nzrv8

nzrv8

Well Known Member
Hi All,

I built my HS a few years ago now, but I was never happy with the final product - I used a home made DRDT dimpler to dimple the skins, and the dimples were not as crisp as they could have been due to flex in the home-made frame. Probably not a big deal structurally, but it was always on my mind. So it has been hanging on the wall of my shop for years waiting for me to get around to drilling the skins off and fitting new ones with crisp dimples.

Due the fuselage now being ready for a HS, I could avoid it no longer!

Yesterday my sisters partner, a helicopter airframe tech, dropped by and asked if he could help with anything.. Sure I said. There is the drill, there is the HS, get drilling!

The skins came off with no problems - He's done this a few times it seems! But as I was measuring the holes afterwards in the understructure, it turns out that the holes in the ribs are slightly oversize - .106 to .110.

But most of the holes in the slightly thicker spars are within spec's for 3/32 rivets at .103 or less.

The holes weren't affected by the drilling out process (most still had primer in them) so it seems they must have been slightly oversized by the original dimpling process. I was using tank dimple dies back then for understructures, but I don't use them now.

My question - would you live with the slightly oversized holes on the ribs, or would you scrap the parts and start again? I have the edge distance in most areas to go up to 1/8 rivets if required, but I do find these harder to drive in difficult areas - Meaning a higher chance of a riveting mistake.

On the other hand - I know spec's are there for a reason! And building a new HS is not a large undertaking, but the parts are expensive to ship to the bottom of the world. Money aside - I do enjoy this sort of 'repair' work, its always nice to fix something rather than buying all new parts.

Any opinions will be gratefully received!
 
A couple of things -

1. No edge distance problems on the hole

2. The skin has holes within spec. so that the strength of the flush head is OK - a large skin hole would have less contact area for the manufactured head.

3. The rib hole is only oversize by 0.007

Since driven rivets will expand to fit the hole, I would just use a slightly longer rivet and make sure that the shop head is not at the minimum spec dimension. If all the shop heads meet a true 1.5x diameter and are not too thin you should have all of the strength estimated in the specifications.

Check with whoever does your Oz sign-off first to see if they agree...:)

You could slightly pre-squeeze the rivets to make them fatter, but with the small error you mention just try it without first.
 
az_gila said:
You could slightly pre-squeeze the rivets to make them fatter, but with the small error you mention just try it without first.
Click to expand...

Ditto to that. It's a technique I was shown fairly early on in my build and it works great for slightly over-size holes. If you simply use a longer rivet so you have more material to fill the hole, you risk clinching it when setting. Then you have to drill out again, hole gets bigger.........

A slight mis-quote, I think, but some General once said something like "Perfection is the enemy of good enough". Early on, I drilled out rivets that were probably OK and then made a mess of re-setting them. If you drill #40 and then dimple, the hole is already enlarged slightly. When you set the rivet, the hole gets slightly bigger as the material fills it. So even with a perfect drill-out, next time you are dealing with a bigger hole. Hence the chance of a poor set is higher. If a rivet really is bad, then I try again once. If that is also bad, then I go for a pre-squeeze with a longer rivet. If that fails, then I will normally go for a drill-out to #30 and an "oops' rivet.

If it is a difficult area to set, you might also think about a MK319 (7/64") pulled rivet. They are semi-structural and on parts of the -10 they are specified as an option on the skins.
 
Thank you both for your replies. I will make up a few test pieces and see if I can get deliberate oversized holes, and experiment with longer rivets and see how they turn out!
 
What I Use

What I have used for many years to solve this problem is the NAS1241 series solid structural rivet (an approved substitute for the AN426 series), that has a body diameter of 1/64" (.015) larger than standard. I use them almost exclusively on thinner joints where the dimpling process enlarges the hole to almost (maybe a couple of thou shy) the oversized diameter - that way the shank completely fills the hole before the driving process starts. On some marginal, hard to reach locations, this "filling of the hole" allows using a shorter rivet by 1 dash number, thereby reducing the clinching possibility.

They do drive a bit harder - but not nearly as much as using an NAS 1097.

These rivets are more expensive, and not all suppliers carry them - but for those hard to get to places, or slightly oversize holes, they are (I think) worth it.

HFS
 
Awesome, thanks for that tip HFS!

I measured every hole in the understructure last night.

90% of the spar holes are within spec. The 10% that are slightly out of spec are between .103 and .110. I will use NAS1241's here.

But - only 10% of the rib holes are within spec on the 6 ribs. Would using NAS1241's over this many rivets in a row present a structural issue, given that their heads are the same diameter as AN426's? I do have the edge distance in most places on these ribs to go out to a 1/8 rivet, but I don't want my riveting to be any more difficult than it has to be.

I will be ordering new skins due to the distortion around each dimpled hole (Caused by my crappy home made C-frame, which has since been retired :) ) and this was the reason for drilling the skins off in the first place, But -

If I was to cleco the new pre-punched skin to the old, already drilled understucture, I run a large risk of further enlarging the holes in the ribs and spars. Is there any ways to minimize this issue?

Thanks again !
 
Pre-Squeeze a longer rivet to take up clearance

Just my 2 cents. When I have had to drill out rivets in the past I normally make sure the hole is clean and then take a slightly longer rivet and pre-squeeze it slightly to enlarge the diameter. It may be a little slow doing many rivets but I've had good success using them since they won't rock around in the hole and rivet well.
 
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