Wings

So I had a good little mini-vacation this past weekend. I even got in a little flying while I was in Raleigh, doing an Arrow checkout up there. I’d hoped to be able to take my niece up for a flight, but unfortunately the weather on Monday and Tuesday wasn’t agreeable. While I was gone, my shipment from Van’s came in, in a nice wooden crate, which Josie got to manhandle into the house by herself (sorry babe). Wednesday night, I moved the crate into the garage, opened it, put everything away, and then tore the crate down. Hooray, more scrap wood! Then I took the rest of that night and last night off.

Tonight I decided to make all the tank stiffeners. These will be attached to the bottom skin between the ribs to provide, well, stiffness. After all, each of these tanks will hold 21 gallons of fuel, which weighs 126 pounds! The stiffeners come prepunched, but they’re also attached together into long strips that have to be cut to size using notches for reference. I finally learned my lesson here after making stiffeners for the tail: I removed the blue film before I cut the stiffeners to size. Much easier to remove the film from four long pieces than from 48 short pieces.

After a lot of work with the snips and a sore right hand, I had a pile of scrap pieces and a larger pile of very rough-edged stiffeners:

Then it was time to spin up the bench grinder to clean up the edges. I gave that grinder a workout tonight – I probably ran it for about 40 minutes straight. When I was done, I had a bunch of nice smooth stiffeners:

I was about 90 minutes into the evening at this point, but I didn’t quite feel like quitting, so I decided to look at fabricating the T-405 inboard tank attach angles. These get fabricated from raw angle and attach to the nose of the inboard rib on each tank. There’s also a reinforcement plate that goes on the inside nose of the inboard and outboard ribs on each tank. Since the attach angles are made to fit the contour of the nose rib, it seemed like a good idea to clean up the reinforcement plates first to use as a guide/pattern. So I broke them out, removed the film, and fired up the bench grinder yet again to take the corners and tooling marks off. Very nice:

From here, I found the piece of raw angle that the T-405s are fabricated from. I briefly considered getting started on them, but it was getting late by this point, and I was a little worried about cutting corners if I decided to try and make them tonight. So I went inside instead; I’ll probably do the attach angles tomorrow. Actually, now that I look at the weather, tomorrow looks clear. If I can restock on primer tomorrow morning, I should take advantage of the good weather and get the right wing ribs primed. Maybe I can get the right wing skeleton riveted this weekend too. Right now the right main spar is living on the workbench because I don’t have a good place to store it. It’d be nice to have that whole workbench back…

Well, we had quite a day out here. We decided to run down to the beach with dogs and make a half-day or so out of it. Unfortunately, things got interesting on the way home – the car overheated. Seems I have a bit of a cooling system leak somewhere. I managed to make it home without any more overheating, but at the cost of driving with the heater running full blast. In the Houston summer heat. Actually, it wasn’t that bad on the highway, except for my feet getting thoroughly cooked. I now know that this car’s heater is tremendously effective.

Anyway, by the time we got home, I was tired again and I took a nap. Then it was finally out to the garage, where I got to work on the T-405 tank attach angles. I’m glad I decided not to try to do these last night; they weren’t terribly difficult, but there was a lot of time-consuming “how should I do this…” along the way. If I’d put some time pressure on myself to finish it last night, I might well have messed something up.

First, the chunk of angle gets cut to the rough length of the attach piece:

One side gets a symmetrical round profile, while the other is shaped to fit inside the nose of the inboard tank rib. I used one of the reinforcement plates as a pattern for this. Here’s the cut line I made on the first angle:

Roughly cut to size in the bandsaw. I did a fair amount of vixen file work to fine-tune the shape, but I can’t remember if this photo is before or after I did that:

Then it’s over to the bench grinder to remove all those nasty tooling marks and make all the corners nice and smooth:

The second angle went a lot quicker, mainly because I could just use the first one as a pattern instead of having to carefully lay out cut lines. Here’s the pair:

These will eventually get drilled to the tank ribs, but that will wait until I can get the right wing caught up with the left. Weather tomorrow looks good, so provided my local Napa has a good stock of 7220 primer, I can finally get those right wing ribs primed and then work on riveting that skeleton together.

I also took some time tonight and read through the instructions for the capacitive fuel sender kit. I can see that these tanks are going to take some time; there are lots of little details that have to be attended to. Fuel senders, access holes, vent lines, caps, drains…fun times!

I went out to the garage with pretty low expectations tonight. I figured I’d aim to shoot all the main wing rib rivets that I could manage by myself, and then just kind of wing it from there. Well, there were exactly three ribs I could do solo, so that took all of about…maybe 30 minutes. Josie is busy working overtime for a big work project, so I couldn’t recruit her to come out and help rivet.

