What's the proper way to repair this?

15 posts in this topic

Posted

Hi guys,

 

Last summer I bought a Kitfox IV and trailered it home from Yellowknife, NWT for 1600km. Someone had advised me to support the rear of the fuselage with blocks to prevent stress on the tailwheel spring with the wings folded. Unfortunately, due to the rough roads in northern Canada, when I got home I found that the tailwheel spring was fine, but the tubes along the top of the fuselage were bent. Now I'm wishing I were just replacing a tailwheel spring.

 

I was busy with other things over the winter, but now that the weather is getting nice, I'm getting the itch to get this Kitfox in the air so I would like to repair the fuselage.

 

I am am pretty mechanically inclined and have taken the Polyfiber covering course so I am comfortable with fabric work.

 

I have attached a photo of the damage. What would be the best way to fix this? As you can see the fabric has some holes already, should I just strip all of the fabric off and straighten the tubing and re-cover the whole fuselage? Or is there a way I can do this repair without doing a full re-cover?

 

Thanks,

Randy (Alberta, Canada)

 

See photo's below:

020.jpg

019.jpg

021.jpg

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Posted

I would strip the area and get a good look at the tubes that are bent.  If the top longerons are bent, there is a pretty good chance that the lower ones and some supporting structure is bent also.  Mine was bent alot worse than yours was and the tubes were kinked.  The chromoly tubes will take quite a bit of bending before it takes a set so you must have whacked some big bumps pretty good!

 

Was there a strap over the fuse right in that area?  It looks like it is pulled  in as well as bent down.

 

If the top tubes are the only ones that are bent, you would be surprised what you can do with a rubber mallet to straighten tubes out.  I would give it a few whacks with a SOFT rubber mallet to see what you can do to straighter it with the covering on before I stripped the fuse.  Just make sure you dont kink or buckle a tube as that is automatic tube replacement time then.  Is there a local AME that is familiar with tube and fabric planes that will take a look at it for you?

 

Just out of curiosity, did you haul this on an equiptment trailer?  I ask this because the plane weighs in around 600# and equiptment trailers are rated at 8000# +  so you have virtually NO suspension on the trailer for the plane.  This sucks big time that your new pride and joy got bent up before you even had a chance to fly it!

 

:BC:

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Posted

Thanks for the reply, there was no strap in the area, but yes the tubing is bent in as well as downwards. Yeah there are a couple of AME's around here that are familiar with tube and fabric planes, but they charge a lot for their work. This plane is registered in the advanced ultralight category in Canada so I can legally work on it myself, but I want to make sure it is safe so I will probably get the help of a professional.

 

I did haul it on an equipment trailer, a photo is attached:

006.jpg

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Posted

Hindsight and all I know, but taking off the front set of tires on the trailer may have helped the situation, too. (For future reference)

If your welding skills are okay, there's a ton of repair references out there on the 'net. (FAA part 43 - I think - has a bunch.)

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Posted

Looks closely at the rear spar attach point.  If you bent the upper longerons, there is a good chance that the carry through tube on the rear spar attach is tweaked too.  I hope for your sake that it is not tweaked, but it stands a better than average chance of being twisted too.

 

Generally it is towed tail first to keep the ram air pressure from filling the wings at the spars and the open root end and balloning the fabric up and pulling it away from the ribs.

 

:BC:

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Posted

for future towing, the modified light boat trailers work well.  Atleast with a boat trailer you will have the trailer suspensoin soaking up the bumps alot more!

 

I straightened out the tubes around the seat truss and under the door frame on mine with a 2X4 and a rubber mallet.  If you cut a groove in the 2X4 to help support the tube the length of the 2X4 you can whack it pretty damn hard without damage to the tube and you can get proble 99% of the bend out if the tube is not kinked or creased already.

 

:BC:

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Posted

Thanks for the information. I did cover the wing roots to prevent air (and bugs) from blowing into the wings.

 

I will inspect the spar carrythrough, thanks for the tip.

013.jpg

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Posted

Here is an old video that I found on You Tube that has a ton of great info on repairs.  It shows some tube straightening jigs that can be built in a few minutes.

