Installing nut plates

No, nut plates aren't those little trays you set out for guests to snack on when they come over on Superbowl Sunday. Nut plates are used to provide a replacement for a nut & bolt arrangement where the back of the piece being attached won't be accessible. Consider the parts that I've been working on for a week now. If you remember, I'm working on the back part of the tail fin to which the rudder will eventually be attached:

Once the skin is riveted to the frame of the tail fin, it will be impossible to get at a nut in there while the rudder hinge is bolted on. So instead of using a nut, we use a nut plate. The nut plate is permanently riveted in place, in this case by using small solid rivets. The smaller #3 rivets are much easier to squeeze than the larger #4 rivets that were used to build the rudder hinge assemblies, but to offset that they have the added complexity of being flush rivets. That means they sit flush against the surface that they are being riveted to. If you remember, they are able to do that because we used the countersink tool to clear away some material around the hole. Note that they aren't quite so easy that I didn't end up having to drill one out. It slipped up in the hole as I was squeezing it and didn't end up flush at all. Quite the opposite.

Here's what the flush rivets look like from the outside (when you do it right):

On the other side, deep under the skin, they look like this:

See how that works? Pretty cool, eh?

Here's how it will look when the rudder hinge thingy gets bolted on:

That will be years from now. I think it's one of the last things you do.

That's it for tonight. The next step involves rounding radii on some of the ribs that give the tail fin its aerodynamic shape, and once that's done they need to be fluted. I'll describe fluting when we get to it, but the point is that I'm going to wait until Saturday when the Certified Fluting Expert that I keep on retainer can make a shop visit.

Thou Shalt Commit Rivetry

I started out the night with some countersinking on the big old getting-tired-of-seeing-it vertical stab spar. Countersinking is done to provide space around a hole for a rivet head to fit down into so it will be flush against the skin. Imagine it as a very tiny toilet bowl. Or, just see if you can pick it out in this picture:

Four of the holes are countersunk. The big ugly black thing to the left of the metal is the countersink tool. It goes in the drill just like a drill bit. The bit is inside the black cylinder and comes out when downward pressure is applied. The amount of bit that comes out is set by adjusting the cylinder by twisting it in or out on the threads. You set it for any given job by testing it on a piece of scrap metal until a rivet sits flush against the metal when placed into the hole. With the precise adjustment made, the bit then works differently on every single hole you try to countersink.

Well, at least it does when I do it.

The countersinking of those holes was the last task before riveting that part (and its mirror image part) onto the spar. That was done with blind rivets. Blind rivets are nice because they don't require access to the other side of the metal as is the case with the solid rivets I squeezed in last night. That's not important with the spar I'm working on, but it will be very important later on when doing things like riveting skins onto wing ribs.

It's really pretty easy. The rivet has what is essentially a nail running through it. If you pull really hard on the nail, it will compress the rivet. If you pull hard enough, the nail will break loose and the rivet will remain in place.

Here's a before and after view of a blind rivet:

You can also see where I did some test holes to set the depth of the countersink tool.

It didn't take long at all to pull the eight rivets that would permanently attach those two parts to the spar:

This is the side that will be visible, so the "manufactured" rivet head is on this side:

This side will be inside the tail, out of sight forever. So this is the side where the "shop" head of the rivet goes:

All in all, a good night in the shop. Nothing broken, in other words.

Eight rivets down, 12,492 to go.