Riveting Ribs To Spar

The ribs are riveted to the spar. I modified the long head of my rivet squeezer to fit loosely over the spar flange. There may be some heavy rivets it won't squeeze without bending but I mostly use it for 1/8" and 3/32" rivets so I don't think it will hurt it. The biggest problem I had was my rivet gun hasn't been used in years and the air valve was stuck. I also discovered that I do so little riveting I don't own any bucking bars. With 9" of snow falling I faked it and moved on. The skin below the tank is riveted to the spar and we're ready to set rivets on the skin and the drag wire brackets. Then the outboard skin can go back on. This was a very long day of going slow and careful. I don't want to have to redo this.

Riveting Spar

I've started riveting the new spar, etc. I could have started this earlier but I wanted to make sure everything fit properly. I've started with the compression members. Then I'll do the ribs and the skin below the tank. I'll get everything I can before the skin goes on. The drag wire brackets will have to wait until the skin is on the spar because the brackets block the rivets above them. I'll trammel the wing before I finish the skin. Well, that's the plan anyway.

Bending Leading Edge Skin

The best way to do this is with a press brake where you push the sheet into a die to form a nice bend at the exact location needed. I don't own a 12 ft press brake. I've faked it where the skin is narrow and long by folding it and pressing it between 2 2x4s screwed to the floor. I just didn't think I had the strength to do that with such a wide skin. The plan here is to form it by pulling the skin down over a tool made by screwing a length of 1" schedule 40 PVC pipe to the edge of a straight 12 ft long 2x8. The screws were spaced about every 18", countersunk and any edge of the head sticking out filed smooth with the pipe.

You can see in the first picture with the old skin setting on the tool showing what the desired shape should be.

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To keep the skin from becoming wavy and to protect the edge of the skin, 12 ft long 2x4s were clamped, with backing blocks, to the skin edges.

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With the 2x4s clamped on, the skin was placed over the tool with the nose of the Leading Edge ligned up with the top of the pipe, red marks on skin and pipe.

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Light crank type cargo straps were than wraped around everything about 2 ft on centers. OK, this took about 10 tries because the 2x8 wants to fall over. By holding the 2x8 with my knees while carefully starting to tighten the 2 straps at each end I eventually got it going. I then realized I should have used a 2x8 on the long side and started over with some extra 2x4s so it would all clamp closer to the bend. .

Eventually the bend was tight enough that the clamps and backing blocks were hitting on the 2x8 preventing further bending. We were able to remove the clamps and blocks at this point because the friction of the 2x4s against the skin prevented them from slipping. If I had some pieces of 2x2 to add right at the bend I could have made this bend even tighter.

As it turned out it is not as bent as the old skin but it is identical the the skins I recently installed on some Cub wings with no problem so they should work just fine. Once they're riveted to the spar and top of the nose ribs the straps can be used to pull the bend tight to finish the underside riveting.

New Leading Edge Skin

The first task was to fit the old skin back on the wing to make sure all the holes still align correctly so it can be used as a pattern for the new skin. Again Cessna did an excellent job making the parts the same. Everything aligns perfectly. While the skin was on I also marked any holes which needed to be ignored or had gotten elongated drilling out the rivets. There were extra holes from drilling out the welds. If a hole was elongated an arrow was drawn on the skin to show which side of the hole to use as a guide when punching the new hole (see the 3rd picture below). After a careful cleaning of the floor the sheet of aluminum was laid out and 15" cut off for the new shin. Because this was thin enough sheet I used a Stanley knife and padded straight edge to score and break the sheet, much cleaner than snips. On thicker sheet like spar webs I use the table saw, noisy but very clean straight cuts. By standing the strip on edge in a curve it is stiff enough to allow easy filing of the edge to clean it up and debur it. You can see some of the markings to help punch new holes correctly. Because of the curve of the old skin I'll work from the inside so all the markings need to be transferred to the inside of the skin. X's mark holes to ignore. The old skin was then clamped to the back of the new skin. The only place I have that is flat is the floor so some 2x4s were used to hold the skins off the floor far enough to work with the Whitney punch. C-clamps and craft sticks, to protect the metal, were used to hold the shins from moving while new holes were punched. I started with the 3/32" holes between the ribs along the spar. They were generally the cleanest holes. As holes are punched clecos are installed and the clamps removed. These are simple matched hole construction methods. Go on line to the EAA Sport Aviation library and look up John Thorp's articles on building the T-18 in the early to mid 60's. Always punch any hole you can reach with the Whitney punch using a punch with the center nib carefully removed. Holes that can not be reached are center punched with a duplicating punch and drilled using the old skin as an added drilling jig to help keep the drill bit from wandering. I will do the holes from the other edge after the bend is made, the skin is fitted and alignment holes located along the other edge.

Primed Nose Ribs

The 4 nose ribs which needed some minor repairs are fixed. All the nose ribs and the outboard leading edge skin are epoxy primed. With the ribs installed the old leading edge needs to be fitted on to make sure it can be used as a pattern to make the new leading edge skin.

New Spar Installed

All the areas where corrosion was removed or cleaning exposed bare aluminum were treated and primed. The spar is now ready to install. Suprisingly the new spar went in easier than the old one came out. Everything fit perfectly and all the holes align. Clearly a testament to how they made the parts and assembled the wings at Cessna. The only rivets which will be oversize are due to old repairs. The most I'll have to do is clean out any primer in the rivet holes and we'll be good to go. With clecos in everything and the drag wire installed the wing is back on the stands until we start riveting. The front drag wire fittings have to be removed to rivet the leading edge to the spar, so it will be back on the tables for that. Until then, the ribs and outboard leading edge have been cleaned and are ready for primer. There are 4 nose ribs which need minor repairs and we'll be ready to fabricate the new Leading Edge. The aluminum is due in 2 days so the nose ribs should be on before the leading edge is ready

Removing Front Spar

I realized I have several 2'x4' plastic tables from Sam's which we use in the shop and for our booth at fly-ins. They are adjustable for height from 24" to 36" so they make nice work tables. Three of them nicely support the wing. After rem0ving the clecos I gently worked the spar loose. With a stool as a support at the root end the spar came off easily.

