Another Wing Spar Question


29 posts in this topic

Posted

This is my first post. I am at the beginning of a build and just mounted the wings on the fuse. I've got kitfox wings with the old spar where you don't have to insert the I-Beam. I understand that scuffing the spar with 220 grit is acceptable according to the manual. However, underneath the strut brace, the spar was prepped with something closer to 80 grit. Since the strut brace is riveted and hysoled to the spar, this is like a "doubler" or reinforcement. Of course, I am not sure.

I believe I have to start over which sucks greatly, but was wanting some concurrence.

Pictures show the scratches-(no other parts of the spar are scratched as such).

Thanks and look forward to your responses and expertise-

Joey

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Posted (edited)

I believe I would try some 100 grit and sand lengthwise on the spar - try to take out most of the deep scratches before you apply 9460.  Especially in the areas the strut attach does not cover.   Orbital or "shaker" sanders are better than straight hand-sanding.  As far as I know, there has not been a Kitfox spar failure without crashing, and no one knows how they were all prepped before adding the strut attachments.  EDMO

Edited by EDMO
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Posted

What Ed said X 2

:BC:

 

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Posted

I also would make sure the drilled holes are deburred...

Cheeeeeers

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Posted

Thanks for the replies!

I did debur the holes prior to riveting... but I won't be able to sand down- I didn't make clear that the wings are installed (glued and riveted). This is where I am - all measurements are perfect on both sides in washout in 12" increments all the way down the wing as well as squareness, length and dihedral.

In doing some reading about these spars, it seemed even the slightest scratch would lend the spar to be useless.

Now, I think I may not scrap it but keep going? In this light, sound reasonable?

Thanks again! 

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Posted (edited)

I figure if you went by the original washout, you will, farther on, wish you had made less washout for a little more speed - But, it's alright - Just Fly it like you had stoled it and have a great time!  ;<)  EDMO

Edited by EDMO
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Posted

keep building on it!  I wouldn't go back and start over.

:BC:

 

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Posted

Thanks guys! I am moving forward. I went from no wings this morning to now I cant wait to mount the engine, Rotax Rick 582.

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Posted

Jeff - the Avid builder manual says that any scratch deep enough to catch your fingernail within 18" of either the lift strut attach bracket or root end pin/bolt bushing hole is at risk of promoting a stress riser. In the Experimental world you fly at your own risk, but ask yourself if you'd like to be the unwitting buyer of a plane with this characteristic. I would not.

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Posted

I second to what dholly says....If it were my aircraft only for my use or to be sold later I wouldn't feel good about flying it or even selling it down the road. I am sure you can buff / sand out the nasty scratches until they're not catching your fingernail anymore....

My 2 cents...cheeeers

 

 

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Posted (edited)

Thanks guys. Back to square one, I've got enough doubts. I do not have an avid manual, and I did not see it mentioned in the KF manual (maybe its in there). 43.13 does have it so no excuse. I would definitely not sell it without full disclosure.

Edited by jeffcoaj

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Posted

The hysol is a structural adhesive and fills the little groves, it is held in place by the strut mount so it cant go anywhere.  If you are that worried about it just drill the rivets, sand it with 200 and re-install it.  you will only loose a couple hours of drilling, sanding and riveting it back on. 

If there was no hysol in there then I could see it becoming a stress riser issue albeit a very remote one.  With the hysol.. I can't see it ever causing any type of issue as is.  That is just my .02 and that along with 5 bucks will get you a cup of coffee these days.  I can tell you that my spars after being put on its back multiple times by the previous owner are not picture perfect.  I beat them up in the air pretty darn good and don't loose sleep over it.

 

:BC:

 

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Posted

Hey akflyer,

 I don't see the adhesif (hysol) bearing any load like the wing spart does.........now imagine a crack in the spar tube under the strut fitting, I am pretty sure the hysol will not keep you in the air at that point. The Hysol filling the cracks will not replace that load bearing aluminum material and/or remove the stress riser. Please don't hesitate to correct me if I am seeing this wrong, I am eager to learn..........because even Einstein once said "Once you stop learning, you start dying......."

