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Rear wing spar dimple cracks

nzrv8

Well Known Member
Afternoon all,

I've been having trouble with #40 dimples cracking on the rear wing spar flanges. Initially, I noticed these only on the RH wing top flange. They were visible with the naked eye, and most (but not all) would clean up with a #30 hole. Approx 15 cracked dimples were identified.

All dimples were done with a hand squeezer and Avery spring back dies. All holes reamed and deburred as I usually would.

I made up a test piece out of the same thickness material, and ran some tests including different dies and deburring techniques (including not deburring at all). The results were inconclusive - the only dimples that had any similar cracking were the ones that weren't deburred first, and even these cracks were much more minor than the ones on the spar flange - they polished out with a pass or two of scotchbrite.

A quick visual check of the LH rear wing spar was made under good lighting led me to believe that the problem lay with the RH spar only.

The decision to replace the wing rear spar was made, and parts ordered.

Fast forward to today. I was getting ready for the final wing assembly on the LH wing. I thought I would inspect each dimple under 10x magnification on the rear spar flange, just to be 100% sure. You guessed it - cracks. Much smaller than the RH wing, and difficult to see with the naked eye. Once again, approx 15 or so, but on both the top and bottom flanges.

Interestingly, every single crack is in the approximatly 5:30 position when viewed with the wings vertical in their stands. So regardless of flange bend direction, or squeezer orientation, every crack runs down towards the floor.

I'm curious to see if anyone else has had a similar issue, and what did they do to fix it? Hopefully the photos attach.

Cheers,

Hugh
 

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Thanks Bob, I agree that the thicker rear spar material doesn't like to be dimpled. But I also know that it can be done successfully (just not by me as it turns out :rolleyes: )
I know it must be me doing something wrong, but I'm damned if I know what it is. At least I have a few spare spars to experiement on until my new parts arrive :eek:
 
I’m getting ready to dimple my rear spars.. is this the way to go? Machine countersinking the rear spar? Is this what everyone else is doing? The instructions say to dimple...
 
use a "light touch" I'm curious.

~Dimple the 0.040 rear spar and “touch up" the dimpled holes slightly with a sharp deburring bit or microstop countersink. This "touch up" operation removes just a small amount of metal to make the skin dimple fit better and is not critical, so it can be done by eye. Use your test patch to test the depth. The usual tendency is to remove too much metal, so use a light touch.~

I guess they want you to dimple the hole and then remove more material from the spar. I wouldn't be surprised that this is often a talking point with Van's. Remember the the minimum bend radius for .040" is 1/8" and if you look at the OP's photos, you'll see stress fractures in the aluminum at the outside radius of the dimple. I concluded the problem is consistent with improper hole preparation prior to dimpling which when dimpled with too much pressure causes the material to crack.
 
~Dimple the 0.040 rear spar and “touch up" the dimpled holes slightly with a sharp deburring bit or microstop countersink. This "touch up" operation removes just a small amount of metal to make the skin dimple fit better and is not critical, so it can be done by eye. Use your test patch to test the depth. The usual tendency is to remove too much metal, so use a light touch.~

I guess they want you to dimple the hole and then remove more material from the spar. I wouldn't be surprised that this is often a talking point with Van's. Remember the the minimum bend radius for .040" is 1/8" and if you look at the OP's photos, you'll see stress fractures in the aluminum at the outside radius of the dimple. I concluded the problem is consistent with improper hole preparation prior to dimpling which when dimpled with too much pressure causes the material to crack.

Thanks for chiming in Dave, good insights. I don't think overdimpling was the issue here, as I used a small hand squeezer - and I'm not strong :). The pressure used to dimple was much less than my pneumatic or c-frame dimplers.

The hole preparation point is a good one though, and I think I have figured out what was happening:

Looking at a #40 reamed, but un-dimpled hole under 20x magnification showed a small patch on the hole wall that the reamer didn't clean up. This patch is in the area (5-6 o'clock) where the cracks were originating after dimpling. It looks like the pre-punched hole diameter was slightly larger than some others that I measured, meaning that there wasn't always enough material around the inside of the holes to completely remove the punch scars during match-reaming, especially when coupled with very slight (and normal) misalignments with the wing skin holes.

