MK IV Aerobat speedwing spar gouge/repair


43 posts in this topic

Posted (edited)

I moved last year and decided to take off one wing to fit inside of a large truck.  Unbeknownst to me, during the drive the position of the wing shifted and rubbed for several hours against a sharp steel bolt.  It gouge the leading edge between .06-.07" deep, the longest of which is about 5" long.  The damage doesn't go through the spar, and there was no bending force as far as I can tell (Basically the metal was just ground away).  The scratch extends back about 29" from the root and is in front of the fuel tank.  This wing was built as a Speedwing with the HH rib spacing and .085 thick spar.

I have a few A&Ps, mechanical engineers, and the folks at Avidkitplanes.com mulling over whether a repair is possible, or if a complete rebuild is in order.  We're considering internal/external bracing with rivets and/or adhesive.  An internal tubular brace, say 24" long with v-cuts on both ends (similar to the strengthening kits, but a complete tube) seems like it would distribute the load away from the area, but I'd like to know what you guys think.  

Thank you,

Justin

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Edited by jacksonbrowne
clarity/typo

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Posted

As a structural engineer, I would say that the scratch is not in a high stress area of the spar (as it is essentally bears vertical flexion loads, the stresses are mostly at the top and bottom.)

Personally, I would make a repair as you suggest. But its your butt in the seat, not mine!

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Posted

My rule of thumb is "if you have to question it, then you have your answer."   In other words, if you have to question a part or repair, fix it right or replace it.  But its not my aircraft or my tail in it. In my work, my name goes in the logbook so its done properly. Of course, someone else is most likely paying the bill. But I use the same guidelines on my aircraft.

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Posted

 

Hey Justin on the avid Catalina the wing spar had to be extended/spliced to make the span 36 ft. I am including the splicing method of the spar (from the Avid company) and it might be an option to repair your damaged spar... 

P1110485.JPG

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Posted

While it will be a pain in the butt, at least it's a lot easier to change out the front spar then the rear one.  JImChuk

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Posted

I would start by removing the fabric around damaged area and clean any glue off with some M.E.K. I would remove the scratches with a 3M pad in an airtool and smooth it out to the point that you would consider recovering it.  I'm not sure the depth of the scratch can be measured effectively the way you show in the picture. You would have to take a measurement of the undamaged pipe and compare it with another measurement at the damage site. 

At face value I don't see this as major repair. If the tube is not dented and they are .085 wall thickness you still have a lot of strength there. But like others have mentioned......its your ass so you need to feel right about the repair, 

 

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Posted

 

At face value I don't see this as major repair. If the tube is not dented and they are .085 wall thickness you still have a lot of strength there

If you confirm hat the dent is less than 10% of the wall thickness, I   doubt the repair solution (from the Catalina spar extension) will be any stronger than the spar in its current state.

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Posted

 

At face value I don't see this as major repair. If the tube is not dented and they are .085 wall thickness you still have a lot of strength there

If you confirm hat the dent is less than 10% of the wall thickness, I   doubt the repair solution (from the Catalina spar extension) will be any stronger than the spar in its current state.

I agree with JeroneF, 

Another option to consider. These planes benefit from a leading edge cuff. The leading edge cuff adds material to the leading edge of the wing to sharpen the leading edge and makes the wing more efficient. I talked to Steven Henry about this and he said it was one of the best upgrades to these tube LE wings. I'm not sure how they attach but it might be worth checking out. Steve could give you more information I'm sure. 

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Posted

Great answers, and I'm glad to see that the consensus seems to be it's not a total loss.  

In other words, if you have to question a part or repair, fix it right or replace it. 

Regarding the above, I agree, but I'm not sure what the "right" repair would be in this case.  It is such a strange area and type of damage that there doesn't seem to be one right way (other than just rebuilding the wing of course).  

I will take this all into consideration and post our repair when complete.  Thank you all very much for weighing in.  This forum has been an extremely valuable resource for a ntiwit like myself.

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Posted

in this area it is more compression than flexion i guess, so the fiber tank helps a lot for this. if it was mine, i would go for repair and special care between each flight

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Posted

That splice will be stronger than the original spar. Go for it. In the future, transport the wings by suspending them lengthwise from the truck sidewall. I hung them at eye level, lengthwise along the sidewall of the truck, using nylon straps around the leading edges. They survived nicely.