Cheeeeers

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Posted (edited)

I don't see any big problem with the marks under the strut fittings, since the fitting, rivets and 9460 bond will prevent any flexing there.  The only real concern might be the scratch that is within 18" of the fitting, especially on the outer end of the wing which will flex more.  I would clean those up as good as possible without getting the spar too thin.  Touch them up with some paint to prevent further oxidation.  Fortunate for you, is having the older spars with the internal I-beam the full length of the wings - like mine - those make it a lot stronger and don't flex as much as the later spars without the I-beam.  EDMO

Edited by EDMO
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Posted

Hey akflyer,

 I don't see the adhesif (hysol) bearing any load like the wing spart does.........now imagine a crack in the spar tube under the strut fitting, I am pretty sure the hysol will not keep you in the air at that point. The Hysol filling the cracks will not replace that load bearing aluminum material and/or remove the stress riser. Please don't hesitate to correct me if I am seeing this wrong, I am eager to learn..........because even Einstein once said "Once you stop learning, you start dying......."

Cheeeeers

the hardened hysol filling those little scratches is packed in tight and held in place with the strut fitting.  I don't think vegas would give you odds on a crack developing there it is so remote.  There is no real area to flex right there, the strut attach fitting makes sure of that.  It has the full length I beam..  I am flying spars with bigger dings and gouges than that.  Are his spars perfect?  No, are there probably hundreds flying that look just like his, more than likely.  I see planes flying every day for years and years that have WAY more issues to worry about that these scuffs and they have never failed or fallen out of the sky.  Take a look at some of the shit flying in the certified world that passes annual every year and you will see that 99% of the homebuilts are better constructed and in much better condition than a super cub thats been beat for the last 60+ yrs. 

You can argue this all day long from both sides of the fence.  Bottom line is, I would build it and fly it without thinking twice about it.  You may not.  Then again, I may fly in a way in which you would say is stupid and careless but to each their own.  There is a reason I am not posting youtube vids daily :lmao:  I use my plane as a tool, not just a weekend cruiser and I beat mine like a rented mule. 

:BC:

 

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Posted

At least we know who to blame for the high cost of mule rent! :rolleyes:

 

Two sides to this fence. Just have to decide what side makes the most sense in your own mind and go with it.

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Posted (edited)

I can only agree. Now I know, there are 2 distinct sides.

I.e., I could argue microballoons mixed with epoxy will weaken a bond (its mostly air and makes epoxy sand easier). Cabosil on the other hand makes bonds stronger and rock hard ( impossible to sand almost). I would never use microballoons in a bond as I've read in some places, and I base this on experience. Also, this is easy to test. And experience is king.

Is there a way to test the wing (on the ground)? I've seen some wing tests with sandbags for strength. 

 

 

Edited by jeffcoaj

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Posted

The testing is done pretty much how you said it . Add weight and wait for something to fail. The real question is do you want to risk damaging a wing and having to rebuild it ? 

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Posted (edited)

It should not fail up to it's g rating.

Edited by jeffcoaj

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Posted

You never know . In my 20 years building and flying experimental aircraft there's a lot of stuff I've seen go wrong that shouldn't have . Not necessarily to my aircraft but others. I go by a thinking of if it doesn't need to be done don't do it unless you're willing to pay for the consequences. It's not my plane so it's not my business just putting my 2 cents worth. 

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Posted (edited)

If it fails at less than rated G, then I'm glad I didn't fly it. I am sure failure should occur at twice the rating or some fudge factor. I would test at less than rated.

Edited by jeffcoaj

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Posted (edited)

If it fails at less than rated G, then I'm glad I didn't fly it. I am sure failure should occur at twice the rating or some fudge factor. I would test at less than rated.

As far as I know, 150% strength of rating is usually followed - But who knows without test results.  The FARs give instructions and G-loads for testing on Part 23 planes assemblies, but "experi-mentals" are in a different world.   EDMO

Edited by EDMO

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Posted

In the words of capt. ron, if its going to happen, its going to happen up there.

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Posted

Replacing that spar isn't going to be easy because that extrusion is not available from any supplier that I know of.  If you replace it with the normal tube and insert, this will have different flexing properties from the remaining rear spar extrusion and will certainly lead to unique handling "quirks" with this aircraft. 

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Posted

Replacing that spar isn't going to be easy because that extrusion is not available from any supplier that I know of.  If you replace it with the normal tube and insert, this will have different flexing properties from the remaining rear spar extrusion and will certainly lead to unique handling "quirks" with this aircraft. 

Totally agree with Larry.  EDMO

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