I made a stupid tonight with E-606PP

Semi-obligatory self-reflective intro: Is it just me, or do these moments always seem to come at the same sort of time? You're out in the shop, you get in a good groove, you're all pleased with your productivity...you might even, at some point, think to yourself, "I'd better be careful, or I'll get carried away and make a mistake..."



Anyway, last night I match-drilled all the left elevator parts, and tonight I set about deburring and dimpling everything. I hit up all the usual stuff: spar, ribs, counterweight skin, etc. That list included my new best friend, E-606PP, on which I proceeded to intrepidly dimple both flanges.

That done, I was about ready to call it a night and head, inside, but I decided to read ahead in the instructions a bit. Well, what do you know...just a couple lines down it specifically addresses E-606PP, along the lines of "Machine countersink the top flange and dimple the bottom flange."

Internal head voice: "Countersink? Just the top? But why...ohhhhhh...the hinge for the trim tab goes there, and needs a flush surface to rest on."

*sigh*

I started to bust out the rivet gun, flush set, and back rivet plate to flatten out all those nice dimples I'd just made, but then realize I was getting right back into the mindset that caused the screw-up in the first place...too much working, not enough thinking.

Then I had another thought: Maybe I could just countersink the hinge piece itself and not have to go flatten all those dimples. Might make drilling the hinge in place a little more challenging, but at least I wouldn't risk messing up a bunch of previously good holes and having to reorder a part.

So, I submit the question to the peanut gallery: Which should I do? Flatten the dimples, countersink E-606PP, hope the holes don't get buggered? Or leave the dimples there and plan on countersinking the hinge?

I'm leaning towards countersinking the hinge, but I want to make sure I'm not overlooking some kind of problem that might result.

Thanks!

I think the consensus you will hear is flatten the dimples with a squeezer and flush sets, then countersink the spar.

Yeah flatten it back out and go from there, I would not attempt to countersink The hinge. Flattening out the metal one time should not cause any problems.

Bird

Yeah, suddenly it occurs to me that (I think) there'd be a problem trying to countersink the hinge. Wouldn't the hinge itself (ie the part that holds the hinge pin, not sure of there's an "official" term for that) get in the way of the countersink cage?

I'd go out to the garage and check, but I'm all comfy in bed now. Getting dressed again would be far too much work.

Emphasize squeezer

KMinor said:

I think the consensus you will hear is flatten the dimples with a squeezer and flush sets, then countersink the spar.

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Use your squeezer, and NOT your rivet gun, the gun will pancake and miss shape your material.

Have fun

Personally I'd not rush to "undimple" the spar. The spar is the key structual item here. You are not only reducing strength by "undimpling" (to a small degree), but then C/S into that area.

The hinge is easily C/S - even if you cannot get the cage on it, just use the deburring tool, ideally with the C/S tool part in it. NB whilst the hinge will be slightly weaker due C/S, I am sure I have C/S hinge in various locations, and a C/S joint is stronger is some ways than flat.

I am not pushing you one way or the other - but as you hint, take the time to the best (which might include easiest) solution from where you are now, not rush back to try and get back to where the plans say you should have been

Back up strip

Andy Hill said:

Personally I'd not rush to "undimple" the spar. The spar is the key structual item here. You are not only reducing strength by "undimpling" (to a small degree), but then C/S into that area.

The hinge is easily C/S - even if you cannot get the cage on it, just use the deburring tool, ideally with the C/S tool part in it. NB whilst the hinge will be slightly weaker due C/S, I am sure I have C/S hinge in various locations, and a C/S joint is stronger is some ways than flat.

I am not pushing you one way or the other - but as you hint, take the time to the best (which might include easiest) solution from where you are now, not rush back to try and get back to where the plans say you should have been

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LIke Andy says, at this stage I think I would c/s the hinge. You can get a cage on it since the ears of the hinge will be on the other side of the surface you c/s.

If you do this I would also add a back up strip of alum. on the bottom. The hinge material is pretty soft and you would not want the shop head of the rivets to be against the thinner parts of the hinge that have been countersunk away.

This backup strip will sandwich the hinge in and prevent any possible rivet tear out under load.

Philip,
you can simply dimple the hinge (that has been done often, it's been discussed a couple of times in other threads).
I wouldn't do that (anymore) on the flap hinge because mine stretched very slightly during dimpling, on the short trim-tab hinge that's not an issue though.
As far as I remember Van's recommended dimpling the hinge as well an said not to countersink it.