The holes appear perfectly circular from the outside, so I'm not surprised I didn't catch it before dimpling.

Perhaps on thinner material these holes would have been fine after dimpling, but the 0.040 is pretty thick..

New spar is coming, and lessons have been learned..

Cheers,

Hugh
 
I struggled to get a decent photo of the inside of an affected hole before dimpling, but here is an example of one after dimpling - no crack has formed in this instance, but you can clearly see the part of the hole wall that wasn't perfectly clean before dimpling:
 

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I just went through this yesterday on my QB wings. I agonized about which way to do it and practiced on .040” scrap. The machine countersink left a knife edge hole when it was deep enough for the dimpled skin to set flat on the test part. Using a reamer to get the hole to final size, touching it with a single edge deburring tool and dimpling it with Cleveland Aircraft Tool dimple dies and a mechanical squeezer resulted in perfect smooth dimples of the correct depth.

I worked up the nerve to try it on the spar and got the same results. Whew.
Mike
 
Nice work Mike. I think if my spar flange holes were reamed on their own without the skins, like I think your QB ones are done, I wouldn’t have had this issue.
 
I went down stairs awhile ago and practiced on some .032". I got the same cracking results and stress exfoliation of the aluminum coating when I didn't properly prepare the hole and really applied pressure with the DRDT-2 - but again, it was only .032. Thanks to you, I'll really look out for this when the time comes. Thank you
 
Afternoon all,

I've been having trouble with #40 dimples cracking on the rear wing spar flanges. Initially, I noticed these only on the RH wing top flange. They were visible with the naked eye, and most (but not all) would clean up with a #30 hole. Approx 15 cracked dimples were identified.

All dimples were done with a hand squeezer and Avery spring back dies. All holes reamed and deburred as I usually would.

I made up a test piece out of the same thickness material, and ran some tests including different dies and deburring techniques (including not deburring at all). The results were inconclusive - the only dimples that had any similar cracking were the ones that weren't deburred first, and even these cracks were much more minor than the ones on the spar flange - they polished out with a pass or two of scotchbrite.

A quick visual check of the LH rear wing spar was made under good lighting led me to believe that the problem lay with the RH spar only.

The decision to replace the wing rear spar was made, and parts ordered.

Fast forward to today. I was getting ready for the final wing assembly on the LH wing. I thought I would inspect each dimple under 10x magnification on the rear spar flange, just to be 100% sure. You guessed it - cracks. Much smaller than the RH wing, and difficult to see with the naked eye. Once again, approx 15 or so, but on both the top and bottom flanges.

Interestingly, every single crack is in the approximatly 5:30 position when viewed with the wings vertical in their stands. So regardless of flange bend direction, or squeezer orientation, every crack runs down towards the floor.

I'm curious to see if anyone else has had a similar issue, and what did they do to fix it? Hopefully the photos attach.

Cheers,

Hugh

Hugh, I needed some time and motivation to find the reference, but the dimple drill sequence as found in MIL STD 403C is to drill the pilot hole then dimple and then drill to final size if needed.

You know what they say about that MIL STD 403C - "If it's good enough for a US Submarine Launched Nuclear Ballistic Missile, it's good enough for an RV-8"
 
You know what they say about that MIL STD 403C - "If it's good enough for a US Submarine Launched Nuclear Ballistic Missile, it's good enough for an RV-8"
Nonsense. An RV-8 is intended to be used more than once.
 
Nonsense. An RV-8 is intended to be used more than once.

Well, apparently not all of them..... besides what was that thing that went from 0 to Mach 10 in 15 seconds? Well - I don't know if was an RV-8, but I'll bet those guys used used MIL STD 403c

On a serious note - in researching something else, I found good stuff that deals directly with this thread.

Read FAA Aviation Maintenance Technician Handbook - Airframe Volume 1 8083-31A, Chapter 4, page 4-44 a paragraph titled "Dimpling Inspection"

I actually re-read Ch 4. All of your questions are specifically addressed in this one chapter. And remember to thank an Aerospace Engineer for their hard work to earn that degree and their devoute determination to write a bunch of information hardly anyone ever reads.

Your tax dollars paid for your copy so you can get yours at this address:

https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/amt_airframe_hb_vol_1.pdf

Remember, I didn't write the book, I just read it.
 
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