 

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Posted

Thanks for posting that video, I'm out of town right now but once I am back home I will do some further inspection and begin the repair process. I hope it is just the top longerons that are bent.

 

I found a photo over on supercub.org that shows a tube straightening jig similar to the one in the video, will use something like this to straighten the tubing:

01010048_zps7737b2a8.jpg

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Posted

I would not be in too big of a hurry to assume that you have to strip the fabric. If the tube is only bent, then I would tend to think that it is more ascetic in nature. I would tend to agree with a 2x4 and mallot would work wonders, assuming the tube is not dented sharply. Also, 4130 tubing has a fair amount of spring to it, so you may have to "overbend" it so that it will relax straight. The stressed tubing does want to return to its "rested" shape. The tubing is rather small diameter, and do not be surprised if you see wrinkled fabric with a change of weight on the tail, or when you lift the tail by the handles.

larry

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Posted (edited)

Two problems I see with leaving the fabric on. Hard to get at the bent tubes, and also, you need to overbend the tubes to get them straight. It's really sad to buy a plane and be all excited about bringing it home and when you get there you see that. OUCH! On the other hand, the poly-fiber system is very simple to learn, and for sure it can be repaired. Take care, Jim Chuk

Edited by Jim Chuk

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Posted

Hi Randy,

In your photos, it looks like the tail is supported with the wood blocks but the fuselage is not level. When trailering or towing your bird, the fuselage needs to be as level as possible because this will transfer the weight from the tail to the main gear. As the tail gets higher, more weight gets transferred to the mains, especially as the fuselage gets past the horizontal position. This plays a huge role in taildragger ground operations. Hope this helps, Bryce

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Posted

The planes I have owned that had bent tubes were also showing cracks in the welds at the nearest joint - I would strip the fabric and inspect the closest joints for cracks before straightening the tubes - fabric should be easy and fairly cheap to replace. You may not be able to fully straighten the tubes anyway without cutting the fabric because you have to bend them more than you want and let them rebound.

ED in MO

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Posted

As you begin your inspection, *don't* cut the fabric.  I've repaired a few places on my III and helped with other fabric repairs.  Your can peel the fabric off.  If it was covered the standard way, peel the tape up from the side enough to show the edge of the top fabric.  Then peel the top fabric off leaving the tape attached.  Preserve all the tapes.  Then peel the side fabric off from around the top tubes.  Hopefully you don't have to take it loose from the bottom too.  If you do have to take the bottom off then try to peel from under the bottom of the sides 1st and *don't* disturb the side or upper tapes: just peel from the inside.  The idea is to leave the finish tapes undisturbed along with that expensive finish.  A thin dull knife helps to separate the fabric from around the tube.  This takes patience but it's well worth doing.

 

As for the bent tubes, I made a simple tool that has a 1/16' chromoly strap and a half "U" block.  The idea is that the strap goes around the tube and about 1 inch away the U presses against the tube.  The is enough steel on the U block than you can get a cheater bar on it.  Draw a sketch or two and you should get it.  I can post a picture.  Anyway, you can slowly work out the bends.  Unlikely that the tube is buckled.  It can be cold worked a long way.

 

When the tubes are strait again, the fabric can be glued back on in the reverse order you peeled it off.  The tape gets glued down last.  If done carefully the only line left is the zig-zag seam at the edge of the one tape that you pulled up on one side.  That can be touched up.  If you're real lucky even that may line up.  If you peeled from underneath even that will be hidden and no one will see it.  Once glued back down tight the fabric has some shrink left in it that can be taken up by carefully using a hobby model airplane covering heat gun.

 

I know this isn't the recommended method but I've seen it work and used it myself.

 

Good luck,

 

Mark Napier

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Posted

My Avid tow set up has tubes that attach at the strut attach point on the fuse and go up to the leading edge spar on the wing and that end is held in place with the same pin as the wing. This supports the wing from bouncing. I know it's too late but all the plans for the factory built trailers back in the 90s were designed to tow backwards with the tail level. Hope your able to get it fixed relatively easily. Please post a thread with the repairs in case anyone else comes across this situation,. 

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