Reflections on fuel tanks

I’ve been thinking about fuel tanks for a while now. The Celebrity normally has a 12-13 gallon fuselage tank, located just forward of the front cockpit Cub-style. In fact it may use a Cub tank for all I know, or a Champ tank. It’s enough fuel for flights as long as I suspect I’ll ever really want to make in the plane, but I’m interested in adding some capacity. I can envision a scenario where I fly somewhere and want to fly home without refueling, or fuel simply isn’t available where I land. Dirt strips, busted fuel pumps, whatever. A couple of wing tanks for some extra fuel capacity would be a welcome addition and give me some added flexibility.

Since the sale of Fisher to, I doubt very much I’ll be buying the tank or tanks from them. I got a price for wing tanks — over $400 each. Nah. They’re simple welded aluminum box tanks, which are mounted in the wings by means of some stainless steel straps attached to the main and rear spar caps.

Welding aluminum isn’t really in the cards for me, I don’t think. I considered building the tanks out of sheet AL, riveted and sealed with tank sealant. I have experience building those for the RV-7 and RV-12, but of course I’d need to design and build these from scratch. That option is not off the table yet.

Lately I’ve been thinking more along the lines of fiberglass. I could mock up the tank using cardboard and then foam board, to get the exact size and shape and work out the mounting attachment details as well as the location and details of the fill, drain, and sump. There are several construction methods possible including making a mold, building it over a Styrofoam plug and dissolving the foam once cured, or building a foam or balsa structure and glassing over the inside and outside for a sandwich construction. I could make it all internal or could make the top surface match the top of the wing, include a flange to attach it to the ribs, and either just paint it or cover with fabric.

Once I get back to work on the wings I’ll do some more exploring. Right now Stu’s got some stuff going on in the shop so the wings are still on the rack. Once he’s ready for me to get back to work, I want to accelerate the pace of construction so this thing is done before I die.

False ribs in

There was a delay of a few weeks in construction progress. I had a horrible respiratory thing for a couple of weeks, and didn’t want to pass that on to anyone. We’ve got a rental house we’d been prepping for rent, a wedding, and a short amount of time given to clear a bunch of stuff out of the hangar, including my Vespa which I’ve been doing some work on. So, things sat for six weeks.

I did get over and glued in all of the false ribs. This actually went pretty easy, maybe easier than doing it the “other ” way of gluing them in before the false spars. I was able to just mark up the main and false spars to make sure the false ribs were aligned, then glue and pin them in place.

Next up: Leading edge plywood.

Bottom geodetic and false spars

Yesterday I got the rest of the compression member gussets glued in, then cut and glued in all but two of the bottom geodetic braces. Those two I couldn’t get in because of a clamp holding a gusset in place. I also got about a quarter or so of the geodetic intersections glued and clamped before running out of epoxy.

Today I finished up the geodetic members, and had enough glue left to install the false spars on the front of the ribs. I think I may have actually used all of the spring clamps I had over there… in fact I know I did, because today I took over another dozen I’d gotten for Christmas a couple years back, that had been sitting in a gift bag in the basement. I think there were a couple left today, but certainly less than a dozen.

Top geodetic glued in

Yesterday I went over and cut all of the geodetic braces for the top of the wing. Unfortunately, I’d neglected to take over any epoxy – and I’d finished up the last pair of syringes earlier in the week. I remedied that today.

My strategy of using the geodetic braces to shim the rib cap to spar cap joint worked a treat, as the Brits say. It saved me time and trouble. Most were a close fit, and a couple pieces needed sanding to slip in snugly.

I thought I was going to use a record number of clamps, but then I remembered the wingtip bow laminations. Still, it looked like a lot of clamps.

Everything glued and clamped in place. Joints that aren’t clamped are a snug fit and don’t need it.

Compression members in (lower left wing)

Over the past few days, I have gotten the rest of the ribs glued to the rear spar. I’ve left them unattached to the top of the main spar, as I’m planning to just run the geodetic braces between the rib cap and spar rather than use a separate shim between the cap and spar.

Today I got all four of the compression member assembles cut, fitted, and glued in place, and glued plywood gussets on some of the joints. I ran out of time before I could get the remainder of the gussets cut and glued in.

The State of the Wing address

The main and rear spars are in place, and half the ribs are glued to them. It’s tedious and intense work, so I’ll finish them up tomorrow.

I dug the CW42 and CW43 plywood pieces for the wing walk out of the crate and tried test fitting them. As with many of the kit pieces, they simply don’t fit. At all. The nose reinforcing pieces will be usable, with some trimming. The rear pieces… not so much. Once the ribs are fully secured I’ll see if they can be used at all, or if I’ll need to order some new plywood and cut new ones. At the very least, the notches for the cross braces are rounded at the bottom from being cut out with a router with a fairly large bit. I can clean those up with a band saw, but the ends are also not right by a long shot. It will take some work to figure out how or if I can use these.

