Broken Tail Flying Wire

[spinningwrench]

Because of the high cost of streamlined flying wires I elected to make my own round wires using the same vendors as American Champion Aircraft and also the New Waco guys. Basically I started with 1/4” 4130 round stock and had it centerless ground down to the root diameter of the 1/4”-28 thread. I then had right and left hand threads rolled on the ends. Rolling produces a much better thread than cutting. All work was done by the above mentioned vendor I only had to supply it cut to the correct length. I sent a sample to Bob he said they looked really good. I calculated that with a batch order price per wire would be around $55. Note that American Champion only uses 3/16” round wire even on their fully acrobatic airplanes and don’t seem to have a problem with wires breaking. These wires actually have relatively low stress on them but possibly lots of fatigue potential from vibration. I calculated that during full utility loading each tail surface sees about a 350 lb maximum downward force. I would have to review the actual force on the wire but it is well below its yield strength.

[Battson]

robcaldwell
Sorry - busy time here so I will start with a brief post.

Summary:
A large manufacturing or material defect inside the steel, combined with aggressive harmonic vibration of the wire, were likely the cause. Think guitar string with a big crack in it.

Detailed analysis:
Those wires failed by 'high cycle fatigue'. Normally it would start at a geometric stress raiser (i.e. sharp leading edge of the wire), but in this case the crack has definitely started at the top of the wire as we see it pictured. The striations show the crack worked its way outwards and downwards, and it did so very quickly. This is a strong indication that the wire failed due to aggressive harmonic vibration causing bending in the up-down direction (transverse to the chord plane).

We see a large vertical shape inside the wire - a flaw, likely caused by the cold drawing process I guess - this 'flaw is unquestionably where the fatigue crack started, spreading outward from there. There are tell-tale 'scour' marks on the outside of the wire, I see, hinting at the flaw hiding inside. For cold rolled steel flying wires, the outside of the wire should be quite uniform dull grey with a tiny dimpled or pin-holed look from the rollers (unless they have been polished out). Those wires of yours appear to have been cold drawn though, they appear to have a uniform shiny linear look - but again, there should be no 'scoured' areas. I have been told by an expert that cold rolled gives a more reliable flying wire, but I cannot discuss the reasons or validity of that.

Suffice to say there was large flaw in the metal with a sharp edges, which greatly raised the stress around that microscopic edges of the flaw. Either bad steel or untidy manufacturing processes probably created the flaw. This appears to have created local stresses inside the flying wire which exceeded the yield strength of the stainless steel at a microscopic level around the crack, every time it vibrated (each cycle) up and down the crack would grow a tiny amount.

I would need photos with light sources from different angles to be sure, or photos taken from different angles - but it looks like the wire had effectively cracked right through when it finally failed. I guess it was cracked 95% to 98% of the way through before it finally broke. This means the tension on the wires was not a major contributor to the failure, apart from allowing harmonic vibration to occur. Most of the cracking was caused by bending / vibration.

Recommendations:

• Replace the wire with a good quality part
• Inspect the remaining wires for unusual "scour" patterns on the outside surface, if found - try and polish them out to see how deep they run. If 30 minutes and wet 2000grit will not remove the scour marks, consider replacing the part or starting an intensive monitoring program - suggest dye penetrant to search for potential cracks every 25 hours (don't spill on the fabric!)
• Adjust wire tension to prevent vibration, remember they are a connected system of four wires, which need to be adjusted together.
• A visual preflight inspection including a firm wiggle on the wire would likely have identified this crack before it broke in flight. Better to break it on the ground by your own hand, than at 10,000ft in turbulence.

[robcaldwell]

Thanks so much for the profound thoughts and insight, Jonathan! I have received the replacement streamlined wire from Mark and returned my plane back home. However, I have also ordered replacement 5/32 1x19 SS cables from Spruce. They will come with MS21259 stud ends installed and pressed in place at their facility. Top wires only. I will maintain the current Steen round bottom wires in place, and inspect as you suggest. New top wires (cables) will arrive tomorrow and I will install before the trip to OSH.

[AZBearhawk272]

Well stated!
Kevin D.
272

[Mark Goldberg]

I forwarded the pictures of the broken wire to Bob Barrows and also to the US distributor of these wires. She will forward the pics to the manufacturer. Bob also thought Jonathan's write up was accurate and could find nothing to disagree with. Bob has requested the manufacture give us a method to test wire tension without the builder owning a tensionometer. Taking into account the exact length of the wires - how much deflection of the wire at a certain amount of pounds of pull in the very middle of the wire. Hopefully the manufacturer will provide some guidance. If not Bob will calculate something. Mark

[Pbruce]

I’m pleased to see everybody working together on this to find workable solutions. Thanks to all contributors and to particularly Mark and Bob for this kind of support. Good work, team Bearhawk!

[Nev]

A few of us in NZ had wires made by Russ Ward at Old School Aero. I asked Russ if he could make a recommendation for tensioning the wires made by him and he responded with the following :

The tension should be about 350lbs

This is derived from the formula F = (4DT)/L where F = pull force, D = deflection, L = length and T = tension.