There was a little corrosion hidden at the area around the tank bay which was where the corrosion problem was. This wing spent to much time parked on its nose. Who knows, a critter may have nested in there at one time while it was stored. It all cleaned up easily from the surface of the ribs, etc. It shows the protection difference of the clad aluminum used for the ribs and the bare extruded spars. I'll treat and prime the cleaned areas so they'll last and hope all the holes match up with the other spar.

Rivets Removed From Front Spar

The first task was to remove all the nose ribs. They need to be cleaned up and treated for minor corrosion and make some repairs. The rivets are now all removed from the front spar so it can be removed. Clecos have been installed to hold it all together while removing the rivets. The wing is now more like a limp flag. I've been able to use my wing stand so far but I need to make some stands or a large table to hold the wing while I swap the spars.

Prime Spar and Finish Leading Edge

Today the rain stopped and it warmed up to 41 degrees. Well, the can of epoxy primer says it will paint and cure down to 35 degrees and if it's humid just let the primer set for an hour instead of 30 minutes. As you can see most of the 14" of snow is finally gone. The primer went on just fine after a final wash with MEK just to make sure it was clean. A few hours later the rain was back but by then the spar was safely back in the attic to finish curing.

By evening the primer was dry to the touch and the attic is only 55 degrees. The leading edge is now off. Removing and cleaning ribs is next. I've tagged the ribs to put them back in the same positions because I want to use the old skin as a pattern rather than mark and drill all the rivet hole from scratch.

Removing Leading Edge on Right Wing

Today was rainy so priming the spar will have to wait.

Today I put the wing back on the stands and started unriveting the leading edge. I didn't want to take the wing apart until the spar was ready to install because an assembled wing is easy to strap to the ceiling in the shop. I think I ran out of storage space a year ago.

Most of the rivets are 3/32" and are very easy to drill off the heads. What I hadn't thought about was the spot welds. The leading edges were originally welded to the ribs. It seemed like a good idea but the welds broke at high angles of attack and high loads so they went back and added rivets. Even with my careful drilling of rivets I won't be able to reuse the leading edge, which is in good shape, just to many holes with rivets and welds.

The welds seem to come apart best by center punching the middle of the weld and carefully drilling with a #30 drill (about the size of the weld spot) while lifting the skin until the weld lets go. Some were already broken years ago. Some brake as soon as you lift the skin a little, and other need to be drilled. You don't have to drill away all of the weld for it to weaken enough to let the skin break lose. You want to leave some skin on the rib since the skin is going to be replaced and the ribs can be cleaned up and reused. We'll clean up the ribs after they're off the wing.

Cleaning up Corrosion on the Front Spar

Now that all the rivets and fittings are off cleaning the light corrion and remainging crud on the spar is easy. First I washed off any remaining paint residue with MEK. To clean the corrosion I glass bead blasted with AC beads at 20 psi. Glass beeds can be sieved and re-used up to 30 times if you keep the pressure low enough not to smash the beads, and blast at a 45 - 60 degree angle so you're scrubbing the surface not just peening it. When I first bought glass beeds the young lady selling them explanied that the pressure should be very low, if you can see something happening it's probably to high. This was slow work but well worth it to make sure there is no corrosion trapped and growing under the expoxy primer when we're done. Also, you're only blasting any spots of surface corrosion. This is a 53 year old spar so there was a fair amount even if nothing serious. The biggest problem was how to get a wing spar in my blasting cabinet. There is a door on one side so I put a metal cutting blade in my circular saw and cut a hole in the other end of the cabinet so the spar could pass through (this will also work for grit blasting the wing struts). I then took some scraps of a stiff foam rubber and made plugs for the holes. I cut the plugs about 3/4" large than the holes and split the edge of the foam so it fit over the edge of the metal making a nice seal. I adjusted my roller stands for my table saw and placed one on each side of the cabinet for the spar to roll on. Next I cut "I" shaped slits in the foam for the spar to pass through laying on it's side. Problem solved, no mess in the shop, put on a head set with something to relieve the boredum and blast away. The spar really cleaned up nice. With the cleaning done the spar was treated with a Phosphoric Acid Etch & Brightener, and then a Conversion Coating to assure no corrosion is left and the primer bonds well.

Removing Rivets From Front Spar

Todays task was to finish removing rivets from the front spar. When it was removed from the wing the rivets were drilled just enough to get the ribs off leaving all the holes plugged with the ends of the rivets (I'm not complaining). For the 3/32" rivets holding the leading edge and ribs a light tap on the drilled off end with a hammer and punching them out with a pin punch worked fine and quick. For the 1/8" rivets in the doubler at the strut fitting I drilled them with a 3/32" drill most of the way through the spar and the punched them out with the pin punch. They just hung up to much with 3 layers of metal to punch out otherwise and I didn't want to increase any hole sizes. I also removed the strut fittings and the drag wire fittings. They'll have to go back on after the ribs are riveted on.

Stripping Wing Spar

We're finally done with end of year tasks in the business and the snow is starting to melt. With the temperature at a warm 51 degrees I got the spots of paint and primer stripped off the front spar. Clearly stripper works better at 70 degrees. I used a stiff propylene brush and a lot of elbow grease and it cleaned up nicely. I wanted to make sure someone hadn't painted over some light corrosion. It seemed better to clean it all up now while the spar was easy to work on.