All in all I’m just happy I didn’t pay full price for this “kit”. I’ve found an awful lot of pieces that, quite frankly, I’d have probably been better off making myself. But at least now I know what to expect.

New month, new wing

Yesterday I made the call to stop work on the lower left wing and start on the lower right. The remaining things to do on the first wing are all attachment hardware and pre-fabric finishing. Some of those things, like attaching the N strut and landing wire brackets, would make it more difficult to store. I think it will be better to build the other lower wing, then varnish and add the attach brackets to both at the same time.

So, we put the wing on a storage rack in a corner of the shop, and I started laying out the parts for the right wing. We figured out the trailing edge pieces and got them clamped to the bench rail, then slotted in the five ribs I had over there and slid the main spar into place. I’ll need to take over another batch of ribs and get them modified as I did the other wing to clear the torque tube. I spent the better part of six months on the first wing, if you count the time spent on spars and wingtip bows. If you count just the wing assembly time, it was more like four months. A lot of that time was spent on the leading edge wrapping and aileron construction. I’m hoping I can speed that up quite a bit and have this one done quicker, even though this one will include the wing walk.

Mocked up, nothing glued yet. Need to take more ribs over!

Wing work through 2/26

Mostly small things over the past week. I cut and installed some more corner blocking that I didn’t get done before, tested a couple new iterations of the bearing block/spacer, and fine tuned the aileron to the opening. Along the way I’ve noted a few new lessons learned…

  • Never trust the plans, or the supplemental sheets included with them. They’re often inaccurate. Like the AL bracket diagrams that call out 3/16″ holes when they will get 1/4″ AN4 bolts… and on and on.
  • Get the torque tubes in place, holes moved or enlarged where needed, and bearing blocks in place and holes drilled before cutting the ailerons from the wing. Otherwise it’s just a bitch getting everything aligned after the fact to locate the bearing blocks. This of course goes along with correcting all of the many problems with the holes that are pre-drilled in those plywood parts.
  • Make sure all of the corner blocking is installed around the aileron bay as early as possible, and certainly before installing the CW40 plywood stiffeners, aileron leading edge skin, and so on.
  • Go over the plan sheet more often and in greater detail to make sure I don’t miss anything that will be a pain to install later on… like corner blocking that would be much easier to plane or sand to shape than to try to cut to match odd angles.
  • Install the compression struts before the geodetics! That cost a couple hours of added time working through tight openings.

Not necessarily a “lesson learned”, but something I’d like to explore. All of the ribs ended up needing a 1/8 shim between the top rib cap and the main spar. Rather than shim them with separate pieces, I want to see if I can maybe cut and install the geodetics at the same time the ribs are glued, or something. Or possibly just use temporary shims, and glue the top cap to the spar as the geodetics are installed. It could lead to neater and better construction.

Wing work thru 2/20

Top and bottom leading edge plywood has been scalloped. That wasn’t as big a chore as I thought it would be. We used a thin stainless scale to establish a curve between each pair of ribs, marked along it with a pencil, and cut the wood with a utility knife. Some cleanup with sandpaper and Bob’s your uncle.

I’ve made a few iterations of the bearing block. Today I’ll print a couple more test pieces and I think I’ll be done. They fit great, the aileron is very well located, all in all I think it’s a lot more precise than a piece of plywood and a chunk of PVC pipe.

I’ve been preparing some of the wing attachment fittings. I have one or two of each part drilled with 1/8 pilot holes; I’ll use those to match drill the rest, then enlarge the holes for the AN4 bolts. The biggest question was hot wo round off the ends. For that I drilled a 1/8 hole exactly 1/2 inch from the disk on my disc/belt sander. Now I can use the tail end of a drill bit as a pivot pin and put a nice radius on the end of the fitting. I’m glad I kept my gray Scotch-Brite wheel on the bench polisher; it makes quick work of cleaning up the ends of the AL bars.

Wing work – 2/16, and a better idea

Yesterday (2/16) I cut and glued in some corner blocking I had missed on each end of the aileron and the ends of the aileron bay. Lesson learned: that would have been a lot easier had I done it before the geodetic bits and plywood stiffeners had been installed.

I’m getting to the point where I kind of need the torque tube bearing blocks installed. Then there are the 3/8” spacers, which the plans call out as pieces of PVC pipe. I figure, since I’m going to CNC machine the bearing blocks anyway, why not just make those and the spacers one piece? I pulled up the design in OpenSCAD and added the spacer. I gave it a slightly larger ID than the bearing block so it doesn’t add drag to the aileron control. I 3D printed a couple samples to use for fit & function testing. If that goes as expected I’ll get a chunk of 1” UHMW and pass the design file to Stew for machining. I’d planned to use some 1/2” UHMW I have for the bearing blocks, but I’ll keep that for now and use it for something else.

3D printed test part. It’s difficult to see but there’s a step in the bore for the torque tube.