Kitfox III, bent tube

[JorgeEC-YUQ]

Hi everybody,

I would like to know your opinion, please see the pictures of my last found...

I discovered a slightly bent tube and a crack when replacing the bungee chords on my kitfox.

Seems like the bungee chord was longer than it's suppose to be and the safety cable made contact with the gear leg thus bending the structure.

Cheers,
Jorge

[Av8r3400]

That is a very common problem with these planes.

Here (http://www.teamkitfox.com/Forums/showthread.php?t=2851&highlight=Truss+repair) is a thread when I did nearly the same exact repair.

[JorgeEC-YUQ]

Thanks for the link Larry,

It seems quiet common indeed. What welding method did you used? What is the material of the tubing?

Thanks!

[avidflyer]

This is also a weak link on Avid Flyers, I would suggest filling in as much of the seat truss as you can with some .040" steel sheet. Jim Chuk

[Av8r3400]

Jorge-- the tubing is 4140 steel, I believe I used .049" wall for the repair. The tube gets sleeved on the inside and rosette welded to hold the sleeves in place.
If you choose to stay with the bungee gear, I would recommend doing as avidflyer suggested and put some webbing in that truss to keep this from happening again.

I put webbing in my side trusses to reinforce for the grove gear on my Mangy Fox project. Something like that would help the bungee truss.

[JorgeEC-YUQ]

Thanks for your replies,

I was going trough the FAR instructions for the repair ... Seems like I need a good welder.

Is it meant to be TIG welded only or what kind of welding machine did you guys used?

Thanks again,
Jorge

[Av8r3400]

Mine is all Mig welded, like the original construction was.

[JorgeEC-YUQ]

Hi Larry,

what kind of "filler rod" did you used for the welded repair?

Thanks!
Jorge


Mine is all Mig welded, like the original construction was.

[HighWing]

I am wondering how many incidents of actual bungee cord failure there has been where the airframe was saved by the cables. Some of my flying buddies cut out the limit cables to avoid the event triggering this thread and just went with the bungees. Since most of us are pretty good at the periodic maintenance schedules, I think I would focus on a timed bungee cord replacement and go without the cables. Wondering if the origInal design fix to prevent bungee cord failure damage has caused more problems than it has prevented.

[Jfquebec]

I agree with Lowell on that...better without the safety cable...change the bungee every year's or 100hr...

[Dave S]

Jorge,

The other guys who have answered your query are really knowlegable about this issue and the particular model so I am not going there.....I'd like to offere some comments on welding methods since that hasn't been touched on very much.

True, the original construction is all electrically welded; but, that does not mean you can't or shouldn't have the repair done with gas welding if it is available or the only option. Electric welding is common because it is fast and economical. Gas welding (by this I mean Oxy-acetylene welding) is entirely compatible with 4130 steel as used in aircraft frames (and the Kitfox) for a really long time. Many of the Classic tube and fabric planes are entirely welded up with gas. Gas welding does cost more in time and materials. It is a method that is also extremely versatile; and, requires a degree of skills not everyone is familiar with (except for those who do it). Aircraft gas welding often utilizes a process called "normalizing" - using the torch to heat up the completed weld and allow it to cool gradually to relieve stresses - hard to do without a flame.

One big deal - you use 4130 gas rod for welding 4130 steel - do not use mild steel rod. A competent gas welder knows this and how to get ahold of the right filler rod. If a welder doesn't understand this - walk away fast. Such filler rods are normally stamped on the end of rod with the 4130 identity - much like other specialty rods used for gas welding.

I will say that welding 4130 materials with a gas torch is one of those processes that work very well...the melt pool flows so well and flows together so well it's almost hard to believe something could work so well.

If you have a competent operator who does gas welding - no reason to rule that out. You can certainly do either gas or electric.

Sincerely,

Dave S
KF 7 Trigear

[jrevens]

Dave has brought up some very good points... I'd like to expand on them a little if I may. I have both TIG & oxy/acet equipment, & used to be pretty competent with both. I don't do enough welding anymore to be really good, but I know people who are. Here are some comments from a friend who is -

"Way back in the dinosaur days when gas welding was the norm and TIG was trying to get a foot hold, TIG got a bad rap because folks were using 4130 rod and it cracked. The reason the TIG welds were cracking and the gas welds were not was because there is a lot more heat in gas welding and the 4130 tubing, as well as the 4130 filler would pretty much get annealed in the process, thereby defeating the benefit of using Cro-Moly tubing in the first place.