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Posted

Splicing spars with a piece of tubing the next size smaller is common practice in the ultralight world. The splices are very much like the method used in the extension diagrams referred to earlier in this thread. We sometimes double sleeved the joints by using a piece of tubing the next size smaller than the splice. If the end of the spar had a hole in it at an attachment point we would also glue in a piece of hardwood dowel to prevent compression at the connection point. A book covering the build practices can be ordered here: http://www.beaujonaircraft.com/

It's got some great information in it but it's only worth the $10 or $20 dollars  I paid for it back in the 70's. Full length spars would be expensive so splicing shorter lengths has been common practice for decades. Many of those early ultralights turned into many of the light sport aircraft that are still being sold today. I'd smooth the outside as mentioned earlier and sleeve it inside. You can always add a layer of fiberglass inside, outside or both if your really worried. Look up Glare for information on composites made of aluminum and fiberglass epoxied together. I think this is overbuilding but sometimes it's needed to afford some people piece of mind.

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Posted

Splicing spars with a piece of tubing the next size smaller is common practice in the ultralight world. The splices are very much like the method used in the extension diagrams referred to earlier in this thread. We sometimes double sleeved the joints by using a piece of tubing the next size smaller than the splice. If the end of the spar had a hole in it at an attachment point we would also glue in a piece of hardwood dowel to prevent compression at the connection point. A book covering the build practices can be ordered here: http://www.beaujonaircraft.com/

It's got some great information in it but it's only worth the $10 or $20 dollars  I paid for it back in the 70's. Full length spars would be expensive so splicing shorter lengths has been common practice for decades. Many of those early ultralights turned into many of the light sport aircraft that are still being sold today. I'd smooth the outside as mentioned earlier and sleeve it inside. You can always add a layer of fiberglass inside, outside or both if your really worried. Look up Glare for information on composites made of aluminum and fiberglass epoxied together. I think this is overbuilding but sometimes it's needed to afford some people piece of mind.

But I have never seen a splice in that section of a spar there always on the outer section of the spar.  Don't dare give any advice on this one.

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Posted

I guess I should add that the book I linked to also contains information on lamination of various materials to form composites and then goes into testing the strength of those composites. Whatever method of repair for this damage can easily be performed on a piece of scrap of the  same material and size and then tested for failure by mounting it in a vise and applying weight to it until it fails. Do the same test with a short sample of regular undamaged spar and see which one is stronger. These airplanes are called experimental for a  reason.

I've got another airplane hanging about ten feet from me as I type this that most of you have never heard of. It's called a Ritz Standard. The original designer was killed in his design. The design was improved and sold for awhile but the new owner of the design pulled it off the market. My point is that just because somebody designed something a certain way does not mean it's the only way to do something. Much of his design lives on in other aircraft.

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Posted

That splice will be stronger than the original spar.

Sure not: as the repair tube is inside the original spar, its section inertia will be lower.

Moreover, the rivet holes will further weaken this area.

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Posted

If it were mine with where the scratch is, assuming it really is a scratch and no more than 10% of the metal thickness, I would sand and polish it out, cover the area with a fabric patch, do my best to blend the paint and never look back.

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Posted

In the first post you state .006 - .007" deep but your picture indicates .069" - that's a major difference and a number of responses are not valid if that is indeed the case. Hard to tell from the pics on my computer.

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Posted

An old machinist here. My eyes are getting bad but it sure looks like a little over six thousandths on my computer screen.

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Posted

That splice will be stronger than the original spar.

Sure not: as the repair tube is inside the original spar, its section inertia will be lower.

Moreover, the rivet holes will further weaken this area.

Yep, you're right Jeromef, my bad. This time I actually looked at the recommended splice and agree. A real repair splice should have an external sleeve the same thickness as the spar but with a larger diameter so it slips over the spar. It should run about 2.5 diameters long on both sides of the repair (5 diameters total length) and should be bonded to the spar with a structural adhesive (Scotchweld is good, I think). The deep scratch should be blended down to a smooth surface, and the splice fitted over it without scoring any of the material, bonded tightly with the adhesive squeezing out.

This is probably overkill, because the area is before the strut, so the spar isn't in much positive bending, more likely it is in compression. I think the highest stress section of the spar is immediately outboard of the strut attachment.