One of my main short-term goals is to get this right wing skeleton finished so I can clear it off the workbench; right now workspace is hard to come by. Between Josie’s unavailability and needing to wait for that replacement rib (which should be here Monday), it seemed that being productive this weekend was going to be tough. So I decided to abandon riveting on the right wing skeleton for the time being, and we moved it over to the wing stand. That freed me up to do some more work on the left tank.

First order of business was to drill the attach screw holes on the outboard edge of the tank. These screws will go into nutplates that will be on the W-423 joint plate. This was pretty straightforward, but before drilling, I double-checked everything to ensure that the tank was in the proper place. It’s been a couple weeks since I last set it up there, and it’d be really annoying to misdrill holes because I didn’t verify alignment and such.

Here’s a picture of a #19 hole for one of the screws. Exciting, huh?

From here, I’d figured I’d pull the tank off and start looking at all the various internal parts. But then it occurred to me – I already had the thing clecoed together, and it was going to need to be match-drilled at some point…might as well take care of that now. First I pulled the tank off the spar, set it in the cradle, and match-drilled the baffle-to-rib holes. For the skin-to-rib holes, I put it back in place on the spar; the cradle limits access to some of those holes.

With that done, I marked the ribs and pulled the whole tank apart again:

I was on a roll, so I figured I’d go ahead and get all the match-drilling done. The only things left were the stiffeners for the bottom of the tank, which I cut and prepped a few days ago. So i got those out, clecoed them in place, and match-drilled them too.

The only prepunched holes that still need attention now are the attach screw holes along the aft edge of the skin. These just need to be drilled out to final size. However, I seem to recall reading something about problems with these holes cracking when dimpled. I think there was a specific approach to prevent those cracks, so I’m going to research that before I do anything.

I suppose tomorrow I’ll really get into the nuts and bolts of the tank’s innards.

Work continues on the left tank while I wait for my replacement rib. With pretty much everything match drilled, I moved on to working on the end rib reinforcements. The outboard end of the tank just gets a plate riveted on its inside face, while the inboard rib get that plate along with one of the attach angles I fabled up last weekend. This angle will eventually attach to the fuselage to help carry the weight of the tank.

To start with, I laid out the rivet hole locations based on the plans:

Then I center punched those locations and went over to the drill press to drill the holes:

Now the angle can be used as a drill guide for the rest of the assembly. The fun here is that according to the instructions you should clamp everything in place and drill it in assembly. Easier said than done, since both the angle and the plate need to be carefully located, but on opposite sides of the rib. After (unsuccessfully) trying a couple ways to do this, I came up with a better idea. First I just clamped the angle in place, then drilled two of the rivet holes. Next I removed the angle, flipped the rib over, and clamped the reinforcement plate in place. Then I used the two holes I’d just drilled i the rib as a guide to drill two holes in the reinforcement plate. Now I was able to cleco the whole assembly together and drill the remaining four holes in assembly.

The finished assembly clecoed together:

I also used the newly-drilled inboard reinforcement plate as a guide to drill the outboard plate, then I clamped that plate in place on the outboard rib and drilled it too.

I wasn’t quite ready to call it a night though. So what next? I decided to look at the capacitive fuel sender plates. I reread the instructions, got out the pertinent ribs for the left tank, and started by drilling the mount holes in the rib for the outboard sender plate. The assembly is drilled to #21 and then the rib holes are enlarged to 1/4”. The reason for the larger holes in the ribs is that the screws that will attach the sender plate will be surrounded by nylon tubing, presumably to ensure the plate is electrically isolated from the rib.

Interestingly, while there are two sender plates per tank, there’s only one wire that connects both plates to a single-terminal connector that will be mounted on the inboard rib. Somehow I was expecting there two be two wires, one to each plate, with the sender measuring capacitance between the plates. I can only assume that in fact the capacitance is measured between the two plates and the tank structure itself. Maybe the outer screw portion of the connector is the “other” terminal? I’m going to ask on VAF about this. Not that I’m worried about doing things wrong – the instructions are pretty clear – but mainly because I’m just curious, and I like to know how things work.

Anyway, here’s that outboard rib with the plate resting on top of the spacers that hold it away from the rib. Basically just a quick look at how the plate arrangement will look:

Tomorrow I suppose I’ll keep moving on the sender plates, and maybe after that, I can do the flanges for the fuel cap and tank drain.