Today, the current thinking for not only aircraft, but anything including Cro-Moly tubing weldments, including NHRA top fuel as well as NASCAR is: Filler rod, ER70s-2 or slightly better ER80s-D2 mild steel TIG rod, and the use of the absolute least amount of heat needed to form a puddle. The rule of thumb is blue/yellow weld, if it's grey it's too hot. A minimum of 70 deg. F ambient temp. and dead still air. And no post weld heating, none. Old plans that call out 4130 rod get ignored."

The "no post weld heating" comment refers (I believe) specifically to the process he talks about in his second paragraph. I believe, especially with TIG or MIG welds and most especially with cluster welds and truss structures like we have in our aircraft, that post heating (with a flame) to relieve stresses is an often neglected but important process. The lack of that is what causes a welded structure to "spring" from it's desired shape when removed from a jig. Some of this built-in stress is also a potential crack inducer. We don't see that quite so much with a gas-welded structure since material furthur away from the actual weld is heated more. That's what I think I know about the subject.

[JorgeEC-YUQ]

Hi,

I was able to find some welders working in aviation but they do TIG only.
I couldn't find anyone familiar with gas welding and aviation repairs around my place. I guess I could find someone t If I would look for a certified mechanic working in general aviation.

Personally I know nothing about welding but I want to learn. I specially like TIG but since I am not planning (or hoping ...) to do many airframe repairs or compromise welds in my kitfox I guess I'd rather go for MIG for general welding of accessories or other stuff in the house (being cheaper and apparently easier to learn?). I like the gas welding as well but better to focus in one I guess.

What method would you recommend to start with anyways?

And yes, those safety cables has to go.

Jorge

[Esser]

My $.02 after working with lots of welders. Welding seems cool and it is a neat skill to have but just pay some guy to do it on the side. It will be way cheaper, way higher quality, and your plane will be ready to fly mush sooner.

Some may disagree with me but I have had guys work for me who have thousands of hours of TIG experience and the stuff they can do for $100 is amazing.

[JorgeEC-YUQ]

Hi,

Thanks to a friend of mine who is aeronautical engineer we have finally located a shop in my area wich can provide with the steel required for the repair. I can tell you is not easy in Spain ...:mad:

Now reviewing this thread I got confused ... Larry you mentioned the steel in the frame is 4140 and not 4130? I guess the procedure of welding would remain the same with both grades?

Also my friend asked me for the "heat treatment" (normalize?) of the original tube since the strength of the new material has to be the same ...

Any ideas?

Cheers,
Jorge



Jorge-- the tubing is 4140 steel, I believe I used .049" wall for the repair. The tube gets sleeved on the inside and rosette welded to hold the sleeves in place.
If you choose to stay with the bungee gear, I would recommend doing as avidflyer suggested and put some webbing in that truss to keep this from happening again.

I put webbing in my side trusses to reinforce for the grove gear on my Mangy Fox project. Something like that would help the bungee truss.

[Av8r3400]

Yes, Jorge, 4130 is "chromoly" steel used in these planes.

4140 is a tool steel. That is my error. :o

[JorgeEC-YUQ]

:D 4130 it is then ...

I am trying to get the materials ready before I come back to my country (I am away now) so I can finally fix my bird .... anybody knows the dimensions of this tube? .049" wall , outside diameter?

[jmodguy]

Jorge
Here's a website that really helps in coping tube ends. I built an engine mount using this method and it went pretty quick. I think I was finishing a tube every 20 to 30 minutes. That included measuring, printing, cutting, fitting etc. It was extremely helpful!

Regards
Jeff

http://metalgeek.com/static/cope.pcgi

[JulietTango]

:D 4130 it is then ...

I am trying to get the materials ready before I come back to my country (I am away now) so I can finally fix my bird .... anybody knows the dimensions of this tube? .049" wall , outside diameter?

Hello Jorge
Did you measure the wall strenght of the tubing? I would appreciate very much having these numbers. I' ve been running into the same problem with bent truss tubing...:-(
And here in Switzerland it seems not to be easyer to get the material for the repair.

Have a nice day!