I dimpled these parts on my 7. No problems dimpling the hinge material with a hand squeezer. It all works out fine.

I would just dimple the hinge or flatten the spar. I don't like to C/S thin material especially when the hinge will interfere with the cage.

tkatc said:

I would just dimple the hinge or flatten the spar. I don't like to C/S thin material especially when the hinge will interfere with the cage.

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The hinge is hardly thin... it is around 0.040. It is a pain to dimple though since it is a soft material and can easily distort and get wavy.

There is no interference with the countersink cage in this case.

Be careful if you are thinking countersinking the hinge material. If the hinge is thin (under .032) then you should dimple. If you countersink anything thinner than 32 (some people say anything less than 50 thou is to thin, but those were opinions of structures guys I have worked with in the past) and are using MS426AD rivets you will run the risk of what is called knife edging the hole. If you are not familiar with this term it means that in order to countersink deep enough to set the rivet correctly, the countersink actually protrudes beyond the bottom of the material and enlarges the hole. For example if you have a .040 thick skin and countersink a #30 hole, to get the rivet to sit flush, your hole enlarges beyond a #30. The base material at the point where the rivet is set is now weaker due to the enlarged hole. This process is avoided by dimpling instead of countersinking or if dimpling is not possible, a Nas rivet can be used which is a reduced head countersink rivet but these are only ment for shear applications. When dimpled, the surface area under the rivet head remains intact creating a good base underneath the rivet. This is why Vans uses dimpling on the thinner skins. With that being said, I would recommend one of the two options already addressed. 1) unset the dimples and countersink per Vans instructions. Use a squeeze as stated above as a gun could distort the metal. As for weakening by undimpling it could be argued either way. If you undimple once you are going to be just fine. If you were to dimple and undimple numerous times on the same hole, it will weaken the meaterial due to flexing. 2) dimple the hinge and carry on. good luck!

MNAv8or said:

Be careful if you are thinking countersinking the hinge material. If the hinge is thin (under .032) then you should dimple. If you countersink anything thinner than 32 (some people say anything less than 50 thou is to thin, but those were opinions of structures guys I have worked with in the past) and are using MS426AD rivets you will run the risk of what is called knife edging the hole. If you are not familiar with this term it means that in order to countersink deep enough to set the rivet correctly, the countersink actually protrudes beyond the bottom of the material and enlarges the hole. For example if you have a .040 thick skin and countersink a #30 hole, to get the rivet to sit flush, your hole enlarges beyond a #30. The base material at the point where the rivet is set is now weaker due to the enlarged hole. This process is avoided by dimpling instead of countersinking or if dimpling is not possible, a Nas rivet can be used which is a reduced head countersink rivet but these are only ment for shear applications. When dimpled, the surface area under the rivet head remains intact creating a good base underneath the rivet. This is why Vans uses dimpling on the thinner skins. With that being said, I would recommend one of the two options already addressed. 1) unset the dimples and countersink per Vans instructions. Use a squeeze as stated above as a gun could distort the metal. As for weakening by undimpling it could be argued either way. If you undimple once you are going to be just fine. If you were to dimple and undimple numerous times on the same hole, it will weaken the meaterial due to flexing. 2) dimple the hinge and carry on. good luck!

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Not Applicable... the hinge is 0.040 material and can easily be countersunk for a 3/32 rivet.
Heck, the FAA and Mil Specs even allow countersinking of that thickness for 1/8 rivets.

My backup strip suggestion would make the non-standard (per the plans) assembly even stronger than original.

You won't find many specifications that allow "undimpling"....

az_gila said:

Not Applicable... the hinge is 0.040 material and can easily be countersunk for a 3/32 rivet.
Heck, the FAA and Mil Specs even allow countersinking of that thickness for 1/8 rivets.

My backup strip suggestion would make the non-standard (per the plans) assembly even stronger than original.

You won't find many specifications that allow "undimpling"....

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I think the problem with this is it's being countersunk to fit a dimple and not the rivet.

gasman said:

I think the problem with this is it's being countersunk to fit a dimple and not the rivet.

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Ahh I'm glad someone read the post and caught it.... a countersink for the dimple will go deeper then a 3/32 and a 1/8 rivet would.

Per Vans, you are allowed to unset and dimple once. Unless the tech guys at the factory are giving out bad advice.

Adding a "stiffener" to the original design is not always better and not recommended. If you would turn to your "FAA Specs" which you quote for Acceptable Methods AC43.13-1B Section 4-50 Paragraph b. ANY repair made on an aircraft structure must allow all of the stresses to enter, sustain these stresses, and then allow them to return into the structure. The repair must be equal to the original structure, but not stronger or stiffer, which will cause stress concentrations or alter the resonant frequency of the structure.