When we left off last night, I’d just started to play with the capacitive fuel sender plates. This morning, I got right back on that. I started by repeating last night’s work for the inboard plate – locating and drilling the mount holes in the tank rib. Next, I needed to install the nutplates for the rib-to-plate attachments. These use flush-head rivets, which are kind of unnecessary since the plate doesn’t butt up against anything, but whetever. If I’d had some round-head -3 rivets I probably would have used this, but I didn’t. I did decide to try something new – instead of using the standard 426 rivets for the nutplates, I used NAS1097 rivets. These are commonly referred to as “oops” rivets, because their flush head is smaller than normal, corresponding to the next smaller size rivet. So for example, if you’re driving skin rivets, mess one up, drill it out, and mess up the hole, you can simply drill the hole out to #30 and install a NAS1097 rivet – the head will fit the existing dimple, while the shaft is larger to fit the hole.

Anyway, NAS1097-3 rivets also work well for nutplates – instead of having to either countersink for the larger head, or else dimple and then worry about maybe dimpling the nutplate, you just make a very shallow countersink and there’s much less drama. The countersink is in fact small enough that it’s easily done by hand. The smaller head isn’t an issue because these rivets see basically no stress – their function is simply to hold the nutplate in place and keep it from twisting while the screw/bolt is installed.

After riveting the nutplates on both plates, I installed them on the ribs using the provided spacers. Here’s the outboard assembly:

For once, this wasn’t just me putting things together just for fun. The assemblies need to be test-fitted in the tank to ensure proper clearance with the stiffeners; the instructions call for at least 3/16” between the two. So I got to work clecoing the two ribs into the skin. The inboard assembly was fine – the outboard one, not so much:

The instructions allow for trimming either the stiffener or the plate. I figured the plate was a better bet since it has about 20 times more material than the stiffener. So I marked where the stiffeners contacted the plate, removed the assembly, marked out cut lines, roughed it out with the bandsaw, cleaned up the cuts, and did the whole test fitting thing again.

Much better:

That’s about all that can be done with the sender plates for now. Actually, all that remains is installing the BNC connector on the inboard rib and running the wire connecting the plates; I’ll leave that until the tanks are closer to final assembly, since doing that bit of work required tank sealant.

So I turned my attention to the inboard ribs. These need to be cut for the tank access plates, which means cutting a nice 5.25” diameter circle. The “standard” tool for doing this is a fly cutter, a nasty little thing that lots of people seem to consider dangerous. I’d looked at getting one before and determined that it could probably be had at the Ace Hardware by the house, so I took a field trip over there. Alas, they did not have such a thing. I started to order one from Amazon, but then got to researching and found where a VAF member had tried a different tool, a hole-cutting mill thingy used by HVAC people to cut holes. Oh, and it was available at Home Depot. It was more expensive than the fly cutter, but I could get it today, and it did seem a lot safer to use. So I made another field trip to Home Depot and came home with this:

Basically, there’s a milling bit that goes in the left part and is attached to a drill. The center pivot is set to the desired hole size, a center hole is drilled, and then you just pivot the whole assembly in that center hole while milling a nice circle. The thing worked quite nicely, and soon I had two nice access holes:

Next we need the access plate screw holes around the perimeter. The plate has prepunched screw holes, so it can be used as a handy drill guide. After drilling the screw holes in both ribs, I clecoed the plate and interior doubler in place just so show the setup.

Exterior side:

Interior side:

Normally these plates would mount both the fuel pickup and the float-type fuel sender, but I’m not using the floats in favor of the capacitive senders, and only one of my tanks will get the normal fuel pickup. I decided to mount a flop tube in the right tank to allow for sustained inverted flight. I don’t exactly have any plans to do a bunch of upside-down flying – which would also require an inverted oil system – but I figure I might as well build this in while I’m doing the tanks rather than finding occasion to retro fit the flop tube down the road. About the only downside to doing this is that the flop tubes do wear out after a while – they get stiff and can potentially not have enough flop left to get to the fuel in the bottom of a tank. So this will be something I’ll have to keep an eye on and potentially replace. General consensus seems to be that the life of a flop tube is five years or so. (I considered putting flop tubes in both tanks, but then I’d lose some redundancy if both tubes got stiff at about the same rate. With a standard pickup in one tank, I can count on the left tank always being able to get maximum usable fuel out of the tank.)

Anyway, the main point of that is that the flop tube, unlike the normal pickup, has its fitting towards the forward end of the rib. So the access plate for the right tank needs no modification. The left plate, however, gets drilled for the AN fuel fitting as well as the anti-rotation bracket. That bracket, true to its name, prevents the fitting from rotating, which could potentially pull the pickup out of the usable fuel, which would be…undesirable.

