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Oblong holes in the rudder skin

idubrov

Well Known Member
I was drilling holes in the bottom rudder fairing attach strip (R-918) through the R-901 skins and R-904 bottom rib.

Since fairing attach strips did not have any holes, I used cleco clamps to clamp all three parts together (attach strip, flange of the rib and the skin). I did not notice that holes in the skin did not align perfectly with the bottom rib, and as a result, the holes in the skin became oblong.

r-901-oblong-holes.jpeg


The worst hole seems to be ~0.122 in diameter. Note that pretty much the whole row of the holes are elongated -- it is not just few of them.

What are my options?

1. I can upsize the holes to the next size, AN426AD4. The rivets heads will be larger, but who cares. The edge distance situation seems to be this: more than 2x on both the skins and the flange. However, on the attach strip it is going to be marginal, less than 2x diameter (0.250), but more than in the MIL-Spec (0.219).

2. I can replace both skins, hoping that everything will match. Not sure if I want to try reusing stiffeners or not. They are not expensive, but I can try to drill out all of the skin rivets without damaging them.

Other options? What would you do?

(I'll ask Van's, too, but interested in other opinions)
 
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I would try and use the oops rivets first in this area. I also think replacing the skin for this mistake is overkill.
 
The structure of a control surface isn't a place to be going off the reservation. If it was just the fairing attach strip then sure...but

I will use oops rivets here and there, but wouldn't be comfortable about using a whole row of them in most situations. The heads aren't big enough and you would be losing a bunch of strength in tension.

Is that a big deal in this area? I don't honestly know, but but as you point out, this area also obviously includes the lowest rib on the rudder. I would guess it sees quite a bit of stress because that's where the cables are pulling when you stomp on the peddles.

per mil-spec the max hole size for a -3 rivet is .103 the max hole size for a -4 rivet is .135. The best course here is to call vans to get their opinion on using a whole row of oops rivets here, but I'm guessing they're going to suggest just upsizing the whole row to -4 rivets, as long as that doesn't blow out your edge distance and based on your original post it seems like you're good.

edit; FYI- if faced with something like this in the future, you could just remove the lower rib and match drill through it instead of trying to keep everything aligned with clamps. Alternately, if the lower rib was already permanently attached, it would also work to poke just a couple of holes through everything, then cleco and clamp it back together. the clecoes would help make sure all the substructure is lined up when you drill the rest of the holes
 
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I will use oops rivets here and there, but wouldn't be comfortable about using a whole row of them in most situations.

Yes, I had the same line of thought. This bottom rib indeed does look important.

Also, I don't think there is much reason for oops rivets here. Since it is the whole row, I think, aesthetically it is going to look just fine with larger heads. I might go further and upsize 4 holes that attach horn brace, too, so the whole row is -4 rivets.

Setting two aftmost rivets is going be an interesting challenge, though...
 
I was drilling holes in the bottom rudder fairing attach strip (R-918) through the R-901 skins and R-904 bottom rib.

Since fairing attach strips did not have any holes, I used cleco clamps to clamp all three parts together (attach strip, flange of the rib and the skin). I did not notice that holes in the skin did not align perfectly with the bottom rib, and as a result, the holes in the skin became oblong.

r-901-oblong-holes.jpeg


The worst hole seems to be ~0.122 in diameter. Note that pretty much the whole row of the holes are elongated -- it is not just few of them.

What are my options?

1. I can upsize the holes to the next size, AN426AD4. The rivets heads will be larger, but who cares. The edge distance situation seems to be this: more than 2x on both the skins and the flange. However, on the attach strip it is going to be marginal, less than 2x diameter (0.250), but more than in the MIL-Spec (0.219).

2. I can replace both skins, hoping that everything will match. Not sure if I want to try reusing stiffeners or not. They are not expensive, but I can try to drill out all of the skin rivets without damaging them.

Other options? What would you do?

(I'll ask Van's, too, but interested in other opinions)

I would up size the rivets to the -4 size. You can also fabricate new rudder bottom attach strips to gain edge distance here. I would also suggest substituting nut plates and screws for attaching the rudder bottom. When [not if] you damage the rudder bottom, you won't end up over sizing those holes trying to drill out the blind rivets. Plus if you plan on a tail/strobe lamp there, it gives you access.
 
I would up size the rivets to the -4 size. SNIP

+1. I guarantee no one will ever notice the larger rivet head.

Note - I’ve seen this when a builder uses a drill with way too high a speed. The #40 drill bit starts going around in a elliptical pattern.

Carl
 
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