Finished riveting the HS skins, the HS is now complete except for riveting in the rear spar. There’s a lot of rivets holding the rear spar in place, but they’re all done with the squeezer so that’s a straightforward job. I’m going to leave the rear spar off and go ahead with the elevators, so that I can work out the grounding strap positions once the elevators are assembled.
I tried to find a way of doing the ten 1/8 inch rivets on the innermost nose rib rear flange with a 4 inch yoke in the squeezer, but couldn’t do it without getting some tooling made up (to turn the 4 inch yoke into a kind of big longeron yoke). I did some practising with the double offset rivet set, taped it up so it couldn’t rotate, and we were able to set the rivets OK that way. Did this very carefully, I didn’t want to have to drill any of these long rivets out.
I pulled all the parts for the elevators, so it’s now back to cutting and de-burring…
Started on the right hand section, for no particular reason. Riveted the nose ribs to the front spar, and then inserted the remaining two HS-904 inspar ribs. I screwed some lengths of pine to the cradles to make it easier to move the entire assembly around. I chose to do the bottom surface first, to get more practise on a less observable part of the plane. With two of us doing the riveting, it was fairly straightforward to rivet the skins to the front spar, ribs up to the stringers, stringers and remaining ribs per the instructions. The results were really good, with just two drill-outs across the entire surface.
The step I’ve been dreading, riveting the Horizontal Stabilizer internal nose ribs. These ribs require the rivet gun, and access is very awkward. There are plenty of horror stories about trouble in this area – such as the one(s) here. So with all this in mind, I put my better half in charge of the rivet gun and between the two of us we managed to produce a good result. It was very slow going, but we got the 40 rivets across the four internal nose ribs all set, with just 3 drill outs. The hardest rivet to buck is the one closest to the leading edge, and although a couple of these are not entirely clean, I deemed them to be good enough. I’ve read of people who have given up on these and installed pop rivets.
I used a couple of different small tungsten bucking bars. To help press the rib flanges against the skin, I placed masking tape on one side of a small rubber grommet, and pushed the grommet onto the rivet so that the base of the rivet shaft stuck to the masking tape, holding the grommet in place. I then positioned the bucking bar with one hand, helped support the skin with the other hand, and then Kerrie gave the rivet a burst with the rivet gun. With the rivet partly set, I then removed the grommet, re-applied the bucking bar, and we completed setting the rivet. This helped to prevent any gaps between the rib flanges and the skin.
I used a 2x rivet gun, with the pressure set way down low to 30psi. The gun barely worked, but the low striking force made it easier to handle. As advised in one of the Forums, I cooked the rivets at 400F before use, but I have to admit I didn’t really notice any change in what was required to set them.
We then set the skins back up in the cradles, dropped the partially built front spar assembly in, and lined things up with just a few cleco’s before calling it a day. The worst part is now over, next step is to cleco everything up, and drive a lot of (much easier) rivets to complete the HS assembly.
Started riveting the Horizontal Stabilizer today. First step is to rivet together the front spar assembly with most of the inspar ribs, inspar web and stringers. I was able to do all of this with the pneumatic squeezer. I then set up the two skins, cleco’d the nose ribs in place, and riveted the “outside” nose ribs on each skin, again with the pneumatic squeezer. That leaves four “inside” nose ribs for another day, a two person job and apparently one of the harder riveting jobs on the entire airframe. Not looking forward to that.
After taking a few days off I set up and primed the HS skins and internal parts. I keep the primer very thin, but went a bit far on one of the skins so I touched it up when I did the rest of the HS ribs and stringers. This was the second major priming session and it went a lot faster this time. Most of the time is taken up with preparation – de-grease, dry, etch, rinse, dry, spray on EAP-12, dry, and only then spray on the primer.
Yesterday and today I countersunk the Horizontal Stabilizer stringers and spars. The stringers were trivial. For the spars, there are a lot of holes to do. I did all of these in the drill press, and was glad to have an industrial quality drill press to do this job. I used masking tape to prevent shavings from scratching the perimeter of the countersunk holes; this worked well with only a couple of holes having small exterior scratches that required a little scuffing to fix up.
The flanges are not perpendicular on either spar, and the spars are different in this respect. I simply used wooden stops to get the spars to around the right angle with respect to the drill head. However, the reference for each countersink is the match drilled hole itself, which is why the wooden stops were just approximate places for the base of the spar to rest.
The book says to make countersinks for dimples around 0.007 inches deeper than flush countersinks for rivets. I dimpled a piece of 0.025″ scrap Alclad, and adjusted the countersink size until no light showed through the gap when the dimpled sample was pressed against the spar. Hard to take a picture of this, but a rivet in the hole showed a nice ring of bare metal around the outside of the rivet head, so that looks about right. Note in the picture that the outer countersinks look out of shape. This is simply the perspective of the close-up photo – they’re all the same and round.
Yesterday and today I de-burred the outside holes of the skins as part of the process of de-constructing the horizontal stabilizer after match drilling. That way the skins were well supported, and I wasn’t de-burring a thousand holes in one hit. I de-burred a section at a time, removing cleco’s as required. I also de-burred the stringers, and de-burred and dimpled all of the inspar and nose ribs, a total of 22 ribs.
Next to do is de-burring the inside of the rivet holes in the skins, and countersinking the flanges of both spars and all stringers.
Yesterday and today I prepared the Horizontal Stabilizer for match drilling. The assembly went together really well. I’ve probably over-cleco’d but I’d rather do that than under do it. Now I know why I bought so many cleco’s.
The workshop arrangement – making two workbenches on wheels (with brakes) – worked out really well on what is the first large assembly job. Easy to set up and good access on both sides as well as in the centre.
Now I have to match drill a zillion holes, moving every cleco in the process.