After drilling those two holes, I temporarily installed the fitting and bracket to get a look at the layout:

The bracket needs to be deburred before it’s riveted on, and it’ll also need sealant between it and the plate (as will the fitting), so no for-real installation just yet. Actually, I need to take care of the pickup itself first. The plans have you make your own pickup out of tubing, but most folks seem to buy prefab pickups from Van’s instead. I’m kind of torn between ordering the prefab – which would prevent me from working on this plate any more until late next week –  or just going ahead and making my own. Decisions, decisions.

In any case, there was stuff to be done around the house, so I called it a day. Got some stuff going on tomorrow, but I may have a couple hours to get out in the garage anyway. If so, I’ll probably get the cap and drain flanges fitted and drilled. That should be relatively straightforward, good for a short day. I can also drill the inboard ribs for the fuel sender connector and the tank vent line fitting – at which point I think I’ll have most of the pre-assembly stuff done, save for deburring and dimpling the tank skins and ribs. These tanks are coming together pretty quickly! (so far…

Lots of stuff to do this morning and tonight, but I did manage to get out in the garage for a bit. After thinking it over, I’m going to order the prefab fuel pickup from Van’s instead of making my own. About the only reason to make my own is so I can do it Right Now instead of waiting a few days. Well, that and paying shipping. It would have been nice if I’d discovered this last week before I ordered my replacement rib…oh well.

So instead I started out with the tank drain. This goes on the inboard rear corner, which is the lowest point due to the wing’s dihedral and the slope of the lower tank skin. There’s already a single prepunched hole for the drain, but the rivet holes aren’t there. So the drain had to be positioned, clamped, and then used as a drill guide for the skins.

Here’s the flange clamped to the inside of the skin:

Getting that thing lined up and clamped was a bit painstaking. Right after taking this picture, I realized that the rivet holes alone the sides didn’t quite line up with the holes for the ribs. This bothered me a bit, and I almost unclamped and redid it. Then I thought if how annoying it would be to try and align the holes as well, and considered how many people were actually going to roll under the wing and criticize my alignment. Yeah, it’s fine as it is. I’m not building a showplane.

The flange actually mounts to the outside of the skin. At first I thought it was odd for this to be hanging out in the wind, but the reason is simple (and obvious in retrospect): The more the drain sticks up inside the tank, the less effective it is at removing contaminants and water at the bottom of the tank. There’s a guy who makes fairings for the drains; I’m thinking I might order and use them. If nothing else, it’ll help hide the not-quite-aligned rivets in the flange.

Here is it in place: (I should have clecoed this from the other side of the skin so you could actually see something other than clecos. Oh well.)

Next up is the fuel cap. Things are kind of reversed here from the drain flange; the large hole and the rivet holes are in the skin, while the cap flange has no holes. So in this case, the cap is aligned and clamped in place and the skin used as a drill guide. What makes this fun is that the top of the flange is curved to accommodate the curve of the tank skin, so it has to be carefully aligned before drilling:

If I thought lining up and clamping the drain was annoying, well…that had nothing on this. I think I needed about five hands to get this done efficiently, but I eventually managed to get it clamped in place:

From there, I just started drilling and clecoing. Once I had a couple of clecos in place, I could remove those bulky clamps, and everything went pretty quick. Here’s the cap clecoed in place:

Next the cap flange gets countersunk for the flush rivet skin dimples:

Then there’s one final bit to be done. The fuel vent line will run from the inboard rib all the way out here by the cap, where it will terminate just outboard of the cap. So there’s a little bracket that is riveted in assembly with the cap to hold the free end of the vent line. This clip has to be fabbed from a strip of .025 sheet. The fun part is making the little curl that the 1/4” vent line will fit into. I puzzled a bit over how best to do this, and eventually settled on a combination of a drill bit, pliers, and the vise.

For my first attempt, I stuck a 1/4’ drill bit in the vise with the base sticking out; this would be my bend guide. Then I just held the end of the strip with pliers as I gradually bent it around the bit. This made for a nice circular bend, but the aluminum has enough springback that the tubing fit a little too loosely for my preference. So I tried again with a slightly smaller bit; this time the bend was a bit too small, but I was able to force the 1/4” bit into it to size it back up a bit. After that, the tubing fit nice and snug:

Then it was just a matter of bending the free end to position the tubing nice and high on the skin, then trimming it and drilling the rivet holes. Here’s the finished clip clecoed in assembly with the cap:

At this point there’s not really anything left to do on this tank that doesn’t involved sealant. I suppose tomorrow night I can do all my hole deburring and dimpling on the skin and rivets. Before I can seal, I need to pick up some supplies, most notably MEK for cleaning mating surfaces. I suppose another Home Depot run is in order. I’m almost tempted now to keep going on this tank even after the replacement rib comes in…