Ongoing spar tunnel annoyances

[Andy Walker]

Hey all...

The saga of my spar tunnel build continues. I've completely rebuilt it now, and I'm pretty happy with it, EXCEPT...

Some of the holes in the tunnel sheet metal and the angle crosspiece are larger than I'd like (I riveted the aft part together before realizing that I shouldn't do that until the gear attach points are in place, so I had to drill out all the rivets again). The largest holes are about #27 in size, there's perhaps 6 out of 50 that size. I tested on some scrap and the rivets will still grab on that size hole, but since this is a structural area I want to make sure it's as strong as it should be. So I have three options:

1) Shrug, assume it's fine, and carry on;

2) Updrill the holes to 5/32 and install larger rivets;

3) Do the whole back side of the spar tunnel over again.

Here are some pics...first, one of the most enlarged holes in the angle (top), next to a normal hole:



Here's the same hole with a rivet stuck through it so you can see the degree of space around the rivet (some of that darkeness is the deburred area though, you have to look close at the innermost black ring that is the gap):



Finally, here is a rivet stuck through the tunnel sheet metal so you can see how enlarged those holes are (all of those are roughly the same size):



I'm actually more concerned about the tunnel holes than the angle holes. All of the tunnel holes are larger from stretching during dimpling, in spite of using a #32 drill bit as recommended by Sonex. Some in the Yahoo group have said that as long as you have enough material for the countersunk head to grab, it's enough, but I'm not sure I'd agree with that.

To do #2 I could use these rivets:
http://www.rivetsinstock.com/rivets/blind-rivets/basic-structural-blind-rivets/q-rivet.html
They have tensile & sheer strength over 1000 each, and so the joints should be stronger than the factory specified rivets, but probably have larger heads and that means removing some additional material to countersink them. I'm guessing the additional rivets will cost about $100.

If I remake the aft tunnel again, the parts will cost $61 + shipping. The good news is I've re-made the tunnel before, and I can do it pretty quickly. It's just demoralizing to have to keep making the same parts (and paying for them!)...


Okay...suggestions, recommendations, recriminations? Am I just being too picky? Is all this "good enough"? Will my Sonex disintegrate in flight if I even think of using these parts? Inquiring minds want to know!

If you got this far, thanks for reading all this!

[Mike53]

After my breaking stem problems,I did a lot of studying about blind rivets. Generally if a hole is oversize you risk the stems breaking off proud,which is just a pain in that now you have to grind the stem flush with the dome.
Your talking a difference of #30 to #27 which is about 16 thousandths of an inch or 1/64" or 1/128" per side so the rivet will easily expand to make up the difference as your test proved. Should you be okay with this considering 6/50 are oversize?
Peace of mind is a wonderful thing but I would think a difference of 16 thousandths of an inch for less than 15% of the holes is okay.Reaming them perfectly round might be all that is needed.
Because of the rivets supplied now ,I now drill up to #31 then to #30 and have not had any stems break proud since :D
I'm curious to know how you drilled out the rivets as you should not wind up with a bigger hole ?
Cheers
Mike

[Andy Walker]

Thanks for the response Mike. Here's the method I use for drilling out rivets:

http://www.eaavideo.org/video.aspx?v=1472379714

I generally use a 1/8" drill bit (not #30!), seems just right for drilling the head off and keeping the hole from enlarging. The reason some of these enlarged was that I pulled the leftover stems from the back instead of using a punch from the front as I usually do. Lesson learned there.

So your post makes me feel better about the angle, but what about those big holes on the sheet metal side. No problem, problem?

[N111YX]

Andy, keep in mind that you do not have to go right up to 5/32 rivets. You can get aircraft quality Cherry Max's at .140 diameter...

http://www.aircraftspruce.com/catalog/h ... rymax4.php

They are expensive but you'll know that you have some kick a$$ rivets in there... 8-)

[Andy Walker]

Andy, keep in mind that you do not have to go right up to 5/32 rivets. You can get aircraft quality Cherry Max's at .140 diameter...

http://www.aircraftspruce.com/catalog/h ... rymax4.php

They are expensive but you'll know that you have some kick a$$ rivets in there... 8-)

Wow, I didn't know about those...Thanks Kip, that's probably *exactly* what I need! I don't see the material listed, are these stainless steel?

[N111YX]

Andy, I must confess that I don't know what they consist of. If they are "certified" for "structural" applications, that's good enough for me for an "oops" rivet...:)

[Sonex1517]

Ok....newbie question....are you pulling the stems from the ones you redo, or all of them? Hadn't heard this before...


Robbie Culver
Sonex 1517
Chicagoland
Aerovee/taildragger
Starting on the wings
Sent from my iPad using Tapatalk

[Andy Walker]

Well, Ted Staton (Xenos builder, if you don't know him) came by the house today to take a look at everything. It looks like I was stressing about several non-issues. The one problem that actually *is* a problem is some out-of-round countersinking on one of my aft spar tunnel angles, which Ted felt (and I agreed) should be replaced. So I ordered that part and I have a good a plan to mate it to my existing parts.

So I'm not going to obsess over this tunnel build anymore and just keep trucking. Woot!