Broken Tail Flying Wire

[robcaldwell]

Yesterday I experienced a broken tail wire while inflight. I did not know the tail wire broke until after I was shut down and began refueling…

The incident happened while in level cruise flight and on auto pilot (2 axis). There was a sudden “pop”. My nose pitched slightly down and to the left. I had no idea what had just happened.

I immediately disconnected AP and reduced power slightly. The airplane felt ok as I touched the control stick left, right, up & down a few times. I changed course and turned to my destination, about 15 miles away (KIPJ). With the wings level I turned the AP on again, still trying uncover what happened. The AP pitch began hunting to maintain altitude. It was very minimal, but the nose was ever so slightly nodding. Even with the AP off, I could not trim and hold altitude. I entered the downwind and made gentle turns to base and final. With 3 notches of flaps, it was one of my best landings!

I taxied over to the fuel pumps and found my right upper streamlined flying wire had broken. See the attached picture.

Mark and I spoke on the phone. He stated this has occurred before, in the same location on a LSA. But never on a 4 place that he is aware of.

This left me stranded at KIPJ, and Mark is expediting a new wire to me now.


FOR DISCUSSION….

Mark mentioned harmonics as a cause to this failure. I’ve read a few threads here about vibration of streamlined H stab struts and tail wires, and broken tail strut threads.

https://bearhawkforums.com/forum/saf...321-tail-wires

I’m curious what others think about the overall direction I am considering with the tail/flying wires.

I am beginning to believe that streamlined wires are more prone to vibration over round flying wires. And the drag reduction benefit of streamline wires over round wires does not outweigh the failure potential.

I am not sure that I could ever position a fixed streamlined wire into the wind such that it would never vibrate. There could be a point in deck angle where no vibration occurs, but a slight trim adjustment will change the AOA of the wire (up or down) now creating an adverse vibration. I cannot imagine this occurring with a round wire.

As I mentioned, Mark will be sending me a new replacement streamlined wire to get me flying again, and to fulfill my Oshkosh obligation which I am very much looking forward to!

I wonder if I can dampen the streamline tail wires for the interim...

But long term, I am considering going to round wires (or possibly cables) only for the added safety aspect. If drag ever becomes a concern, I may consider fairing the round wires using something like this. https://antisplataero.com/products/u...ine-fairing-14

I’ve also read some discussion here from Isilverone and others about using velcro strips, or turbulator tape on the bottom of the H stab struts to reduce vibration. I will be looking into this as well with my new found sensitivity to tail vibration and cure for any and all vibration.

I would also like to know more about swagged cables for tail wires. Especially for the uppers and as a replacement for steamlined wires. If anyone has experience making and using these, please comment!

Thank you so much!

Rob

[Sierra Skydog]

Yikes, glad it only had minimal control effect for you. Thanks for the anti splat link, wasn’t aware of their products and like the fairings

[Bissetg]

Rob, Glad to hear all worked out OK. How did you tension your tail flying wires?

[robcaldwell]

Per Bob's instructions... https://bearhawkforums.com/forum/bea...g-flying-wires

[Nev]

Glad you're back flying Rob. You might be onto something with the aerodynamic wires. Could you tell us how many hours the aircraft had before the flying wire failure?

Do you recall the tension on the flying wires ? (As per Bobs specs/ Not very tight vs very tight )

This has been a discussion point on several occasions with my own aircraft. Currently mine are "quite tight", even after I've reduced tension somewhat. Sorry I don't have the actual tension.

I'm wondering if one factor affecting the flying wire longevity could be if they're tensioned against the lower horizontal stabilizer strut, and whether this might have contributed to failure of the struts that occurred on several aircraft. This might be done intentionally when rigging to "rig out" an out of square horizontal stab for instance. Mine was out of square when assembled and couldn't be easily corrected.

I believe that one Bearhawk also broke the bolt that holds the flying wires through the top of the vertical stabilizer.

Perhaps we could gather data points here from others that have had a broken flying wire, or broken a strut (the two failures might have a correlation).

In addition, has anyone broken a lower flying wire, or is it only the upper wires that are breaking ?

[robcaldwell]

Nev, I have 60 hours on the plane. I installed the wires per Bob's instructions.

[Mark Goldberg]

To put this in context I believe Rob is the first person to have a tail wire break on a 4 place. With lots of planes flying. Generally, wires break because the tension is too low or too high, or a crack got started and grew until it failed the wire. I had wires break on my LSA twice, but Bob determined it was the rigid way the homemade ends held the wires. After replacing the homemade ends with AN665's and tabs like used on the other models - there were no more problems.

Generally - when something happens to just one plane - it usually does not convince me that something is wrong with all the planes. When something happens twice (like on my LSA) then obviously something different needs to be done.

Curiously - the wire that broke twice on my LSA was the right upper wire also. In almost the exact place as Rob's wire broke. I suspect that is a high vibration area. Mark

[Nev]

One other thought - I recently had a small vibration through the elevators caused by the pitch trim tab hinge having play in it. I could feel the vibration through the airframe, although it was barely perceptible. It was easy to fix the play (discussed in another thread) and that removed the vibration.

