8-11 -> 8-12: Riveting HS ribs and spars to the HS skins

The next step was the final assembly of the HS - it was a long road to get here (almost 50 hours of work), but this gives a great sense of satisfaction to get this far!

The manual called to rivet the nose ribs into the skins first. As the skins were flexing out quite a bit, i used tape on the top of the skins to help hold them together (taping them the same width as the rear spar), and used a homemade "wire clamp" to squeeze the skins in toward the top of the nose ribs.

"Wire Clamp" from the outside. You just twist the wire ends like this to adjust the clamping pressure. 

When i was at Narromine, my TC, Peter Pendergast showed me a lot of RV-14's, and pointed out the large impressions left around each of the rivets on the empennage (and HS especially). These were all about maybe 1" in diameter, and were little concave impressions around each of the rivets. He told me this was due to the air pressure being too high when riveting; the pressure of the gun actually deforms the skin and underlying rib when the rivet is being set. So i decided this was a goal for my build - to get this right. In some cases when this happens, you can hit it from behind to push the dent back out - but not in the case of the HS (as there is not enough access).

My rivet gun came with what i thought was an "air regulator" on the gun, like this one:

I set this to #2 on the gauge and began riveting in the nose ribs. Each time i set a rivet, i was checking to see how much deformation there was on the skin / ribs. As you can see, there was a lot of deformation occurring - just like the completed aircraft i was shown at the airshow.

I had a chat to Pete, and he recommended removing the "air restrictor" from my gun, and simply setting the shop regulator to a lower pressure. I looked around on the google forums, and found a pressure table for various rivet sizes. I started with these settings, and i was quickly able to work out what was working for each rivet size. I like this method, and it has improved the riveting a lot i think.

A much better result once i used the regulator to control the gun pressure. 

I have already dropped a bucking bar into the skins once, so i made sure i used some rags just in case. This photo also shows the little plastic tube i have been using on the rivet tails sometimes - where there is a rib flange that is sitting off the skins (like in the image below), i use the tube on the tail as i can then push on the bar to push the flange toward the skin. I leave the tube a little longer than the rivet tail (using the rivet gauge to measure and cut it), and once the rivet starts to push out of the hole, i stop it pushing out with the mushroom set of the rivet gun. I then maintain this pressure balance until the rivet starts to set, and then release a little of the bar pressure. Most times the rivet won't set fully - so i remove the tube, and set it a bit more. This usually results in a rib flange being nice and right against the skins.

A close up of the plastic tube on the rivet tail, before bucking

I then put the whole spar and rib assembly into the skins - this thing is HUGE (and heavy with all the clecos in it). 

Next, I riveted this using LP4-3 rivets to the nose rib flanges. After the dints i got in the skin of the rudder from using a crappy cheap rivet gun, i purchased a very small Kinchrome rivet gun. This thing is great, fits in tight places, and doesn't jump a heap when the rivet stem breaks off. I made sure i added cardboard to protect the skins just in case. 

I also had a little visitor into the shop, who signed his name on the forward spar to posterity!

Next up i riveted the spar to skin rivets, then the rivets from the spar back to the stringer, then the stringer itself, and finally the rivets between the stringer and the trailing edge.

Almost all the rivets were set with the gun and bucking bar (using a wooden spacer on the bar), but i used the squeezer in any place where it would fit (the end ribs etc).

I am really happy with how the rivets are turning out - the rivet lines are looking nice and flat with none of the aforementioned deformations.

There were a couple of tricky rivets on the inspar ribs, which were hidden behind the flanges of the stringers - you literally can't see these rivets at all. I just placed the bar as best i could using feel, and then riveted them. I then checked them with a mirror and a torch. Overall, i am having a good run on these rivets - only maybe 10 had to come out. I have been drilling them out as soon as they set, so i don't forget or miss any.

The final step once the skin to ribs were finished, was to remove the assembly from the cradles and lay it flat on the bench. There were 5 rivets per side left to do - the AD4-7 and AD4-8's which connected the modified inspar and nose ribs to the spars and spar doubler. This is one thick bit of kit and these were very long rivets. I started out using 40psi on the regualtor, and quickly realised that i needed more. I was using the offset 6" cupped set on the forward side (manufactured head), and the bar on the back side, wedged in against the ribs. I used some cardboard to prevent scratches. These were some tough rivets! At 60psi they took about 3-4 seconds to set - the whole assembly was walking across the bench. For the most part, the rivets are just acceptable. A couple are very slightly bent over, but given the thickness of the material, and how hard they would be to remove, i have elected to leave them in.

On the left side, i clipped the middle rivet tail with the bucking bar when i was bucking one above it - This damaged the shop head slightly as per the pictures below. I spoke with the TC, and we both agreed that the rivet will still do it's job with this small error, and the chances of me destroying the hole in the spar and doubler are high - so we elected to leave it alone.

The bucking bar just clipped the middle rivet.

These are the 5 modified ribs / spar rivets

These are the 5 modified ribs / spar rivets

As you can see this is one large bit of aircraft! At this stage, all that is left is to rivet in the rear spar. I am toying with the idea of waiting until a TC can have a look inside the assembly, before i go ahead and rivet. I think i will leave it a few days, then inspect the rivets again and make the final decision as to if i wait or not. I am tending to think that all is good in the world at this stage and i should just finish it off - but not 100% on that decision.