4x.5 x350/42 = 16 lbs on a fish scale.

So a 12mm deflection should read about 15-16 lbs on a fish scale,
They have been known to break if left too loose.

Not sure if this helps or if it can be applied to wires made by other manufacturers.

[svyolo]

K&S aka "Hardware Store" aluminum sells some pretty slick airfoil shaped aluminum tubing that would cover those flying wires nicely. I believe someone also sells them in carbon fiber.

[zkelley2]

Burl's aircraft(owns the TC to the Aeronca Sedan) in Chugiak, AK has the machines to put rolled threads on round wires if you're looking for another vendor. I think it was about $50 a wire for 1/4" SS wire when I had him do mine.

[ttibc@msn.com]

If the small rod end mount needs to be bent to align properly with wire end is anything special required or just bend to fit?

[Bcone1381]

I left my rod ends straight and adjusted the angle of the links.

Screen Shot 2022-07-17 at 2.28.27 PM.png

[ttibc@msn.com]

I wasn’t clear but that is my intention.
Thanks,
Bill

[way_up_north]

My son is considering buying a CH-300 homebuilt… so I’ve been reading up on the history of that plane.. it has suffered from aileron flutter in certain circumstances… this has been fixed but there were some interesting reports leading up to the fix

one report was that the pilot felt no vibrations on the stick …. But looked out the window at the aileron fluttering up and down 1/4 inch …

these wires are invisible back there and a lot could be going on in certain circumstances

placing a go pro back there watching areas of interest.. could shed some light on what’s going on

[Battson]

Yes, you can see the tail wires out the windows of the 4-place, and you will see them vibrating.

You will not necessarily feel the vibration through the controls or the airframe, you probably have to look for it.

[Brian Mason]

Hello Gents.
Ive seen AM/FM radio antennae on cars and trucks that are round wire with a spiral protrusion wound around spaced 1 or 2 inches along which deters the antenna from swinging back and forth in the wind, especially with a layer of frost on them. If you’ve ever seen a set of Citizen Band radio antennas on a semi truck waving back and forth as it goes down the highway, this is what I’m referring to. Wouldn’t this work as well to deter harmonic resonance in flying wires?
Brian

[Bcone1381]

That insightful. I have an antenna with a spiral and wondered why.

Harmonics and flutter are similar if not identical. Things have to be right for flutter to happen and it's mysterious. Sing the right note and the champagne glass breaks. Any other note wont effect it. That spiral wire must change things on the antenna. I saw velcro on a tail wire at OSH this year that stopped the flutter/vibration. A change in shape, tension, length, a wire's angle of attack, or who know what, can induce or eliminate harmonics. One thing I lean towards believing is that our wires strength meet all design parameters, but high fatigue from harmonics will teach us a thing or two. We'll get it figured out.

[Cguy]

My experience may be of some interest, though it was on my RANS S-7S. The stock tail bracing setup is round, not streamlined, stainless "aircraft" rod. Years ago,, back when RANS was using 3/32" 7x7 cabling for the task, on my first S-7, I got the bright idea, after seeing an ad in a hang gliding magazine, to buy some PVC streamline fairings that simply snapped over the cables. Drag is drag, less is better I figured, even at 85 MPH.

First flight, I felt a vibration in the stick, opened a door and looked behind, and they were buzzing! Took them off, moved on. 20 years later, I see Anti Splat's ad for rod fairings, bought a set, glued them on. After about (I'd have to check the log book) 20-40 hours, I landed back home after a long XC over very remote country, and noticed my left side, lower rod had broken off, and was swung back against the tailwheel. Now, this was the same rod that I had bent in a mountain landing when a rock bounced off it and went clear thru my hor stabillizer, putting a slight bend in the rod. I remember taking it off and eyeballing, deemed it repairable, and gently straightened it out with a rubber mallet. Flew with the repaired rod 500 to 700 hours, but it broke only after installing the fairings. After my first experience of flutter years earlier, you can bet I videoed the Splat fairings, and they looked fine, no buzz in the stick, and also no speed increase whatsoever, but I wasn't really expecting much, I'd be happy with the same speed and a bit less fuel burn. Edit: the slight bend in the rod was mid span, no where near the threaded end fitting, my thinking at the time was that the fitting end had not been stressed.

I took off all the Splat fairings, repaired the broken rod with 3/32" cable, and have flown it about 100 hours since, and expect no further issues. My theory is that the slight bend stressed the fitting, and maybe (or not) the fairing somehow stressed it a bit more. The threaded ends are quite brittle, I put the unbroken end in my vise and tapped it sideways a bit, then back the other way, and SNAP, it broke. Did the same with a AN 3 bolt, it took many more, much more extreme bends, before finally breaking. The good news is....losing one hor stab bracing rod didn't send me into a death spiral, it flew fine, I'm done experiementing back there.