[Ray Strickland]

For whatever it’s worth, I followed Dan Shilling’s (Subaru powered 4-place in Alaska) lead and used round control cable with swaged end fittings made for me by Aircraft Spruce. Draggy, but cheap. 500+ hours w/o problems.

[robcaldwell]

I would be interested in knowing more about these cable wires. Are they adjustable?

[alaskabearhawk]

I don't really have anything to add from a BH perspective of streamlined wires. But I do know there are thousands and thousands of streamlined wires flying on float rigging, biplane rigging, etc, etc and have been for years. If it was a common point of failure I would think they would have been replaced long ago with something else. Just a thought...

[Battson]

I focused my Engineering degree on failure mechanics and fluid dynamics, so I have a little knowledge here; plus experience with several similar issues on our Bearhawk 4-place.

robcaldwell
If you can send a detailed photo of the cracked surfaces, we can tell you a lot more about the cause of the failure. I am talking close-up macro photos, almost microscopic detail - in good light.

Round wires are every bit as likely to vibrate, probably more likely than streamlined wires. The streamlined ones are more likely to crack though, because of their geometry and the way they are made.

If you have the tension wrong, the flying wires can vibrate like crazy. It's about harmonic frequencies. Think about a guitar string, that's your flying wire.

Too much stress on the wires reduces the "minimum flaw size" required to start a crack - and every piece of metal has flaws. The stress this can come from over-tension in the wires, or wires which are flighting against the rest of the structure. These are ideal conditions for cracking to occur quickly. High frequency vibration with lots of force applied.

Any surprise change in pitch associated with a little 'pop', always assume you've lost a strut or wire on the tail and limit maneuvers until you can confirm what's happened.

[Battson]

How did you tension your tail flying wires?

robcaldwell commented
Per Bob's instructions... https://bearhawkforums.com/forum/bea...g-flying-wires

robcaldwell
I found my streamlined wires vibrated like crazy when I followed these instructions. I had to lower the tension considerably to stop the vibration.

[Ray Strickland]

Rob,
I used 5/32”, 1X19 stainless cable.
Select end fittings. Measure pin-to-pin distance. Call ACS and talk to their control cable maker.
About 6 years ago, I think the price was about $100 each.
My remembrance is that Bob was impressed with the workmanship.

[robcaldwell]

Thank you for posting the specifics, Ray.

When you says "pin-to-pin", I assume you are referring to clevis pin to clevis pin.

I have compiled the details and part numbers to construct this assembly. I will be ordering these today.

Stud End   Thread ¼-28   $8.35 Each

https://www.aircraftspruce.com/catalog/appages/ms21259.php?clickkey=16787

MS21259-5RH 
MS21259-5LH
 
Wire Cable = 5/32”   1x19   $2.19 / ft.

https://www.aircraftspruce.com/catalog/appages/controlcable.php?clickkey=9629

SS 05-04500
 
Terminal Assembly   Thread ¼-28   $24.00 Each 

https://www.aircraftspruce.com/catalog/appages/an665terminals.php?clickkey=8137

AN665-34R
AN665-34L

[Ray Strickland]

Rob,
Yes, clevis pin to clevis pin.

[schu]

I did the same using cables, but I skipped over the fork and pin and simply notched the link to accept a welded tube that is threaded. This way the threaded end threads directly into the link. There is a picture around somewhere on the forum if you search for it.

[Battson]

In addition, has anyone broken a lower flying wire, or is it only the upper wires that are breaking ?

A lot of rocks have bounced off my lower wires (round) and they have barely been majorly damaged. The important thing is to dress out any minor damage to prevent cracking. One hard hit should be fine.

The geometry of a streamlined flying wire and the way they are made makes them a little more prone to cracking, compared to a round wire which is more crack-resistant, but this should not be a concern. Streamlined wires are very common in so many aircraft types and rarely crack. The problem here is probably related to an unusual flaw in the wire, combined with the way they've been set up. If Rob shares some photos on the fine details, we can make some better judgements.

[Nev]

I found my streamlined wires vibrated like crazy when I followed these instructions. I had to lower the tension considerably to stop the vibration.

This is a very good data point - I wonder if Bob is aware of this ?

[robcaldwell]

If Rob shares some photos on the fine details, we can make some better judgements.

I will try to get some better pics (close ups) of the break tomorrow.

[robcaldwell]

Battson Using my iPhone, I was able to capture some grainy pictures of the broken ends of the streamlined wire. There is an obvious crack(s). This would have been difficult to identify during preflight as the crack was located on the bottom (underside) of the wire.

25E10267-5F0E-4F52-8F4B-E15A3A8062F7.jpg



5B56B100-F822-4713-A412-60522D671976.jpg



C1E618AE-278A-4C05-BEAC-6DC41DD73580.jpg

[Nev]

Thanks for posting this Rob.

[Battson]

OMG who made those wires.... They almost look they have been drawn, I would have expected cold-rolled...?