer is attached, then the assy. Is placed in the tank and the washer welded to the tank and the elbow fitting. The Fuel lines are installed as shown  below,

Dia3:

Install an indicator light or a low-pressure gauge (0-10#) in the Aux. Fuel line to give a warning when the fuel pump is starting to cavitate, allowing the electric pump to be shut off before any damage can occur. Replace the 2 position Gas Gauge switch with a 3 position switch that will show Left, Aux., Right. Note that the pump is located at a lower level than the bottom of the tank. A 3 way valve is fitted into the line from the left engine. Same type of valve  as the main fuel selector can be used, HD 108-6 will work fine.

Dia4:

The Filler is centered 4” back from the radius line and 2 ½” from the edge of the tank. The fuel supply line is centered, 1 ½” from the edge, 2 ½” from the break line. AC sender is centered 4 ½” back from radius line, no height given below top edge. (I think 1 ½” below would work.) Sender is located on Left side of tank. Radius for front roll is 3 1/4”, = 7 ”inches for radius.

Dia5:

Make 2 baffles with fan tails around the edge and break to fit inside the tank . No figure is given for the flange on the baffle or the end plates. I would go with 1”, Use as few rivets as possible on the end plates, if you feel confident about your ability to weld this unit together, a good place to start would be to get the baffles tacked, then welded in place with ¼” runs every inch or two, that way the whole tank is more likely to stay straight. I intentionally did not show the entire radiused end with fan tails as I figure you can see what is needed. The end plates are made up the same as the baffles but without the drain and surge holes. Break to fit into each end of the tank, drill de-burr, Cleco and rivet in place. Before installing the tank be sure to leak check, and get a 337. If you are contemplating an Aux. Tank discuss this with your mechanic/AI before the project, it helps to talk to the local GADO folks as well. This can solve a lot of problems that could occur down the road.