I have seen on countless times in GA where an A&P thought it would be better to make it stiffer because stiffer is better, only to induce stress into the area of repair, causing a cracks to show up around the repair as stress will fatigue the weakest point in a system.

If you have some spare hinge (or same thickness of metal), countersink a scrap piece and check for knife edging before you drill on your good hinge.

MNAv8or said:

If you have some spare hinge (or same thickness of metal), countersink a scrap piece and check for knife edging before you drill on your good hinge.

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This is pretty much where I'm going with this. It sounds like flattening the dimples and then countersinking is the preferred method, but I'm still a little concerned that the material might not flatten completely. I'm pretty sure I've got some scrap of the same thickness, so I'm going to dimple that, then flatten and countersink to see what the result looks like. If it's nothing ugly, I'll go with that.

I think there's also plenty of extra hinge material in the tail kit, which I can also use for testing the alternative countersink method if need be. But I'm pretty sure there's not going to be a problem with flattening the dimples and countersinking; I just want to do a test run to make sure before I do any more potential damage to my real pieces.

If you use a squeeze set, it will pretty much flatten out completely. But I understand your hesitation. If you are in the process of experimenting anyways, take some scrap the same thickness as the one you have already dimpled and do the same type of experiment on it with unsetting a dimpled hole, this will give you an idea of what it will look like. While you are right, the material will not return to a completely flat look, but it will be pretty close.

Also, you comment about worrying if the countersink cage will sit down on the hinge, did you know they make a reduced head countersink cage? Its pretty price but check it out for future reference. http://www.browntool.com/Default.as...ductName,ProductNumber&Level=a&ProductID=1860
I have a couple of them and they work great!

Keep us posted on how it turns out!

MNAv8or said:

If you use a squeeze set, it will pretty much flatten out completely. But I understand your hesitation. If you are in the process of experimenting anyways, take some scrap the same thickness as the one you have already dimpled and do the same type of experiment on it with unsetting a dimpled hole, this will give you an idea of what it will look like. While you are right, the material will not return to a completely flat look, but it will be pretty close.

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Just got back from a quick experiment in the garage. Dimpling and un-dimpling a similar piece of scrap looks perfectly acceptable to me. I'm not even honestly sure exactly what I was worried might happen, but after the experiment there doesn't seem to be any reason not to un-dimple and countersink E-606PP.

Like you said, it doesn't flatten out completely, but seems fine. Worst case, if the hinge doesn't sit perfectly flat, I could just do a very slight countersink on the hinge. But it doesn't look like that will be necessary.

Also, I have no idea where I got the idea that there was lots of extra hinge material in my tail kit. The piece in there appears to be just the right length to fit the trim tab. For some reason I had this mental image of a piece of hinge about twice as long as needed. Senility strikes again...

Its easy to be worried when working with something new. Most of the owner/ builders that I have worked with want absolute perfection in their work which drives a lot of the worries and questions on here. I remember being a wet behind the ears A&P structures guy and tasked with putting extended range tanks in a brand new Piper Malibu with a value of about 1 million dollars. I was working for a Piper Dealer and it only had 10 hours on the aircraft, I had to punch about a 4 inch hole in each wing to install the new fillers. Talk about being nervous! A fellow tech was laughing at me because the closer I got the drill to the wing, the more I shook lol. But as you become more comfortable with the process and with your abilities, you will have no problems with anything that you come across. I don't think anything of walking up to drill holes in a brand new multi million dollar biz jet now days. You will still run across some things that don't make sense but everyone on here is absolutely awesome when it comes to help.

Well, my poor spar didn't make it. After flattening the dimples, there was a very noticeable bow in the spar. I initially fluted the bow out, though it occurred to me later that probably would have caused problems with the hinge sitting flush against the spar web. In the end, it didn't matter, because I somehow managed to wallow out a couple rivet holes while trying to countersink. I had the piece clamped to my workbench by the lower spar web, which I think allowed the web I was countersinking to vibrate, which in turn contributed to ruining the holes.

In the end, I decided it just wasn't meant to be. Tomorrow I shall order a replacement.

Still, I made the day useful; I set about bending the ears on the trim tab and elevator skin. I figured I should go ahead and do that, so if I ruined something else, I could at least combine parts orders and save on shipping.

They turned out OK though.