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Need Advise Over Counterweight Screwup

330Jock

Well Known Member
After reading all the posts I still screwed up my elevator E-703 tip rib when I drilled the counterweight. Take a look and let me know! Second time of course was a charm, perfectly in the middle. Is this 703 toast or is there a fix?

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This is what I did.

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I would just open up those two holes to one larger clean hole and then install a doubler. You should be good to go.
 
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Bill - I think that is a good choice. Arnie, I feel your repair is probably just fine, makes sense, but do we know?

I will try to keep this short, ha......

Also, to preface, I am not a certified mechanic. This discovery process I am sharing here is how I was trained by one, so I encourage "real" mechanics to chime in and tell me if I am all wet here. This is my understanding but I am no expert and want to learn.... Please, blow holes all through this if you find them....

I view airframe repairs on Experimentals the same way I do on Certified aircraft. This is certainly arguable.
So, in my view, one should follow an acceptable standard such as AC-43-13-1b, or documented airframe repair manuals, which are acceptable under AC-43........ Engineers did all of the hard work for you. Or, you can have your repair analyzed by an airframe engineer $$.

A repair should be documented with the reference used to justify the method. "replaced flange with .025 tab repaired per AC-43-13-1b chapter 4 page blah, blah, blah.....

This is where the sluething comes in. You have to find comparible damage with a repair. That can be difficult. I couldn't find a "replaced the entire flange" repair in either AC43, or a couple airframe repair manuals I have. I did find repairs to flanges and splices and by combining the two came up with a repair that I feel you could document and justify.

First, what rivet to use? AC43 states diameter to be roughly 3x the thickness of material. So, we could use a 3/32" rivet, but 1/8" round head are far more common in our kits, so 1/8" is just fine if we can meet edge distances.
Now, how many rivets? If we are treating our newly added flange like a single splice, our chart in AC43 for .025 2024T3 ALCLAD says 4.9 rivets per inch and it assumes 1" overlap splice. We can multiply that per the chart by
.75 and get to 3.7 rivets per inch. Let's drop that to 3 rivets per inch. Whoa, time out, that is a bunch of rivets. On center we can meet our minimum of 3x diameter called out in the standard and still put them in a straight line, but holy cow, that just does not look right. On to the next option....

Both of the airframe manuals I have cover flange repairs and splices. One of the manuals has a chart to reference for splices. That chart shows for this type of repair, in .025 material with 1/8" rivets that we can use a 4D - 6D spacing. Ok, that is much better. I can now space my rivets at 3/4" o.c. and can document where I derived this repair from in an airframe manual. AC43 does not say it has to be the airframe manual of the airplane you are repairing. As long as it is of the same structure and type, we are good to go. That still seems like a lot of rivets.

So, next step, look at the construction manual of my RV6. They show the flanges to be riveted to spars at a minimum of 1 1/4" spacing with 1/8" rivets. Bingo, that is the one I am going to use. However, I have lost 25% of the strength of the flange with the splice. I still think this spacing is adequate. However, I do not think I can keep it, so I would put 3 rivets across that newly added flange.

I now have a document to reference, "repaired per Van's builder plans page blah blah blah, xxx" spacing of 1/8" rivets to support flange, AC43xxx blah, blah...." It does not look like Arnie'e repair meets this spacing. Is it still ok? I am sure it is. However, the above process is what I would follow when trying to determing an acceptable repair. I do not personally accept repairs made by "common sense", "It looks stronger", or any other subjective means. But that is just me.

Sorry for the long post but I see a lot of questions coming through here about repairing mistakes. Although the answers are not always obvious, there are resources to help and with a little work you can get to where you are going and feel good about it.

Of course, the other acceptable method... call Van's and have them engineer the repair or show them what you are doing and get a sign off on it. That is accepted in AC43.....
 
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