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Posted

Many of us fly the STOL version with spars everywhere thinner-walled than likely your scratch goes down to.  It would be nice to validate, maybe with a straight edge to span the scratch.  The highest stress is likely at the strut junction, in bending; the rest of the spar is wasted strength, really excess weight due to the design simplification of using a tubular spar.  With the speedwing this bending moment is much less than with the STOL.  The likely next highest stress is roughly midway between root and the strut attachment, where it wants to bow upward and is helped a little by compression of the spar, essentially reacting the tension of the main wing strut.  The cabane struts don't add much stiffness at all to the wing spars.  They're only there to protect the main struts in negative G conditions., to stabilize them against column failure.  Like the others with engineering backgrounds are saying, just dress out the scratches, and patch over the fabric, and you'll be fine.  I say this with confidence as an ex-professor of aerospace engineering, and designer of a couple of my own hang gliders with tubular spars, where it was my butt on the line.  The Avid (even the STOL) wings are quite strong.  My STOL wing, with me flying alone, is good to better than 6 Gs based on Avid's static test.  I nust admit I have not analyzed, so don't know about the H-stab, though.  I don't know if anyone has bent one as yet, though, but I doubt it.  Oh yeah - but it's not my butt...

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Posted

 Like the others with engineering backgrounds are saying, just dress out the scratches, and patch over the fabric, and you'll be fine.  I say this with confidence as an ex-professor of aerospace engineering, and designer of a couple of my own hang gliders with tubular spars, where it was my butt on the line.  The Avid (even the STOL) wings are quite strong.  My STOL wing, with me flying alone, is good to better than 6 Gs based on Avid's static test.  I nust admit I have not analyzed, so don't know about the H-stab, though.  I don't know if anyone has bent one as yet, though, but I doubt it.  Oh yeah - but it's not my butt...

Really???? 

Almost a .070" gouge out of a .083" thick spar needs some definite surgery to make that safe.

You guys recommending smoothing that out need to look at the first pics again and take note the amount the digital caliper is open. 

The original post stating .006-,007" doesn't come close, apparently using that mode on the caliper throws people off. You can barely see a .007" gap between the caliper jaws.

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Posted (edited)

Yes, the jaw gap is obvious, and look at the decimal point location in all three measuring images. 0.069" - sixty nine thousandths.

Edited by marcusofcotton

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Posted

I wiped the  finger prints off my screen and zoomed in. It is far deeper than first reported.

I agree with Turbo in his patch suggestion. Even sleeved you will hardly see the  patch once you get the fabric and paint added back.

 

.06 measurement.jpg

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Posted

I was looking on an Iphone and took the .007 at face value. I still can't tell anything from the photos on my full screen.

The depth needs to be verified before polishing out like I suggested. even worst case of needing a patch, it seems like an adequate patch could be riveted to the outside with sufficient overlap without having to totally rip the wing apart. Then just patch the fabric and repaint the small patched area.

Hopefully....The plane really looks pretty otherwise and it looks like it is painted with polytone which is super easy to repair and blend paint.

Best of luck!

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Posted (edited)

You're all correct; .007 was a typo it is ~.07".  I just edited that to correct it.  Obviously I don't have the right type of caliper for measuring on a radius, but I thought that might help give a general idea of the damage.  Here is the response my A&P got from Mark Mendick at Avid:

. . . I sat down with another seasoned Avid builder and we came to the consensus that the damage was too detramental to the integrity of the wing to try and patch or install a stiffener. I ran some rough simulation on a CAD program and the stresses in that area show signs of premature fatigue failure. . . 

I would normally assume a supplier just wanted to sell more parts, but Mark was very open to the idea of trying to fix the wing or even possibly just replace the spar.  Since the Hysol doesn't seem to have a heat release method, it doesn't seem like there's a way to do this.  Obviously they'd like to err on the side of caution, but I'm a little torn between what I feel would be a more than adequate solution vs. just playing it safe and redoing the whole thing - the classic homebuilt dilemma.   

I'm going to look into a few options.  I'm also considering adding wing extensions if the fabric comes off to have a little bit better climb and takeoff performance.  Again, I appreciate everyone weighing in.  I will conclude the post with the route I decide on and report back on our progress.

 

Edited by jacksonbrowne

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